6,500-year-old oven with heating and hot water system is similar to modern technology

6,500-year-old oven with heating and hot water system is similar to modern technology

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Archaeologists in Croatia made an incredible discovery during an excavation at a Neolithic site in Bapska, which experts say is one of the most important in Europe; they found a 6,500-year-old oven, complete with a heating and hot water system, which worked in a similar way to a modern-day AGA cooker, according to a news release in the Croatian Times . In addition, they found a smelted piece of iron ore that is thought to date back thousands of years before man apparently learned to smelt iron.

Lead researcher Marcel Buric, from the Department of Prehistoric Archaeology at Zagreb's Faculty of Philosophy, explained that prehistoric houses at the time were made from wattle and had roofs made of hay, so using an open fireplace was dangerous. As a result, they came up with an ingenious solution, which enabled them to cook food, have hot water and central heating in their dwelling.

The 6,500-year-old oven had a covered stone frame and worked in a similar way to an AGA. Credit: EFE

"It was permanently heated all day long and as the residents came home after a day in the fields they ate hot food cooked by the oven, washed in warm water, and went to sleep in a room heated by the same kiln. Just like some kitchen ovens today," said Mr Buric.

The ancient prehistoric oven worked in a similar way to a modern-day AGA, a heat storage stove and cooker, which works on the principle that a heavy frame can absorb heat from a relatively low-intensity but continuously-burning source, and the accumulated heat can then be used when needed for cooking and other purposes.

The 6,500-year-old oven unearthed during an archaeological dig at a Neolithic site in Bapska, Croatia. Credit: Croatian Times

Also within the Neolithic dwelling, archaeologists unearthed the cremated remains of a 15-month old baby, and a set of deer antlers on the walls of the home, which are the thought to be the oldest known examples of hunting trophies.

However, the research team made another very rare and important discovery at the site - a smelted piece of iron ore by the kiln, thought to date back thousands of years before man learned to smelt and work iron.

"It's not possible to say what it was used for but it is a significant find," said Buric.

Surprisingly little information has been released about this amazing discovery despite its potential to change the current timeline regarding metal-working skills and technology. We hope that photographs and the results of complete testing of the iron will be released in due course.

Featured image: An outline of the Neolithic dwelling in Croatia with the 6,500-year-old oven shown in the bottom right section. Credit: EFE

Simply put, it’s a giant pile of compost with tubing that is coiled throughout the compost pile and then filled with water. The water within the tubing heats up substantially (and relatively quickly).

Jean Pain in the process of building a compost water heater

To give you a glimpse of what a real-life compost water heater is capable of, here’s Ben Falk at Whole Systems Design in Vermont showing off their first generation compost water heater:

They used a mix of wood chips and horse manure to build this compost water heater, and got hot water with temperatures of 140 – 145 degrees Fahrenheit (

60 degrees Celcius). Not bad.

Exploring different heating options

There is a range of options for home heating and they all have pros and cons. Some are more suitable for some homes than others. Annoyingly the choice isn’t straightforward.

Heat pumps

A heat pump is basically the same as a fridge in reverse. Rather than making the inside colder and transferring the heat outside it does the opposite – it extracts heat from the environment outside the house and pumps it into the house. Roughly speaking, for every unit of electricity you use, it will provide 3 units of heat. It's therefore by far the most efficient form of heating there is.

Remarkably it does this even when it’s freezing outside – in fact, heat pumps are now the most fitted heating device in chilly Sweden.

There are 4 types of heat pump:

Hybrid heat pump works alongside your gas boiler. Smart controls pioneered by UK company PassivSystems enable you to maximise your carbon pollution savings (although currently their controls only work with a few brands). The controls do this by switching between the heat pump and gas boiler to use whichever is lowest carbon at the time. So, when electricity from the grid is low carbon the heat pump is used to provide your heat. When the electricity grid is being powered with lots of fossil fuels, the gas boiler will be cleaner and the controls switch to that. On really cold days the controls can use both simultaneously to keep your home nice and warm.

Air-source heat pump extracts heat from the air outside (even when it’s cold!) and uses it to heat the water in your radiators and in your hot water tank if you have one. The heat pump needs to be outside your property. It doesn’t make the water as hot as a gas-fired boiler, so to ensure your house is warm enough it runs for longer. You're also likely to need to increase the size of your radiators. The fan in the heat pump (which also needs to be outside the house) will make some noise, but no more than the background hum of a fridge. Of course the pump will be busiest during the winter months when you are less likely to be outside anyway. This is the option I plumped for.

Ground-source heat pump extracts heat from the ground, so requires a garden for a trench. It is more expensive than an air-source heat pump, but also more efficient and quieter.

Air-air heat pump blows warm air into your house rather than hot water. If you have multiple floors to heat you will need more than one of these.

Costs of heat pumps

Heat pumps will give you an impressive 50-60% reduction in your greenhouse gas pollution footprint, and they shouldn’t increase your energy bill. In fact, if you live in an off-grid property currently powered by oil or LNG (Liquefied Natural Gas) and you switch to a heat pump, the greenhouse gas pollution saving will be greater because oil and LNG are particularly polluting fuels.

But they do come with an upfront cost, including installation. As a rule of thumb, a heat pump is expected to cost around £10,000 to buy and install, depending on what work may need to be done. The cost of my air-source heat pump was £11, 392.50, which included fitting a hot water tank for baths, showers etc.

A government grant is available to cover some of the cost, though not for air-air heat pumps. The amount of grant will vary by technology and how efficiently it will operate in your home, but is likely to cover at least half the cost. Friends of the Earth has joined more than 20 organisations from construction, energy and civil society sectors in calling for government to pay the full cost for poorer households, among other measures. You need to use an accredited installer to get the grant – they’ll be able to give you an estimate once they've inspected your property.

Pros: Heat pumps are a very efficient way of providing heating, using roughly 1 unit of electricity to produce 3 units of heat they are also eligible for a government grant.

Cons: Heat pumps do involve some disruption to your house, eg pipework, and some systems will need bigger radiators. You'll need to be happy with a heat pump outside your house making some noise, although it’s only about as loud as a fridge, and will be working hardest in winter when you are less likely to be outside.

Heat batteries

New on the scene are heat batteries, which can store the heat produced by your heat pump for later use. The heat can then be used to provide instant hot water for showers and baths, as well as heat your radiators. This approach means you can use your heat pump when the price or carbon intensity of electricity is low - often during the middle of the night - and use heat when you most need it (e.g. first thing in the morning or the evening). They also do away with the need for a hot water tank.

As well as working with heat pumps, they can also be used with solar thermal panels, solar PV panels, or charged directly by electricity. They are also small – about a third of the size of a hot water tank and small enough to fit in a standard kitchen cabinet.

Pros: Heat batteries are compact, much better at storing heat than a hot water tank and therefore more efficient, and they enable you to use low cost electricity.

Cons: The upfront cost of installation is likely to be higher than fitting a hot water tank, but they will reduce running costs by 30-40% by using off-peak low-cost electricity and reducing heat losses, so will save money over time.

High-heat retention storage heaters

These are much better insulated than the old fashioned storage heaters. This means they're much better at providing the heat when you need it.

The switch to high-heat storage heaters is straightforward, so long as you have an electric supply near to where you want to fit the radiators. They come with individual room controls and the better models match energy input to weather conditions. However, we have not found an independent review of the different models.

Currently high-heat retention storage heaters won’t reduce your greenhouse gas pollution compared to your gas boiler (especially if you already have smart heating controls). But in a few years’ time they will, as the electricity grid gets more power from renewables, so if you need to replace your boiler it’s worth opting for these (if you don’t want a heat-pump).

Caveat: they will cost you more to run, because even economy 7 electricity is more expensive than gas. The additional cost is likely to be around 20% more according to our calculations.

High-heat retention storage heaters don’t heat your water. If you have the roof space, solar thermal panels can help you with that, and government grants are available (for thermal panels) under the Renewable Heat Incentive. You will need to use an accredited installer to get the grant, which should cover at least half the cost of around £5,000. High-heat retention storage units are a lot cheaper than heat pumps to fit though, costing around £7,000 if you have 8 radiators.

Pros: Cheaper than heat pumps to fit. Easy to install. In a few years' time they will reduce your carbon footprint. Their gradual release of heat makes them ideal if you're at home some or all the day.

Cons: No government grants available for high-heat retention storage heaters.

Electric radiators

These radiators use electricity to provide you with heat when you need it. They are cheap to buy and fit compared to the options above. They might cost less than a new gas-fired boiler. Many will be programmable and have smart controls.

However, they will cost you an arm and leg to run – potentially tripling your energy bill.

For at least the next 5 years (and probably longer) they will generate more greenhouse gas pollution than your gas-fired boiler. This is partly because they use electricity at peak times, when the grid is mostly powered by fossil fuels.

If you're concerned about the environment you won’t want to opt for electric radiators. And beware the sales people: we've seen a number of eyebrow-raising claims.

Pros: None that I can think of.

Cons: Will increase your carbon footprint for at least the next 5 years.

Infrared heaters

Infrared heaters are pretty new to the market They provide warmth by heating objects rather than the air – like sitting in the sun on a winter’s day, you can still feel warm even though the air is cold around you. That doesn’t mean your house will be cold, because as your household belongings and the fabric of the house (such as sofas, floors, walls, etc) warm up, they will radiate the heat back out.

One company selling these heaters (Herschel Infrared) gave us illustrations that suggested heating bills will reduce by around a third compared to conventional electric radiators. If this is true, they will out-perform electric radiators in both costs and carbon footprint, although they still wouldn't be as good as heat pumps for reducing your carbon footprint.

One advantage of these heaters is that they are super thin and light-weight – they can be located on your ceilings pointing down, printed over to appear to be pictures on your wall, or hidden behind mirrors.

The retailers claim they will be no more expensive than fitting traditional electric heaters.

Pros: Can look great, and offer savings compared to conventional electric radiators, according to the manufacturers.

Cons: Because infrared heaters are new on the market there is little or no independent evidence on how well they work in practice. Like conventional electric radiators they will use peak time electricity, not Economy 7, so are likely to be more expensive to run than storage heaters and will increase your carbon footprint for at least a few more years.

100% Efficient

Ever since the thermoelectric effect was first described by Thomas Seebeck in 1821, thermoelectric generators have been infamous for their low efficiency in converting heat into electricity. [1, 3-6] Today, the electrical efficiency of thermoelectric modules is only around 5-6%, roughly three times lower than that of the most commonly used solar PV panels. [4]

However, in combination with a stove, the electrical efficiency of a thermoelectric module doesn’t matter that much. If a module is only 5% efficient in converting heat into electricity, the other 95% comes out as heat again. If the stove is used for space heating, this heat cannot be considered an energy loss, because it still contributes to its original purpose. Total system efficiency (heat + electricity) is close to 100% – no energy is lost. With appropriate stove design, the heat from electricity conversion can also be re-used for cooking or domestic water heating.

Modern Beginnings: Chimneys And Stoves

After the 14th century, chimneys appear in written literature. However, their use seems to have spread very slowly. Chimneys were still rare enough 200 years later that one English architect, upon encountering functioning chimneys at Bolton Castle, exclaimed: “I muche notyd in the hawle of Bolton, how chimneys were conveyed by tunnells made on the syds of the walls… and by this means … is the smoke of the harthe in the hawle strangely conveyed.”

Early chimneys were very large, so as to allow a chimney sweep to climb into them. But the size precipitated such vicious drafts that room divider screens sometimes had to be used to shield the occupants.

Stove heating soon advanced beyond the crude devices first used. The first freestanding warm-air stove was probably the “Furnus Acapnos” or “smokeless stove” invented by Dalesme in France in the late 1600s. Dalesme introduced fresh fuel in the same opening as combustion air, directing all combustion products over already-burning fuel, a design that ensured complete combustion.

Although the smokeless stove was a great advance, it and other heating innovations were accepted slowly, for “…few housekeepers are philosophers enough to be willing to undertake the management of a machine requiring especial mental effort, where the advantages are not directly visible to the senses.”

The earliest stove in North America was probably a cast iron box stove invented by Dr. John Clarke of the Massachusetts Bay Colony about 1652. This type of stove had originated in Holland and was imported into England after 1600. By the mid-1700s, cast iron box stoves were being manufactured by a number of eastern Colonial American foundries.

Stoves continued evolving throughout the 1800s. Notable improvements included the base burner stove invented by Eliphalet Knott in 1833, and the airtight stove invented by Isaac Orr in 1836.

A stove with thermostatic draft control was invented by F.P. Oliver in 1849.

By the time of the Civil War, cast iron stove manufacturing was a large and well-established industry, particularly in the northeastern U.S. By 1900, thousands of different designs (many approaching pieces of art in their appearance) were produced by dozens of manufacturers.

The Truth About: Heating and Cooking with a Wood Stove

Winter is here in North Carolina. Frosty grass every morning. Freezing fingers during barn chores. It’s time to get cozy and talk about heat.

First of all, wood stoves are not an automatic environmental impact win. You should know that right up front (more about it later). We always consider environmental impact when we choose but we also take into account other things: comfort, safety, labor commitment and especially resilience. Our wood stove was chosen mainly for resilience.

A merry fire. I found that tea kettle in my aunt’s yard. The black contraption next to it is a fan operated by a peltier motor, which turns heat into electricity, saving me the trouble of shutting off the fan when the fire dies. I was skeptical of this inexpensive little device, but it’s been going strong through one and a half heating seasons, and it really does move the hot air back to the bedrooms. I’m impressed.

I grew up in Alaska, where the weather is legitimately trying to kill a body at least nine months of the year, and sometimes also on random June afternoons. As a child, one of my family’s worst encounters with hypothermia was on a random June afternoon. We had electric heat set in the mid-60. We had a two-story living room which conclusively proves that some architects are idiots. It felt about 45 degrees sitting on the couch. I’m not a well-insulated person. Maybe that’s why I’m so concerned with heating resilience, even now that I live in the south.

It gives me great comfort to know I’d be warm if the power went out, but I was pretty nervous when I designed my family’s little house with a wood cook stove as our sole source of heat. I’d done short stints in wood-heated buildings, but never long-term. What if I hated it, but I was stuck with it because we were out of money at the end of the build? What if I couldn’t manage a wood-cooked dinner on time with two little kids? What if I was miserable because it just couldn’t keep the place warm enough? But into our third winter, I have to admit it’s one of my favorite parts of the house.

Part of the reason is comfort. I have some chronic pain, and a mid-60s house is pretty uncomfortable to me in the evenings when I finally sit down. It’s the worst part of the day for people with spine issues, when muscles are tired and tight. I was shocked to learn that different temperatures in different zones and at different times is much more comfortable than 67 everywhere always. By the time I’m done eating dinner, my back absorbing all that radiant heat, I’m much more relaxed than I used to be in a cold house. When I wake up in the farthest back bedroom on a winter morning, I’m still cozy even though it’s chill. Even arriving home late from a week-long December trip, it’s nice enough in an hour or two. What a pleasant surprise!

My husband made me this graph with data he collected so I could illustrate for you exactly how comfy I am. See that dip at week 5? We went to visit my in-laws for a week. Thank you husband!

There was a learning curve with cooking to be sure, but not a terribly steep one, probably because our model seems very well designed. It takes a smidge longer to heat up than an electric stove, requires more finesse, and gives you less immediate control over the temperature. That might all sound bad, but the ultimate result seems to be better. I used to burn dinner once in a while, which I hated because I despise waste, and I’m sometimes running later than some people would like (it’s my husband- my kids couldn’t care less).

I’ve burned two things total on the wood stove, which is a significant reduction. The reason is probably that the wood stove is a natural check on frustration. It just takes as long as it takes, and there’s no way to turn the knob up when I’m impatient only to regret it later. Dinner takes me an hour no matter what I cook, whether I’m using a nice gas stove, any of the terrible electrics I had in rental houses, or my trusty Coleman two-burner at a campsite with a cutting board balanced on my knees. Lo and behold, with the wood stove it usually takes me about an hour from striking the match to dishing out food. It takes the same time, but subjectively it feels less rushed.

Cooking apple cinnamon oatmeal on a resilient stove is pretty much the same as cooking it on a regular stove. When I need more space I take the fan and kettle off the stove top.

Never mind burning food how about burning myself? I admit I used to do that too once in a while on our former electric or gas stoves. Again, I’ve done it twice on the wood stove, another significant reduction. I think the reason is it’s impossible to forget the wood stove is hot, because it’s hot. Not just to the top but also to the front, which is a boon to chilly thighs, and also demands constant remembrance and respect. Our littlest kid was two and a half when we started heating with wood, and even she had no problem learning to walk around the other side of the table. No child in my house has even come close to getting scorched. If she’d been littler, I would have had to engineer a serious safety barrier.

Our stove is the Vermont Bun Baker. You can cook on top of lots of different models that are more efficient and cheaper, but the Bun Baker comes with an oven. This was a silly thing for me to spend a lot of extra money on, since I’m really not a baker. What can I say about this absurdity? When designing a house for the first time it’s easy to get fixated on maximum functionality. I definitely let my anxieties during that difficult process seep into my actions in weird and surprising ways.

The oven works fine. I do use it, but only in late December, January, and February, because the stove doesn’t run enough to get it hot in the other heating months. I use my sun oven in the summer and I personally don’t miss baking in the shoulder seasons.

Another advantage disguised as a drawback is that it takes time and attention to heat the house with wood, which encourages me to slow down and hibernate. Spring, summer and fall border on frenetic because there is a lot to do and I’m naturally inclined to go go GO! When I come inside to make a winter dinner, settle the mind and sit on the tile floor to light the stove, it’s a sort of relaxation. It’s a ritual that focus me on my task. If we were the type of household where no one is home except to sleep and bathe, rather than the type that has reorganized our lives around home and being there, it might be pretty inconvenient to heat with wood.

Our stove is fitted with the optional water jacket that lets us draw hot water off of it. This is a scary idea to lots of people because wood stoves used to heat water in sealed, pressurized loops. When water turns to steam it expands in volume 16,000 times, turning your metal tank into a bomb. People died. That isn’t possible with our setup.

Not pretty, but totally functional. The grey tank is our electric hot water heater, not usually on. The blue tank is a pressure relief system. The red device is the efficient electric pump, and that thing surrounded in bubble wrap is the heat exchanger. The fill valve is the yellow handle. The power supply is the green cord. The wood stove is on the other side of the wall.

In our system, the hot water is in two loops. One loop is pressurized from the city water to the water heater to the sinks and shower. The other loop is open to the air (“vented”), and this is what gets directly heated by the fire. The water heats in the jacket, and as it heats it becomes less dense, rising naturally up a copper pipe to a heat exchanger. From the bottom of the heat exchanger it returns via another pipe to the jacket (you can see this copper return pipe in the picture at the top of the post, and the riser pipe insulated with silver stuff next to it). This is called a thermosiphon and it operates only by gravity. So cool! There’s a fill valve and a little bit of clear plastic extension sticking up from a T at the top, so I can see that I’ve filled it enough.

The other side of the heat exchanger is plumbed to the pressurized household water in a loop operated by an efficient electrical pump with a temperature-sensitive switch. The switch senses the temperature of the water in the unpressurized stove-side loop and turns on the pump at 130 degrees. The circulating pressurized water draws heat off the unpressurized loop through the exchanger.

On cool, one-fire winter days this setup acts as a preheater, reducing the electricity needed to bring the water up to full heat. On colder two-fire days it heats the water entirely, for just the minimal electrical cost of running the pump.

On the one hand, I’m extremely proud of this little system. The idea got hold of me and just wouldn’t let go. I had to try it. I read everything I could find on thermosiphons, and based on that I basically guessed about which makes and models of pump, heat exchanger and switch I needed and how to arrange them, and darned if the thing doesn’t do exactly what I hoped it would. It definitely saves us electricity, helping to invert our usage. See the graph below: we use the least electricity in the coldest months when other households use the most, therefore driving less extra demand for electricity at peak times, which is more emissions-intensive. That’s about the most successful outcome for one of my plans, ever.

My husband the data fiend made this graph of electricity use comparison between my house and my parents’ house, which is in our neighborhood. Thank you husband! You’ll notice we’re lowest when they’re highest. How is our usage so low?? The wood stove is part of it, but don’t worry, I’m writing a whole post about the rest as we speak.

On the other hand, there are some significant drawbacks. The littlest one is that cool water is drawn into the tank as you shower, but it sits at the bottom and doesn’t get to the tap until the hot runs out or unless the pump mixes it up. If the pump clicks on while you’re in the shower, the water cools off as the loop mixes cool water into the hot in proportion to the time you’ve been showering. This is easily remedied by unplugging the pump before I get into the shower. But, if I forget to plug it back in once I’m clean, the stove loop will overheat and spill steam out the vent into the room the water heater is in. No big deal, but not exactly a great feature.

A bigger problem would occur in a winter emergency. The whole point of the wood stove is that we still have heat if utilities fail due to bad winter weather. If the power went off, we could be fine. We have a little solar generator that would run the pump. But if the water also went off for a really extended period we’d be in bigger trouble, and here’s why:

If I fire the wood stove with the jacket empty it’ll warp, break the watertight seal and make the whole setup useless. Also, the energy has to be drawn off that water, or it’ll steam the room pretty good which could lead to mold over time. Finally, the water must be drawn out of the water heater and new cool water added, because while it would be impossible for that little exchanger to heat 40 gallons to steaming and explode the water heater, it’s still scary to have that much water so hot. It would take days to get too hot, even in cold weather. But I’m not willing to bet we’ll never have a utility interruption longer than a few days.

Without running water, the water heater would have to be hand-filled and hand-emptied. It could definitely be done with some modification and the right set of funnels and elbows, but it’s not the sort of thing I want to be fooling with in an extended wintertime emergency.

Assuming our minds and bodies were occupied with more essential tasks such as parenting in a blackout, we’d be faced with the choice of draining the jacket, running the stove and letting the whole setup fail, or letting the house cool off. Even with low solar gain, cold days and no heating, our data says the house doesn’t drop below 54 degrees, which is totally survivable. Just not comfortable for small children and people with spine problems, who would already be under some stress.

We would be reluctant to let the setup fail because of the third drawback: it was freakin’ expensive. By the time I paid the premium for the optional jacket, bought the components and paid a plumber to solder it together, this is an $800 piece of technology.

It would have been cheaper and more edifying if I’d hooked it up myself, but I was absolutely frazzled after 17 months building our first house. I wasn’t confident I could learn soldering quick enough and well enough to not screw the whole thing up. It was Thanksgiving, my relatives were coming and we needed to start heating and cooking inside immediately. I had an undiagnosed selenium deficiency causing spectacular fatigue and hair loss. My husband was working his standard 50+ hours a week, and I was home with an unusually challenging four-year-old and also a two-year-old. Basically, life got in the way.

It’s disappointing, though, that one of the coolest things in my house isn’t really scalable or even a very good demonstration because it’s prohibitively expensive. “The bougie-est water heater ever,” I think my husband termed it. He’s completely right. It doesn’t return enough either in saved money or in reduced environmental impact to justify the cost, complexity or embodied energy in the parts. But I just had to try, you know?

While we’re being honest about drawbacks, let’s talk about the drawbacks of a wood stove itself. First of all, burning wood releases carbon dioxide. How much carbon dioxide, for any particular stove? That’s really hard to tell without equipment to measure. One best guess is 2.5 tons per cord of hardwood such as oak or walnut (a cord is a neat stack 4 feet wide by 4 feet tall by 8 feet long)

We currently burn some walnut because one fell next to the house. I sold the usable portion of the trunk and root ball we’ve burned limbs and also sticks as kindling. We also burn some hundred-year-old pine out of the houses we’ve torn down. This softwood is less dense than hardwood so it probably emits less. It’s too insect-damaged to reclaim even for my funky aesthetics, but it’s better to burn it than bury it in the landfill where it would emit methane, a shorter-lived but much more effective greenhouse gas.

However, there are some contortions in our collective reasoning about burning wood that we should examine. It’s generally considered a renewable resource, which sounds great and it is, sort of. Trees do regrow, but only if they are allowed to regrow. If the soil isn’t too damaged by their felling, if the land doesn’t get used for something other than forest causing some pretty wild swings in soil carbon, if the trees planted to replace them are a good fit for the ecosystem and are cared for so they survive (I’ve learned the hard way that young trees are fragile). If, and this is a big if, we don’t cut trees faster than they can regrow.

On our land this isn’t a problem more trees fall than we could ever use for firewood. Many are left to support the creatures that thrive on rotting wood. We won’t even finish burning ancient framing out of defunct houses until the end of next winter at the soonest. But in other contexts, failure of forest regeneration is a problem.

Another issue is that, even though fresh-fallen tree carbon isn’t ancient carbon, it is nevertheless sequestered carbon. It’s held safely in living bodies rather than dangerously in the atmosphere, and we’re letting it out in one hot bright flash. We imagine that this doesn’t matter because a fallen tree would give its carbon back to the air anyway, but that’s not what happens. A fallen tree is fed upon, and some not-insignificant portion of its carbon therefore remains sequestered in other living bodies or in the fluffy forest soil humus (that’s the organic matter in soil). Some is breathed back into the sky, but not all of it, and not instantly.

Processing and transporting wood also has energy requirements. It’s got to be cut and split somehow, and while we split by hand rather than by machine, we’re not badass enough to cut much by hand yet. It’s a very small part of the carbon footprint of our wood, because like I said, the tree fell right next to the house. For a wood stove that needs its feed logged with larger machines and then moved dozens or hundreds of miles, it’s not so small.

Burning wood also releases things other than carbon, some of which are quite nasty, health-wise. Lifespans were shorter when American cities were wood-heated, and they still are in developing nations for exactly the same reason. Keeping the smoke out of the house is critical. Keeping the smoke in the neighborhood at low-enough levels is critical. In short, if you don’t have a patch of land large enough to produce more fallen trees than you could ever burn, you and your neighbors might be making the area too smoky for good health. It’s not all bad some of those wood-originating aerosols are cooling the planet, even as carbon warms it (here’s a thorough explanation). But that doesn’t mean we should breathe it.

So what’s the bottom line on wood stove carbon?

Some data from my parents’ house allows me to guess how much electricity might heat our little home, sans-stove. They live right up the street, so the climate is comparable. Overall their insulation and air leakage probably isn’t much different my attic insulation is definitely better, but then again I have a French door that is currently rather poorly sealed (I’m getting to it, I swear).

Our living area is half the size, which doesn’t halve our heating needs because our surface area to volume is larger, and therefore so is our heat loss. Our house is banked into the earth, which reduces some of that loss. They only have small children there sometimes, so their doors get left open less often. (“Why is the door open?” must be my most-uttered winter phrase, second only to, “Yes, you DO need socks. It is 31 degrees. Put on socks.”)

It looks like my folks emit maybe a ton of carbon a year strictly for electric winter heating. I’m not sure exactly how much more they use for winter cooking and hot water, because I haven’t yet used our handy Kill-A-Watt on any of their appliances. If their percentage of household energy used to heat water follows the typical pattern (about 14% of the total) they might be emitting another ton and a half there, just in the winter. They run their electric stove and/or oven at least two hours a day, which means over half a ton from cooking, for a total of at least three tons.

We burn about a cord and a half of wood per winter, which could be about four tons, given that wood cook stoves aren’t the most efficient. This also does all of our winter cooking and much of our winter hot water, in addition to heating. We’re less efficient in terms of carbon emissions, but maybe not by too much. It’s worth it to me, for the peace of mind.

How do you heat? If you have a moment, I recommend you do your own little experiment and check out your carbon emissions by using your electric bill. Compare the difference between, say, October and February if you live in the south, or maybe June and January if you live in the north. See the average difference in kWh/day, then multiply that by heating days, and multiply again by 1.45 lbs/kWh. That’s the average for a unit of electricity from the U.S. grid Mike Berners-Lee gives in How Bad Are Bananas? The Carbon Footprint of Everything. When you do your math remember that’s 2000 lbs to the ton, not 1000.)

Or take the number of cords of hardwood you burn, multiplied by 2.5 tons/cord (if somebody has a really awesome source that supports or contradicts that number, please send it to me). How do your heating carbon emissions look? Is your heat source more or less resilient in case of a power outage, water outage or other likely emergency? What are the other drawbacks and advantages? Tell us below.

Honeywell L8148A Aquastat problem

Have a Honeywell L8148A connected to my FHW boiler. The aquastat is not firing up the boiler. The boiler is dead cold.

I have 24v across the T screws. I have 120vac on L1, ground on L2, L3 jumpered to L1 (factory installed).

Temperature dial is set to 160. High limit is unchanged from the factory setting (190 I think)

Boiler does not fire up to heat to low limit.

I set my thermostat to 90. Its only 75 in the room, I hear the thermostat click on to call for heat.

The boiler still doesn't fire.

I do not get 120vac across C1/C2, nor do I get it across B1/B2.

Also do not have 120vac on either B or R

I ran down to the supply house yesterday afternoon to pick up another aquastat, figuring this one went bad.

Just finished swapping it out. Same problem. Doesn't turn on the burner to get the boiler up to operating temperature. Voltages check out on T1/T2 and L1/L2/L3.

Lost. Not sure what else can be wrong? Help?

Thanks. Disconnected the T wires, and powered up the aquastat. If I jumper the T leads, it will fire the boiler.

So, this would suggest I have a bad zone valve somewhere?

Depends how your system is configured. If you have zone valves with individual thermostats and end switches, then yes, may have a failed switch (or the actuator) or could be a break in the wiring.

If none of the zones fire the boiler I'd suspect a wiring issue. Could also be a limit (hi temp or whatever) depending on how they are wired in.

Thanks. I don't mess with the zone valve wiring. The house has three zone valves, two feed new construction and the third feeds a hydro/central-air air handler which services the "old" section of the house.

If it were simply zone valves I could handle the wiring, but the air-handler has a half-dozen transformers and switches in it which also connect to the thermostat and zone valve. The wiring to handle cooling, heating, turning the fan on and off, etc. is beyond me.

My guess is the wiring problem is contained in the hydro-air unit, I'll have to trace out the circuit with my multimeter when I get home tonight and see if I can determine where the 24vac current is originating from.

I've been looking for a reason to ditch the hydro-air unit (we do not use central A/C -- way too expensive to even contemplate given electricity rates) and instead subdivide the rooms the hydro-air unit services into several zones with baseboard heat.

This may be a good opportunity to do just that, for the cost of some baseboard, a few zone valves and thermostats, and a zone valve controller, I could do the conversion over the long upcoming weekend.


Typically hot water storage tanks are wrapped in heat insulation to reduce energy consumption, speed up the heating process, and maintain the desired operating temperature. Thicker thermal insulation reduces standby heat loss. Water heaters are available with various insulation ratings but it is possible to add layers of extra insulation on the outside of a water heater to reduce heat loss. In extreme conditions, the heater itself might be wholly enclosed in a specially constructed insulated space.

The most commonly available type of water heater insulation is fiberglass, fixed in place with tape or straps or the outer jacket of the water heater. Insulation must not block air flow or combustion gas outflow, where a burner is used.

In extremely humid locations, adding insulation to an already well-insulated tank may cause condensation leading to rust, mold, or other operational problems so some air flow must be maintained, usually by convection caused by waste heat, but in particularly humid conditions such ventilation may be fan-assisted.

Most modern water heaters have applied polyurethane foam (PUF) insulation. [ citation needed ] Where access to the inner tank is a priority (in cases of particularly aggressive minerals or oxygen levels in the local water supply) the PUF can be applied in encapsulated form, allowing the removal of insulation layer for regular integrity checks and if required, repairs to the water tank.

In a solar water heating system, a solar hot water storage tank stores heat from solar thermal collectors. [3] The tank has a built-in heat-exchanger to heat domestic cold water. In relatively mild climates, such as the Mediterranean, the (heavily insulated but metal-wrapped) storage tanks are often roof-mounted. All such tanks share the same problems as artificially-heated tanks including limestone deposit and corrosion, and suffer similar reductions in overall efficiency unless scrupulously maintained.

While copper and stainless steel domestic hot water tanks are more commonplace in Europe, carbon steel tanks are more common in the United States, where typically the periodic check is neglected, the tank develops a leak whereupon the entire appliance is replaced. [4] Even when neglected, carbon steel tanks tend to last for a few years more than their manufacturer's warranty, which is typically 3 to 12 years in the US. [ citation needed ]

Vitreous-lined tanks are much lower in initial cost, and often include one or more sacrificial anode rods designed to protect the tank from perforation caused by corrosion [5] made necessary since chlorinated water is very corrosive to carbon steel. As it is very nearly impossible to apply any protective coating perfectly (without microscopic cracks or pinhole defects in the protective layer) [6] manufacturers may recommend a periodic check of any sacrificial anode, replacing it when necessary.

Some manufacturers offer an extended warranty kit that includes a replacement anode rod. Because conventional hot water storage tanks can be expected to leak every 5 to 15 years, high-quality installations will include, and most US building/plumbing codes now require, a shallow metal or plastic pan to collect the seepage when it occurs.

This method stores heat in a tank by using external heat-exchangers (coils) that can be directly tapped or used to power other (external) heat-exchangers.

The chief benefit is that by avoiding drawing-off domestic hot water directly, the tank is not continually fed with cold water, which in 'hard' water areas reduces the deposit of limescale to whatever is dissolved in the original charge of water plus relatively trivial amounts added to replace losses due to seepage.

An added benefit is reduced oxygen levels in such a closed system, which allows for some relaxation in the requirements for materials used in the hot water storage tank and the closed water circuits, external heat exchangers, and associated pipework.

While an external heat exchanger system used for domestic hot water will have mineral deposits, descaling agents extend the life of such a system.

Another method to store heat in a hot water storage tank has many names: Stratified hot water storage tank with closed water circuit, stratified thermal storage, thermocline tank and water stratified tank storage but in all cases the significant difference is that pains are taken to maintain the vertical stratification of the water column, in other words to keep the hot water at the top of the tank while the water at the bottom is at a distinctly lower temperature.

This is desirable in places with a wide climatic range where summer cooling is as important as heating in winter, and entails one or more of the following measures:

  • Different heating and cooling loops must send the heated or cooled water in with as low a velocity as possible. (This necessarily entails heating and cooling loops having velocity controlled pumps and tube ports with the maximum feasible diameter.)
  • For cooling applications, cool water is sent out from the bottom and warm (return) water is fed in at the top.
  • Heating applications get hot water out at the top and return cool water to the bottom.
  • "Stratification-enhancing" devices within the hot water storage tank (but if the water inlet velocity is as low as possible this might not be needed).
  • A more advanced heat control system[8] is required.

When a stratified hot water storage tank has closed water circuits, the water temperatures can be up to 90 to 95 °C at the top and 20 to 40 °C at the bottom. Calm, undisturbed water is a relatively poor heat conductor when compared to glass, bricks and soil.

(Illustrated by a still lake, where the surface water can be comfortably warm for swimming but deeper layers be so cold as to represent a danger to swimmers, the same effect as gives rise to notices in London's city docks warning 'Danger Cold Deep Water).

Accordingly, an arbitrary volume of hot water can be stored, as long as the stratification is kept intact. In this case there must not be vertical metal plates or tubes as they would conduct heat through the water layers, defeating the purpose of stratification. When effectively employed this technique can maintain water as high as 95 °C (i.e. just below boiling) yielding a higher energy density, and this energy can be stored a long time provided the hot water remains undiluted.

Depending on the purpose of the installations, water exchanges tapping different levels allow water temperatures appropriate to the required use to be selected. [7]

In many solar heating systems the energy parameters can be read as a function of time, from the 'dwell' time necessary to transform daylight into heat, at its peak the maximum hot water temperature near the top of the tank. [1]

When flow starts from the uppermost outlet, cold water enters the tank at the bottom. This drop in temperature causes the thermostat to switch on the electric heating element at the bottom of the tank. When the water at the top of the tank is drawn off the hot water at the top is displaced by relatively cooler water, the top thermostat turns the top element on. When the flow stops, the elements stay on until their settings are met. [9]

While it is common to have the top and bottom thermostats set differently in order to save energy, the fact that hot water rises means the thermostat controlling the upper element should feed the hottest supply, while the lower element the warmest.

If the thermostats in such a system are reversed - warm feed from the top, hot from the center - it may not only affect the energy efficiency of the system, feeding scalding water to a domestic hot water outlet may be dangerous, or if directed to warm-feed washers damage them beyond repair.

Hot water can cause painful, dangerous scalding injuries, especially in children and the elderly. Water at the outlet should not exceed 49 degrees Celsius. Some jurisdictions set a limit of 49 degrees on tank setpoint temperature. On the other hand, water stored below 60 degrees Celsius can permit the growth of bacteria, such as those that cause Legionnaire's disease, which is a particular danger to those with compromised immune systems. One technical solution would be use of mixing valves at outlets used for sinks, baths or showers, that would automatically mix cold water to maintain a maximum below 49 C. A proposal to add this to the building code of Canada was unsuccessful. [10]

How Do Warm Air Heating Systems Work?

Warm air heating, or warm air central heating, is the process of turning cool air into warm air by passing it to a heat exchanger via a fan and then blowing it into rooms through vents in the floor, walls or ceiling.

The heated metal plates within the exchanger heat the cool air as it passes across, turning it into hot air. Once the air has been warmed it is passed out to the remainder of the building. This continual process carries on until a predetermined temperature on the thermostat is reached.

The system then remains on standby mode until the temperature drops below a limit (set by the occupants of the house) at which point it starts warming the air again.

Beyond fireplaces: Historic heating methods of the 19th century

Thanks to modern heating systems, we can enjoy the cozy picturesqueness of a fireplace without depending on it to keep our homes warm. But that wasn’t the case in 18th- and early 19th-century America.

“Up through about 1800, the wood-burning fireplace—very popular with English settlers—was the primary means of heating a home,” explains Sean Adams, professor of history at the University of Florida and author of Home Fires: How Americans Kept Warm in the Nineteenth Century. “The problem was that winters in America can be much harsher than in England. The weather quickly exposed how inefficient fireplaces are at heating a room.”

The majority of the heat in a fireplace goes up and out of the flue. What little heat does make its way into the room gets concentrated directly in front of the firebox, leaving the rest of the room quite cold.

A fireplace with a Franklin Stove insert. Photo by Robert Khederian

In 1741, Benjamin Franklin sought to improve the efficiency of the fireplace. He introduced a cast-iron insert for the firebox—called the “Franklin Stove”—in The Papers of Benjamin Franklin, volume 2. While it didn’t fundamentally change the design of a fireplace, it addressed his theory about heat.

“Franklin believed heat to be like liquid—he was trying to keep the heat in the room as long as possible, or else it would rush out of the room,” explains Adams.

The Franklin Stove had a series of baffles, or channels, within the stove to direct the flow of air, to keep as much of the heat circulating in the firebox and flowing out into the room as possible. However, the design had problems.

“The stove had to be very tight,” explains Adams. “If there were any leaks, smoke leaked out into the room. Wind would also blow the smoke back into the room. It wasn’t considered a real success.”

Toward the end of the 19th century, the inventor Count Rumford devised a fireplace designed along a set of proportions so it could be built on a variety of scales.

"In the fireplaces I recommend," Count Rumford writes in a 1796 essay, "the back [of the fireplace] is only about one third of the width of the opening of the fireplace in front, and consequently that the two sides or covings of the fireplaces. are inclined to [the front opening] at an angle of about 135 degrees."

The Rumford fireplace efficiently burned wood while its characteristically shallow firebox reflected as much heat as possible out into the room as possible. The handy design of the Rumford gained a strong following.

Thomas Jefferson installed eight of them at his country house Monticello. Rumford fireplaces became so mainstream that Henry David Thoreau wrote about them in Walden as a basic quality of the home, alongside copper pipes, plaster walls, and Venetian blinds.

By the 1820s and 1830s, Adams explains, coal was quickly becoming a dominating fuel type. Stoves that could burn either wood or coal—the type being pushed was Anthracite, or “hard” coal—became popular.

Iron stoves were not new technology. While English settlers brought fireplaces, German settlers had iron stoves that did a good job of heating a space.

An example of an elaborate iron stove. Courtesy of Library of Congress.

But what was new was the type of fuel: coal. Adams explains that since coal was so different from the familiar fuel type of wood, it took a little while to gain popularity.

“Coal was first marketed in a similar way to how some new technology is marketed today,” says Adams. “You needed early investors willing to take the risk. It was billed at ‘the fuel of the fashionable,’ which would revolutionize home heating.”

To match, coal stoves became highly decorative, featuring intricate ironwork and decorative finials to make them just as desirable as they were utilitarian.

Coal became mainstream in post-Civil War America. Wealthier families might have burned coal in basement furnaces—with specific rooms dedicated for coal storage—while poorer families might have used little stoves in individual rooms in their home.

The architecture of the home also changed as heating technologies shifted. While Colonial houses of the 18th century needed big chimneys to support multiple fireplaces, houses built in the later half of the 19th century only needed ventilation space for stove pipes. That translated into skinnier chimneys.

Inside, mantlepieces sometimes remained as a backdrop for the stoves. Even though they were technically no longer needed, they continued to act as a focal point in a room.

A mantle that was never designed to surround a fireplace but rather be a backdrop for a coal stove.

Also coming into play in the 19th century was steam heating, which first appeared in the 1850s but gained popularity in the 1880s. Adams explains that this is just another form of coal heating, as coal would be used to heat the water that turns into steam.

Steam heating was first used in institutional buildings like hospitals but then moved to residences. One of the most elaborate examples of a steam-heating network in the 19th century was at Biltmore Estate, the Vanderbilt-owned mansion in Asheville, North Carolina.

“Richard Morris Hunt, the architect of Biltmore, needed to heat roughly 2,300,000 cubic feet of space for the 175,000-square-foot house,” says Denise Kiernan, author of The Last Castle: The Epic Story of Love, Loss, and American Royalty in the Nation's Largest Home.

Kiernan explains that the subbasement of Biltmore, which was completed in 1895, had three boilers capable of holding 20,000 gallons of water each. Those boilers created steam that circulated to radiators in a network of shafts around the house, a system that seems simple in theory but quickly intensifies when one realizes that the network had to heat 250 rooms.

“Of course—this heating system had help from 65 fireplaces, some more utilitarian, others wildly elaborate,” Kiernan adds.

Heating the largest private home in America was no small feat: In The Last Castle, Kiernan reports that 25 tons of coal were burned in two weeks during the winter of 1900. To prepare for the winter of 1904, the Vanderbilts placed a coal order for 500 tons to be shipped and ready.

Biltmore estate in Asheville, North Carolina. Courtesy of The Biltmore Company.

Regardless of how elaborate or rudimentary the heating system of choice was in the 19th century, something that seemed to connect all methods, whether it be wood or coal, was a reliance on oneself to light the fire and supply the heat. Something that changes in the 20th century, when national grids of electricity and gas fundamentally changed how we heat our homes—but that’s a different story.

“The hearth becomes industrialized throughout the 1800s, but people still wanted to make the fire themselves,” theorizes Adams. “Now, we’re very comfortable with the idea that we can flip a switch to turn the heat on, but that wasn’t the case a century ago. They were close enough to that era of open, roaring fireplaces that people wanted to control their own heat!”

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