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It's well-documented that adding thermal mass to a woodstove will improve efficiency.

You can burn smaller splits of wood hotter, causing more complete combustion. The additional heat output won't over-heat the room, because it is buffered by the thermal mass. The thermal mass can keep the space warm long after the fire has gone out.

Many woodstove users will fill the firebox with wood in the evening, and then close the damper, leaving the wood to smolder overnight. This can keep the home warm for a long time, but it is wasteful of fuel, pollutes the air, and deposits creosote in the chimney. This practice becomes unnecessary with sufficient thermal mass.

High thermal mass designs I have seen require a dedicated footing in the ground. This is a significant additional expense and inconvenience vs. a conventional woodstove, which can rest on a conventional framed wooden floor.

How can I use thermal mass to increase woodstove efficiency without overloading the floor?

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I have seen rocket mass heaters, and they're pretty cool, but 1) they require a lot of developed expertise, creating test stoves repeatedly while you refine your design, and 2) they require careful tending while in operation. Woodstoves are well-understood, easy to find, and easy to operate safely, so I want to start there. – Jay Bazuzi Jan 29 '13 at 23:42
Don't trust the Internet designs of "rocket mass heaters". They don't meet any building code, anywhere. They aren't designed by professionals. They can kill you. Carbon monoxide from poorly design wood stoves kills about 1 million people a year. – Jeff-Inventor ChromeOS Aug 16 '14 at 13:40

(Corrected)The "thermal mass" value you're interested in increasing is essentially the same (exceptions mentioned in this other answer) as the mass (essentially weight, for those not into learning physics!) of your material multiplied by its "specific heat capacity".

That doesn't help much when it comes to keeping mass down. However, there is a way around it. You can use liquid mass (water is cheap, and very heavy) and store it elsewhere. You just need a way to gently circulate it past the stove and store it in a tank somewhere (one that can handle the temperature increasing somewhat higher than your average water tank!).

If your tank is located outside of the space you want to heat, you'll need to insulate it heavily, along with the pipes leading to it. In that case you may also need panels for radiating the heat back into the building, which you'd circulate the hot water through.

Using a liquid to distribute heat is a common method of heating a building; sometimes oil is used and a furnace is running while the building needs heating though. Using oil almost certainly isn't going to be sustainable though, especially when you want to store a tank of it.

Using water, all parts of the system will need to be protected from corrosion (from the inside, especially). It will also need to be able to handle the changes in pressure as the water warms up (and a sudden increase if the water happened to boil by overheating).

Other than that, adding mass to any other part of your building that is reasonably warm will help, especially if the insulation is good. This may be an option too. It doesn't have to be right next to the stove (though the closer it is, the more effective it will be)

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No, from a physics perspective, thermal mass is not exactly equal to mass. – EnergyNumbers Jan 30 '13 at 6:59
thermal mass = mass x thermal capayity. – mart Jan 30 '13 at 7:30
@EnergyNumbers, thanks, I learned something new today! I've provided a brief correction to my answer and referred to yours for more detail. – Highly Irregular Jan 31 '13 at 21:45

There are various ways to include thermal mass with a lower weight. Different substances have different specific specific heat. So, for a given weight, a material with twice the specific heat, will have twice the thermal mass.

Water has one of the highest specific heat values of any known material, so for a heat buffer with high thermal mass, low weight, and no phase changes, water is one of the places to start. The solid alternative would be to look at materials that are used in storage heaters (heat banks) - clay, ceramics, (maybe magnetite).

Phase-change materials are another way to do it. A high latent heat of fusion means that a lot of energy is needed for each kg of material, in order to turn it from solid to liquid. So, to get a good thermal buffer, you'd need to find a material that has a high latent heat of fusion, and has a melting point at standard pressure that is in a useful range for you - and that's probably around 30°-70°C. Such a phase change material will store lots of heat in relatively low weight when it melts; and release it again when it solidifies.

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Great physics lesson. Got any suggestions of high specific-heat materials that would be a good choice here? Cheap, non-toxic, etc. – Jay Bazuzi Jan 30 '13 at 18:21
@JayBazuzi well, water is cheap, non-toxic and has an extremely high specific heat. I've updated my answer with alternatives too. – EnergyNumbers Feb 1 '13 at 18:49
Paraffin is great for storing heat, read – Peter Ivan Feb 3 '13 at 20:41

A simple incremental addition is to add a top cover to your stove. Presuming you don't cook on the stove this is a nice addition that most woodstoves would be better to have.

You will need an assessment of the load bearing capacity of the floor. That information should be available in the building specifications for your home. If you don't have them (most people don't) you can get a copy for a small fee from the local government office that issues building permits. If this information is unavailable a competent carpenter, structural engineer, or architect can tell you with sufficient accuracy after examining the building. Use this information to determine the weight of top cover your floor can support.

Contact companies in your area that build custom stone countertops. Tell them you want a slab cut to fit the top of your stove. Granite and soapstone are the common choices, but there may be other options available where you live. They can cut/form a slab to shape of a thickness that produces a final weight your floor can bear. The finished product just sits ontop of your stove.

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Sounds roughly similar to this: – Jay Bazuzi Feb 8 '13 at 20:34

Some years ago I rented a house that had a wood burning fire place in the rear of the home. It was a fairly smart layout as the fireplace was in the family room and there was a 3 step up area to the kitchen, then small raise in each ceiling as you went through the home, then the stairway going up to the bedrooms in the front of the home. This allowed heat to rise, then flow from room to room while new cooler air would sink and slowly move back to the room being heated. The rear where the fire place was would still get too hot when we tried to heat most of the home using the fire place, even with a fair amount of brick built-in around the fireplace.

My solution at the time was a cheap temporary fix. We had a couch and love seat along 2 of the walls in the family room and by moving them out less than 4 inches more from the wall I started to save round 3 liter bottled water bottles we bough a fair amount of for the water itself. They would stack on top of one another so I started lining the wall behind the love seat and couch with them stacked 2 high. By the time I was finished filling the area with refilled water bottles the temperature would stay reasonable an extra 4+ hours after the fire went out and we didn't feel overheated like we did before adding them to the room. So we could run the fireplace all day if we wanted, then let it go out and maintain a decent temperature most the night. All cheap and hidden out of sight and with the weight distributed. An added bonus was that the dogs could no longer hide behind the couch which we were trying to keep them from anyway.

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Welcome to Sustainable Living! I've taken the liberty to edit your answer and add some punctuation so it is better readable. – THelper Aug 17 '14 at 6:50

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