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I could build a steel box and fill it with the heads, then put a steel plate on top of that, so I can still use my heat powered fan on it as well?

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Thermal mass is almost always best implemented by using water.

Cast iron has only 460 J/kgK heat capacity whereas water has 4190 J/kgK. That's nine times bigger.

Not only is the heat capacity of water nine times bigger than cast iron, but also water production never releases any carbon dioxide as the water is already present on this planet (well theoretically desalinating seawater might use some energy but in most places desalination is unnecessary), whereas steelmaking releases carbon due to the fact that iron is reduced using coal. In the future, steelmaking will use hydrogen, but today, if you start hoarding cast iron, you are preventing other far more sensible uses of iron from using recycled iron and they have to use new iron that releases carbon dioxide during production. Also, if you use cast iron, you have to haul heavy solid material. If you use water, you only have to haul very lightweight empty water containers, and you can fill them with water at your home.

So, doesn't make any sense. Use water instead.

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  • Using water could raise the indoor humidity, which depending on the climate may be a health concern. I've heard of others using bricks for this purpose (one reason why fireplaces, hearths, and chimneys are made of brick).
    – LShaver
    Dec 16, 2021 at 17:20
  • @LShaver I mean water in enclosed containers. Don't let the water evaporate!
    – juhist
    Dec 16, 2021 at 17:48
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    You need to take care that "closed container" doesn't mean "pressurized container". Creating a pressurized hot water system using a wood stove as a heat source is incredibly dangerous. "Kill you and anyone nearby" dangerous. If you use a low-pressure container then you've got at least a "gallons of boiling water spreading uncontrolled" in the failure mode. If you use a high-pressure container then you've got at least a "high velocity projectiles and high pressure steam" problem. You don't want either one of these problems. Dec 17, 2021 at 2:03
  • @LShaver Are there climates where high humidity is a problem when you would be operating a wood stove? I have trouble thinking of one. The air outside is cold. You heat it up in your living space. The relative humidity drops dramatically. Perhaps it could be a problem in a building that is so well air-sealed that the humidity has a long time to build up ... but then the air quality is going to be extremely unhealthy, too, and also what's going to supply the fire with fresh air? Dec 17, 2021 at 2:05
  • I certainly don't encourage putting a container that can become pressurized in an area where temperatures is likely to exceed the boiling point of water. Each container should either be placed in an area where you are 100% sure it's not possible the temperature exceeds the boiling point of water, or the container should be such that it releases the pressure gradually, without becoming a bomb.
    – juhist
    Dec 17, 2021 at 17:14
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Possible , the density of iron/steel permits storage of more heat in a specific volume than water. However, cleaning engine heads will be difficult. They are likely to smell bad whenever heated. And modern cast iron is engineered to be thin so there is not as much mass as it may appear; but you can get the same efficiency with something like steel rebar - it is clean and can be stacked neatly, is readily available. Cost to be determined based on what a junk yard may charge for heads as the have a reuse value. Water, of course is cheap and versatile; I use a few 5 gallon carbouys in my shed to store heat

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