Adding thermal mass to stovetop

[yukiginger]

I have a Lopi wood stove in my basement, which I installed this year. It is older, used, and therefore not under warranty. One of the recent posts here got me to thinking about adding some thermal mass to the nice, flat stovetop. I have some cast iron weights (you know, the kind for building muscle) that are lying around. I know it wouldn’t look very good, but what if I laid a few of these on the stovetop, adding 40 or 50 lbs. of cast iron to the already thick stovetop? Shouldn’t this help radiate heat longer after the fire has died down? I’m especially interested in whether there is any safety issue with these weights going through such temperature swings.

Thanks for any advice. Also, any cheap alternatives that might work, like a dense stone that could be cut to the approximate size? I suppose soapstone would work really well, but a piece that size would probably be pretty costly.

MarkG

 

[Roospike]

Sure it will work ...................but ...................

The stove being in the basement you might not see much improvement . Where the extra add on mass would help is on the 40°-50° days the heat will linger longer from the stove to keep the chill off . On the 30° days/night and lower the little extra mass is not going to help a whole lot unless you have a lot of mass and a lot of mass would be very hard to "add" to a stove.

 

[begreen]

surround it in cement blocks filled with rocks like the barrelstove in the other thread.

 

[kevinmoelk]

I believe there are other threads that touch on this subject. Do a little poking around yukiginger and you might come up with some more ideas. One the recent things I read was of someone adding large jars full of water around the stove. I thought that was a good idea. Especially since when in the off season you could pour out the water and stash the jars away until the next year.

-Kevin

 

[wg_bent]

This one has been discussed before, and the basic conclusion is that your stove isn't designed to do that, and altering the transfer rate of heat out of the stove could damage the stove. I don't think weights would alter that too much since the surface contact patch is pretty small, but a flat stone that has good contact likely would alter it. Just look at the Morso line of stoves that have the soapstone options (3400 series I believe) No direct contact, just placed on the outside of a "convection" flow.

I have a fair amount of thermal mass in front of my stove (my hearth) that gets fairly warm, and it cools off at just about the same rate as the stove does during it's burn cycle. I don't believe it would help much.

Thermal mass has to be really significant to be truely effective such as in a masonry heater. Thermal mass of something like 12,000 lbs. Maybe Marty know what his weighs, but I'd bet it's like 500x of a wood stove.

 

[yukiginger]

Thanks for all the advice. I might give it a try.

MarkG

 

[Corey]

A little searching around and it looks like the specific heat capacity of iron would work out to about 0.12 BTU/lb degreeF.

Basically what that is saying is that your 50 pounds of iron cooling from say 500F to 70F (430F drop total) would give off:

50 x 430 x 0.12 = ~ 2600 BTU. It is nothing spectacular...but there are substantially better materials.

As info - surprisingly, on a pound for pound basis, aluminum can store almost twice as much heat as iron (0.22BTU/lb F for aluminum vs 0.12 for iron) Water blows them away at, of course, 1 BTU/lb F - although the upper temperature is limited! Concrete and/or sand is about 0.80 and brick about 0.84.

Corey

 

[Martin Strand III]

Corey is on the mark.

Adding thermal mass with metal, which has a high heat transfer property, will help some but not as much as adding thermal mass with another material, such as water and brick which have much lower heat transfer properties and higher specific heat capacities.

Remember, an "ideal" heater would have high burning efficiency but only "moderate" heat transfer properties, the latter which delays heat escape to your home. This feature evens out temperature fluctuations (aka "indoor weather"), desirable for home occupants.

In the real world, a metal stove which is heavier (700 lbs) will retain more heat and release is somewhat slower to the room than a 350 lb metal stove. This is one factor to look at when purchasing a stove. Some mfg's add masonry (granite, soapstone, etc) to their metal stoves to help "bank" the heat and add aesthetics.

For this application, "more is better".

Aye,
Marty