Can Thermal Mass Make a Difference

[peakbagger]

A reminder, thermal mass is great for retaining heat but its needs to be backed up by insulation on the outer envelope of the house. I have seen a lot of primary and secondary homes built up in my area with the stereotypical "massive stone fireplace with stone chimney" on an exterior wall where the exterior wall is built around the fireplace structure rather than behind it. This lets the stone be visible outside the building as well as inside. Sure they look great, but from a heating point of view this arrangement is a disaster. Stone does retain heat but it also conducts it readily. If there is not a proper insulated thermal break between the interior and the exterior of the building, these fireplaces can be a major heat loss to a house. If they are inside the outside envelope of the house like the old center chimneys or the more modern masonry stoves (aka Russian fireplaces) they can be an asset as long as the stack draft is dealt with.

In the case of the OP, if the cabin was better insulated and air sealed (very difficult in the long term with a log cabin due to settling) with a lower heating load, it would cool down slower overnight requiring a smaller stove and less wood to maintain the temperature. That is not criticism, just an observation. Insulation and sealing cost time and money so when energy was cheap or its just a seasonal usage, the decision is usually made to use more heat. On the other extreme, Passive homes are designed to have a very low heating load due to lots of insulation and sealing and they are heated just by the sun entering the windows usually with stone or occasionally water based thermal mass used to absorb the heat and release it during the night. The tradeoff to minimal heating costs is a much higher initial cost and if the money is borrowed higher mortgage payments.

 

[EbS-P]

I have been running without my blower because I’m lazy. Have not burned more than 12 hours in a row yet. Without the blower is takes longer to heat up the room. But the stone/bricks are noticeably warmer and radiate longer.

From a new construction cost perspective you would have a hard time selling me on a massive masonry structure to use as a heat storage device. I should try running the stove harder without the blower and see what it does. It spends 91% of the time on the lowest possible air setting.

Think of all the mass of all the things in your house. With the exception of a giant two story central chimney (I have one too) all our stuff probably out weights and has greater thermal storage than any extra we add. I’m a fan of using the whole house as thermal storage. In the summer I’ll cooling down once air temps drop below 81 and my AC gets better efficiency. Turn the tstat back up when we wake up. During the winter. I’ll heat the house up on the heatpump or woodstove while we are up warmer than I’d really want. Then shut everything off and got to bet.

I keep trying to think of aesthetic was to add 99k+ btus of retrofitted thermal storage. (Just to pick a number) I think this is the amount where it could be noticeable and actually useful. My only good idea is an indoor water garden think pond but no fish as I don’t think they would like the temperature swings. But that brings humidity issues in the summer. So maybe I’s seasonal. Then it’s just going to grow lots of algae so…. Yeah probably not.

A reminder, thermal mass is great for retaining heat but its needs to be backed up by insulation on the outer envelope of the house. I have seen a lot of primary and secondary homes built up in my area with the stereotypical "massive stone fireplace with stone chimney" on an exterior wall where the exterior wall is built around the fireplace structure rather than behind it. This lets the stone be visible outside the building as well as inside. Sure they look great, but from a heating point of view this arrangement is a disaster. Stone does retain heat but it also conducts it readily. If there is not a proper insulated thermal break between the interior and the exterior of the building, these fireplaces can be a major heat loss to a house. If they are inside the outside envelope of the house like the old center chimneys or the more modern masonry stoves (aka Russian fireplaces) they can be an asset as long as the stack draft is dealt with.

In the case of the OP, if the cabin was better insulated and air sealed (very difficult in the long term with a log cabin due to settling) with a lower heating load, it would cool down slower overnight requiring a smaller stove and less wood to maintain the temperature. That is not criticism, just an observation. Insulation and sealing cost time and money so when energy was cheap or its just a seasonal usage, the decision is usually made to use more heat. On the other extreme, Passive homes are designed to have a very low heating load due to lots of insulation and sealing and they are heated just by the sun entering the windows usually with stone or occasionally water based thermal mass used to absorb the heat and release it during the night. The tradeoff to minimal heating costs is a much higher initial cost and if the money is borrowed higher mortgage payments.

Specific heat and thermal conductivity are different properties.

 

[Ashful]

A reminder, thermal mass is great for retaining heat but its needs to be backed up by insulation on the outer envelope of the house. I have seen a lot of primary and secondary homes built up in my area with the stereotypical "massive stone fireplace with stone chimney" on an exterior wall where the exterior wall is built around the fireplace structure rather than behind it. This lets the stone be visible outside the building as well as inside. Sure they look great, but from a heating point of view this arrangement is a disaster. Stone does retain heat but it also conducts it readily.

So, I've done a bit of reading on this, as most of the houses I've owned are the exterior masonry "disaster" you describe here. Heating and cooling engineers, in calculating the heating requirements for houses like this, assume the stone is holding a near-constant 53F, the local temperature of the earth into which the walls are sunk. Sure they conduct, but the path to ground is so strongly dominant over the path to air, that the walls really don't seem to vary much in temperature with outdoor weather.

We see this in measuring the temperature of our own exterior stone walls, at least on the first floor, they hold almost consistently 53F through the majority of winter. They warm up a bit in summer (can't remember number, something near 60F), but the change of just ~10 dF takes weeks, they don't change temperature with each warm front or cold front passing through. Our summer/winter weather change is probably near 50F, but the walls change only ~10F, due to this strong ground-sinking effect.

I say all of this because, while I'm sure that interior stone work is better than exterior stone work when talking about the typical mass of fireplace on the first floor of a home, I'm not convinced the actual difference between the two is anywhere near as dramatic as you imply. They're both sunk to earth, which seems to be a strongly-dominant path over any path to the outside air.

PS - I've never taken the time to measure my second or third floor walls in different weather conditions, I guess I should out of curiosity, and to complete this picture. I would expect they vary more than the first floor walls, due to longer ground path working against constant "weather path".

 

[EatenByLimestone]

When I met my wife, I was into fish. I had 9 tanks around my house, centered around my living room. It was frequently talked about on the boards that the tanks provided a buffering effect.

 

[joop]

don't find any benefits with my rocks.

 

[EatenByLimestone]

so, without heat, your first floor would stay, high 40s, low 50s all winter long without heat?

 

[joop]

maybe ashfuls, i know mines not

 

[EatenByLimestone]

It surprised me too! Maybe soil type makes a difference here? Soil moisture might be more conductive?

 

[joop]

summer is very stable,winter not a chance

 

[EatenByLimestone]

My basement stays low 50s all year long. Maybe wall thickness is the difference. Cut block vs fieldstone...

 

[joop]

2ft field stone,0 to -7c comfortable more or less temps put on the coat weird

 

[bigealta]

As for a pond, there is a guy that piped his AC coils into his pool. Heats the pool while cooling his house.

 

[Ashful]

When I met my wife, I was into fish. I had 9 tanks around my house, centered around my living room. It was frequently talked about on the boards that the tanks provided a buffering effect.

Yahbut... you're heating those tanks with electric resistive heaters, right? I guess differential between tank water and room air is pretty small, though.

so, without heat, your first floor would stay, high 40s, low 50s all winter long without heat?

Me? Not sure, never let the place go totally cold, but I'd suspect warmer than the 53F walls, based on normal human activity.

It surprised me too! Maybe soil type makes a difference here? Soil moisture might be more conductive?

Our soil is very wet all winter, hovering just either side of "saturated", depending on number of days since last storm. I imagine Schenectady must be very similar?

My basement stays low 50s all year long. Maybe wall thickness is the difference. Cut block vs fieldstone...

Our basement holds closer to 60F, but only because there's an oil-fired boiler working down there to make hot water or firing to heat the few zones too far from the wood stoves. Without the boiler, I suspect the basement would assume 53F or very close to it, the temperature of the walls, floor, and surrounding earth.

 

[Todd]

Nifty experiment. Kinda labor intensive looking

Wonder what indoor morning temps would be if you had a reflective corner heat shield and more indoor air circulation?
Your stove has already got a significant heat sink of cladding. Dunno.
For your next experiment (and our continued entertainment ) Perhaps.....

I thought about some old rusted metal roofing back there but the wifey didn’t like that idea, she likes the block look.

The iron clad sides block quite a bit radiant heat and I don’t think they retain much heat after the fire dies due to the natural air flow between them and the steel stove body. My IR readings are almost 100 degree difference between the outside of the clad and the steel body behind it.

 

[Todd]

This morning I woke up to a 71 degree cabin and 18 outside. The blocks were still 90-100. Stove top was 150 with a good bed of coals.

I used to heat this place with a little 602. It did well but had to feed it every 4 hours in cold weather. That little stove really threw off the radiant heat. I’m wondering how that stove would work out with a bunch of mass surrounding it.

 

[Ashful]

I thought about some old rusted metal roofing back there but the wifey didn’t like that idea, she likes the block look.

Why not just trim a sheet to hide behind the block, where no one will see it? Make it an inch smaller in all directions? Foil-backed foam board would be much better, still. Easy to trim, very effective insulation/reflector barrier combo, available in different thicknesses.

 

[EbS-P]

This morning I woke up to a 71 degree cabin and 18 outside. The blocks were still 90-100. Stove top was 150 with a good bed of coals.

I used to heat this place with a little 602. It did well but had to feed it every 4 hours in cold weather. That little stove really threw off the radiant heat. I’m wondering how that stove would work out with a bunch of mass surrounding it.

Not going to change the burn time much. You could block and hold some radiant heat but it was already going somewhere.

 

[begreen]

I agree, a more radiant stove would get those blocks hotter. I did remove my rear and bottom heat shield to get the blocks hotter and it did. Those side panels will stay put.

Hopefully that was just a temporary trial. Without the shields the stove is in untested area with little idea of the hearth requirements for insulation or clearnance. That puts the clearance requirements in the 36" in all directions territory and unknown for the hearth insulation requirement.

 

[Douglas3Stoves]

I have about 280 pounds of iron rail tie plates on the top and sides of my Drolet Spark II. It does seem to somewhat even out the heat blast/cool off inherent to these types of stoves. I'm sure it's not in the territory of Woodstock or Blaze King however.

 

[Ashful]

If you've air-gapped them off the stove body, as it appears you have, I'm sure they do a great job of increasing convective coefficient, and minimizing amount of heat radiated into that block wall. It looks cool, too!

I just looked up specific heat of soapstone versus cast iron, and... wow! I knew soapstone was good, but had no idea it'd be nearly double that of iron. For those who have blocked out the trauma that is chem101, specific heat is the amount of energy to shift a mass by a temperature, such as BTU/(lb·°F). Put otherwise, how much heat energy it can store per pound.

 

[Douglas3Stoves]

I suppose I could have elaborated more but I'm new here and I don't want to come off as a "me,me,me" kind of guy. I've been a lurker for years and the knowledge gained here has been invaluable to me. A big thank you to the regulars on here.

Anyways, yes there is a 1/8" air gap between the stove top and first layer of plates. The plates don't have much contact between layers, even on the flat sides, there is a diamond shaped waffle pattern. I originally wanted to use slate or granite slabs but couldn't find anything like that for free at the time. All walls of this room are on the interior of the basement.

Perhaps this was a somewhat silly project. But I vote that greater thermal mass is an improvement, in this particular situation.

 

[Todd]

I have about 280 pounds of iron rail tie plates on the top and sides of my Drolet Spark II. It does seem to somewhat even out the heat blast/cool off inherent to these types of stoves. I'm sure it's not in the territory of Woodstock or Blaze King however.

View attachment 320759

Homemade iron clad stove, love it!

 

[Todd]

I’ve added some more blocks as well as bricks to the hearth pad. I figure I have close to 2000lbs of mass there including the concrete, stove and hearth pad.

We had some nice warm days in the upper 40’s and instead of my usual low and slow 12 hour burns I tried a single hotter firing getting the stove top into the 750+ range. And internal flue temps 800-900. This heated the concrete up to 160 degrees and brought the cabin up to 81 pretty quickly. I let the fire die out and the next day the cabin was still 70. This was a little too hot for most of the day 🥵 You could really feel the radiant heat! Could be an option for shoulder seasons instead of burning low and slow.