Can Thermal Mass Make a Difference

[Todd]

This year I decided to stack 14 ea 63lb concrete pavers behind my stove not only to protect my logs but also give me a little more thermal mass. I also placed a 100 pounder underneath the stove.

This year has been milder than last but it seems this extra mass has helped a bit. So far my cabin rarely gets under 70 degrees in the morning. Mostly I’ve been burning 12 hour reloads and the blocks will warm up to as high as 150 then by reload time they’re down to the 90’s. I think it helps even out the heat over those long burns and the cabin seems warmer in the mornings.

 

[kborndale]

It's going to help a little keeping warmth when the fire goes out, but on a cold start the pavers are going to make it take longer to heat the place up.

 

[Ashful]

You could calculate the actual BTU's stored in that stone, taking into account temperature, mass, heat capacity, and see how much of a real difference it is making. The effect is real, but I'd suspect the actual nudge to be pretty small for just a few hundred pounds of stone.

Our walls are made from stone, and our stoves sit inside large stone fireplaces. Once heated, they do continue radiating awhile, but we're talking several thousands of times more total mass than you could be free-stacking behind a stove.

 

[Todd]

So I have about 1000 lbs of blocks, heat capacity of concrete is .2? I tried looking up possible BTU calculations but the math is confusing to me.

Before I had these blocks the logs got quite warm too but I think because they are exposed to the outside a lot of that heat was lost. These blocks sit inside with air gaps in between the logs and blocks so maybe that makes a bit of a difference? Also the corner install V shape maybe helps reflect some of that radiant heat out into the room?

 

[Todd]

It's going to help a little keeping warmth when the fire goes out, but on a cold start the pavers are going to make it take longer to heat the place up.

Yeah there is a little difference with a cold start but the stove is a little oversized for the cabin so it’s not really that bad. We were gone last weekend and it took about two hours to get the blocks up from 65 to 120. Cabin was at the same temp but was in the mid 70’s in two hours as well.

 

[peakbagger]

Here is a chance to use my recently renewed engineering license

Heat capacity calculations are pretty easy, just follow the units. The specific heat is .2 Btus per pound *Deg F. In math terms thats 0.2/pounds * Deg F. If you are heating the blocks throughout the pile to 150 deg F and the desired room temperature you want to maintain is 70 degrees than the temperature difference between the heated blocks and the room is 150-70= 80 degs F. You have 1000 pounds of blocks so you need to multiply the specific heat of 0.2 * 1000 pounds *80 = 16,000 Btus (note that the units cancel out to just BTUs. We would need to have the heat loss from the room to calculate how much it makes a difference and for how long.

Masonry is not that great for specific heat but it can hold very high temperatures if you had a way of getting them heated up to high temps. The far better heat storage media for this application is water, which has a specific heat of 1.0 so the same amount of water would hold 80,000 Btus . Folks can and do fill barrels with water for thermal storage. Water is around 8 pounds per gallon so you need 125 gallons of water. One thing to keep in mind is unlike masonry, water can boil and its volume changes as its heated so the tank either needs to be vented or flexible enough to take the change in volume (most 55 gallon drums do)

The difference between water and masonry is the rate of heat loss, (aka conductivity) the masonry is going to give off heat slower than the tank of water. For overnight heating that may be a factor. Passive solar homes use that technique, sunlight comes into a window during the day and shines on heavy masonry floor or wall. It slowly heats up sucking heat from the room so it does not overheat and at night slowly lets that heat back into the room long after the sun has set. Sometimes its worth experimenting on a mix of the two like encasing some water containers inside a masonry wall.

Back in the late seventies there was technology for storing more heat with less weight using phase change materials (PCM). It worked but I am not aware of anyone actively selling the systems anymore.

 

[Todd]

Thanks for the math peakbagger. Yeah water is pretty incredible thing. I actually looked into using water but couldn’t find any slim metal containers that would look nice behind the stove. Maybe a couple old cast iron radiators would look ok, definitely not a couple 55 gl drums lol.

Not sure what my heat loss is, lots of variables. I do notice on colder days once my stove top temps fall into the 300’s the cabin temps stop climbing, stay steady for awhile the slowly fall. Maybe this extra mass slows that fall a bit?

 

[EbS-P]

I think unless it’s tons of mass its effects are small you are burning the same btus. Now the logs aren’t heating (probably the biggest impact you have made )up you get more radiant heat which feels better. I bet if you put a thermometer out of line of sight it would not measure any difference. Just my feelings

 

[begreen]

This may not be too effective. The F45's cast iron jacket is already serving as thermal mass surrounding the steel firebox. It greatly reduces the radiant heat from the sides and the back is shielded. A better test might be with an unshielded stove like the Drolet Austral III.

So far, it's a mild winter here too. We were in the low 50s yesterday. My garlic is already 6" tall.

 

[Todd]

This may not be too effective. The F45's cast iron jacket is already serving as thermal mass surrounding the steel firebox. It greatly reduces the radiant heat from the sides and the back is shielded. A better test might be with an unshielded stove like the Drolet Austral III.

So far, it's a mild winter here too. We were in the low 50s yesterday. My garlic is already 6" tall.

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.

 

[Todd]

I think unless it’s tons of mass its effects are small you are burning the same btus. Now the logs aren’t heating (probably the biggest impact you have made )up you get more radiant heat which feels better. I bet if you put a thermometer out of line of sight it would not measure any difference. Just my feelings

Sure, I don’t think my cabin temps are any higher than before but the radiant heat is better. It’s a little more intense than before when sitting in my recliner about 10’ away. I have high ceilings so most of the convective heat was going up there. Probably should of gotten a more radiant stove for this setup.

 

[bigealta]

My f400 sits in a Large Brick/masonry fire place. With the stove going for multiple reloads it "warms" some of the bricks. And this is nice and it helps. After the fire has gone completely out, The masonry will stay warm to the touch for maybe 1/2 a day. Remember you feel cold when your body looses heat to cold surfaces, if the air is cool but the walls (or surfaces) you are standing next to are warm then you feel warmer.
The interesting thing is the exposed ends of the fireplace are 2 full bricks wide (Away from the fireplace opening). These exposed ends or far sides of the fireplace masonry Never even get warm, even when burning 24/7 in the room that is 80F. The mass is to great and too far from direct exposure to the woodstove sitting in the fireplace.

So the key to mass storage is not necessarily size but more importantly direct close exposure to high heat.

And the brickwork is exposed on the second floor and where it tapers down and is closer to the flue the brick gets quite warm. Warm enough to heat the room it goes thru.

 

[Todd]

Nice looking install bigealta! Too bad Jotul didn’t redesign the F400 for the 2020 regs, I may have bought one but weren’t available when I purchased my F45.

 

[bigealta]

Well it sounds like the f45 and f55 are better performers. No way am i getting 12 hr. burns. But even with the shorter burn times i do like this f400. My climate is not that harsh so lack of very long burn times is not that big a deal for me. I like lighting fires, just never seems to get old.

 

[bigealta]

And if i remember correctly you got a crazy good deal on that f45?

 

[Nick Mystic]

Thermal mass makes a huge difference with our heating. Our Jotul F600 sits in front of the fireplace with about four inches of the rear of the stove sitting inside the fireplace opening. This chimney is 8 feet wide, four feet deep, and over 30 feet tall. The photo shows the living room, which is on the second floor with another lower floor worth of chimney starting down there. When I bought the house the owner, who had it custom built, told me there were over 6000 bricks in the chimney. Of course, inside all that brick is the cinder block structure, so I can't imagine the total weight of all that mass. When I installed the stove I put in stop off plate with insulation laying on top of it, but the SS liner is uninsulated until it reaches the exterior portion of the chimney above the roofline where it is insulated. The chimney is in the middle of the house with the kitchen directly behind it. Once the stove gets heated up in very cold weather when it is burning 24/7 the brick wall in the kitchen will register 95F on my infrared thermometer. The bricks directly above the stove in the living room will register up to 160F to just below the mantle (which has a heat shield on its underside). In the morning the chimney around the stove will still still be over 100F.

I have a Woodstock Classic woodstove on the first level that runs up a flue to the right of where the F600 is sitting. In really cold


weather (highs in the 20s) I'll burn both and really get some heat flowing!

 

[Todd]

And if i remember correctly you got a crazy good deal on that f45?

$1999 back in 2020.

 

[kennyp2339]

Back in the late seventies there was technology for storing more heat with less weight using phase change materials (PCM). It worked but I am not aware of anyone actively selling the systems anymore.

Local Rural electric co-op by me offers electric boilers w/ this tech, mass stones, they heat up during ff peak and store the heat, lots of happy people w/ this type of setup

 

[Todd]

Thermal mass makes a huge difference with our heating.View attachment 320570 Our Jotul F600 sits in front of the fireplace with about four inches of the rear of the stove sitting inside the fireplace opening. This chimney is 8 feet wide, four feet deep, and over 30 feet tall. The photo shows the living room, which is on the second floor with another lower floor worth of chimney starting down there. When I bought the house the owner, who had it custom built, told me there were over 6000 bricks in the chimney. Of course, inside all that brick is the cinder block structure, so I can't imagine the total weight of all that mass. When I installed the stove I put in stop off plate with insulation laying on top of it, but the SS liner is uninsulated until it reaches the exterior portion of the chimney above the roofline where it is insulated. The chimney is in the middle of the house with the kitchen directly behind it. Once the stove gets heated up in very cold weather when it is burning 24/7 the brick wall in the kitchen will register 95F on my infrared thermometer. The bricks directly above the stove in the living room will register up to 160F to just below the mantle (which has a heat shield on its underside). In the morning the chimney around the stove will still still be over 100F.

I have a Woodstock Classic woodstove on the first level that runs up a flue to the right of where the F600 is sitting. In really cold


View attachment 320570weather (highs in the 20s) I'll burn both and really get some heat flowing!

Wow that’s definitely some major mass there! Looks great!

 

[EatenByLimestone]

I’ll just throw out there that just because it doesn’t make a huge difference, doesn’t mean it’s not worthwhile.

 

[Ashful]

Mass... here's some rough math for you: 3.5 stories at 9.5 ft. average = 33.3 ft. x 130 ft perimeter x 1.8 ft. average wall thickness x 166-168 lb/ft3 = 1.3 million pounds, before adding in the fireplaces, their foundations and chimneys, or subtracting window and door holes. I remember coming up with almost exactly 1.0 million pounds with more careful accounting many years ago, but now I wonder if that was right, if the door and window subtractions actually outweigh the fireplace additions.

Whatever... the dudes who dug up and then threw up this stone, back before diesel power, could probably teach us some things about eating an elephant. Of course, that's all exterior-exposure stone losing a lot to my back yard, so not such a nice storage unit as that behemoth of a fireplace Nick Mystic enjoys.

 

[bigealta]

$1999 back in 2020.

Ok a good deal. Someone else grabbed one for $500 if i remember correctly. I think a builder or new homeowner was getting rid of a like new condition F45 or maybe it was an F50?

 

[bigealta]

Nick that’ a nice looking set up. You have me beat by 1 brick. Your's looks to be 11 across while I only have 10.

 

[EatenByLimestone]

The house next door was the original farmhouse for the area. The house was stick built, but all of the interior walls were brick thermal mass.

 

[moresnow]

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.....