Calculating Bricks for Winter Overnight Heat Storage in Stove

[Pete_L]

Please see if you agree with how I am calculating the number of bricks required to reduce cooling off of my house overnight in the winter when I'm not there (sleeping) to feed logs into the stove.

The estimated weight of each log that I would normally be burning in my stove is 8.6 pounds. Assume that burning three of such logs overnight would prevent room temperature dropping from 75 deg. F to less than 60 deg. F. BTU of air dry wood equals approximately 5000. Three logs weigh in total 25.8 pounds, and thus produce total heat of 129,000 BTU.

The heat storage capacity of clay brick equals 0.21 BTU/pound-deg. F. Each brick weighs 4.6 pounds. The outside surface temperature of my stove is 600 deg. F. Room (air) temperature equals 75 deg. F. Thus,

(BTU/ 1 brick) = 0.21 X 4.6 pounds X (600 deg. - 75 deg.)
(BTU/ 1 brick) = 507

Then

Total required bricks = BTU of 3 logs/ BTU per brick
Total required bricks= 129000/ 507
Total required bricks = 254

Putting bricks along the sides, on top, and underneath the stove, I can fit about 100 bricks. Any more bricks would have other bricks between them and the stove. So my question is, if I place 100 bricks next to my stove, will this cause my house to be noticeably warmer in the morning, or would adding the bricks only slightly or not noticeably improve the room temperature in the morning?

 

[hutchphd]

You do know that water holds 1.0 BTU/LB/F and circulates to heat evenly. Seems a better idea on many fronts.

 

[jrmichler]

Your calculations are good, but the assumptions behind them are not. If you surround your wood stove with bricks, the stove will get hotter. This is bad for the stove and chimney because they are not designed for this. The stove thermal efficiency will decrease.

The bricks will not reach the temperature of the stove. The brick temperature will reach a temperature about halfway between the stove surface temperature and the room temperature. As the bricks cool down, the rate of heat transfer decreases. The house will get cool anyway. The overall effect will be to help a little bit.

You don't say at what the outside temperature the inside drops from 75 deg F to 60 deg F overnight, nor do you say if you leave the wood stove full of wood and how long that wood lasts. You may want to take a close look at the insulation in your house.

For comparison, my house is well insulated. The measured cool down rates are:
Cooldown rate 1-29-2012 from 73 deg F down to 65 deg F in 8 hours at 0 F OAT.
Cooldown rate 2-5-2012 from 72 deg F down to 59.5 deg F in 37 hours at 30 F OAT.
Those cool down rates were measured by shutting off the furnace, so residual heat from the wood stove was not involved.

I once designed a hydronic wood fired heating system with propane backup for a friend. The calculations called for 1100 gallons of water thermal storage to meet his specifications. He built it and it worked as designed. He could build a fire in the evening, and the four zones held the setpoint temperature until the following evening at outside temperatures down to about zero deg F. Colder temperatures required a morning fire. If he had put the same time and effort into insulation, he would have been better off.

 

[sophiecentaur]

You may want to take a close look at the insulation in your house.

Agreed. Any other strategies come after that. (And, of course, even that can be surpassed by running the house cooler and wearing thicker clothes. You may say that's a glib answer but that approach also benefits the environment.

You do know that water holds 1.0 BTU/LB/F and circulates to heat evenly. Seems a better idea on many fronts.

I came across a form of night store heater which used water as the storage medium but the fact that it wouldn't work at higher temperatures than 100C. The factor of about five in the increased specific heat capacity of water should balance the requirement for five times the internal temperature difference. I never found out why the water storage didn't take off - except for sheer the bulk of water required.

 

[Pete_L]

You do know that water holds 1.0 BTU/LB/F and circulates to heat evenly. Seems a better idea on many fronts.

Yes, thanks, I did see in my chart of thermal properties that water has a specific heat that is five times that of brick. The problem with using water is that the containers for it would have to be metal, and finding metal containers of the proper size to fit to the dimensions of my stove would not be easy. Also there is the possibility of a container developing a leak.

 

[Pete_L]

Your calculations are good, but the assumptions behind them are not. If you surround your wood stove with bricks, the stove will get hotter. This is bad for the stove and chimney because they are not designed for this. The stove thermal efficiency will decrease.

The bricks will not reach the temperature of the stove. The brick temperature will reach a temperature about halfway between the stove surface temperature and the room temperature. As the bricks cool down, the rate of heat transfer decreases. The house will get cool anyway. The overall effect will be to help a little bit.

You don't say at what the outside temperature the inside drops from 75 deg F to 60 deg F overnight, nor do you say if you leave the wood stove full of wood and how long that wood lasts. You may want to take a close look at the insulation in your house.

For comparison, my house is well insulated. The measured cool down rates are:
Cooldown rate 1-29-2012 from 73 deg F down to 65 deg F in 8 hours at 0 F OAT.
Cooldown rate 2-5-2012 from 72 deg F down to 59.5 deg F in 37 hours at 30 F OAT.
Those cool down rates were measured by shutting off the furnace, so residual heat from the wood stove was not involved.

I once designed a hydronic wood fired heating system with propane backup for a friend. The calculations called for 1100 gallons of water thermal storage to meet his specifications. He built it and it worked as designed. He could build a fire in the evening, and the four zones held the setpoint temperature until the following evening at outside temperatures down to about zero deg F. Colder temperatures required a morning fire. If he had put the same time and effort into insulation, he would have been better off.

Thank you.

That's a good point that surrounding the stove with bricks would increase the temperature of the stove for a given size of fire in the stove. Wood stoves are most efficient running in the range of 400 deg. F to 600 deg. F. Wouldn't it be possible to make smaller fires in the stove so that the stove surface temperature doesn't exceed 600 deg. F?

A temperature drop from 75 deg. F to 60 deg. F is anecdotal for winter temperatures around here. I haven't done any exacting measurements of what happens relative to the nighttime low temperature and room temperature in the morning. Typically around here in the winter, temperatures are in the teens to single digits Fahrenheit. Occasionally the outside temperature can go decades below 0 deg. F.

Dong any more insulating to the house than I have already done would be very costly and time consuming. My house is a trailer (aka mobile home) with exterior walls that are only 3.5 inches thick.

Agreed that the temperature of the bricks could not match the surface temperature of the stove, but how much of a temperature difference I think would depend on the extent to which the bricks retain heat. Clay as opposed to concrete brick I believe is much better at retaining heat, but I don't have any numbers about that.

 

[hutchphd]

Your own figures tell you this is a hopeless proposition. At very best the 100 bricks might be at 400F. So you have stored the heat equivalent of a single smallish stick of wood. Just not enough.
Better to have some good dry sticks and a match (or some banked coals) in the morning !

 

[sophiecentaur]

Yes, thanks, I did see in my chart of thermal properties that water has a specific heat that is five times that of brick. The problem with using water is that the containers for it would have to be metal, and finding metal containers of the proper size to fit to the dimensions of my stove would not be easy. Also there is the possibility of a container developing a leak.

Bricks are a good method for electrical storage heaters because the elements are buried deep inside the inner bricks. that ensures good thermal contact and the elements will operate at very high temperature. A steel / iron stove is not designed to operate red hot. Buried inside insulating bricks it could distort and crack. (Leak of flue gases?)
Heat transfer for brick can only be by conduction (obvs) so you need a good fit to get contact with the inner bricks. You could easily end up with a lot of luke warm bricks. Would the trailer floor support them? A leaking water jacket is a risk but there are many suitable designs for thermal storage tanks and a car heater matrix on top and at the back could be lagged on the outside. Water Thermal storage can be remote, in a convenient place. The stove jacket could still operate at around 150C (optimal) .
It’s true that houses with thick brick walls have good storage properties but that’s a total system and has to have good insulation too.

 

[Pete_L]

Your own figures tell you this is a hopeless proposition. At very best the 100 bricks might be at 400F. So you have stored the heat equivalent of a single smallish stick of wood. Just not enough.
Better to have some good dry sticks and a match (or some banked coals) in the morning !

That's what would be a correct assessment, I think, and what I was afraid of.

 

[Pete_L]

A leaking water jacket is a risk but there are many suitable designs for thermal storage tanks and a car heater matrix on top and at the back could be lagged on the outside. Water Thermal storage can be remote, in a convenient place. The stove jacket could still operate at around 150C (optimal) .

Please, my Jotul 602N (wood stove) is located in my living room! I would prefer that it not look like my basement.

 

[hutchphd]

Start working on plan B ! As an undergrad in Ithaca I lived in a farmhouse where snow would blow up through my bedroom floorboards on a lake storm. Made me stronger. Colder, too.

At least you have a nice stove!

 

[sophiecentaur]

Please, my Jotul 602N (wood stove) is located in my living room! I would prefer that it not look like my basement.

And a pile of bricks would be what - an art installation? I looked at images of your stove and it does look nice (top or rear flue exit?) but . . . .

You have already decided that you need something in there and that something needs to be a certain volume. That will impinge on the look of the stove, one way or another. If you surround the heater with bricks and if the system is doing its 'storage' job, it will take a couple of hours before any heat from the stove gets out to you. I don't see that compromise being acceptable. If you were to use a water based heat exchanger, it would be 'noticeable' but you could isolate it from the storage tank with a simple tap, giving you only a slight delay in useful heat output. The tank could quite safely be covered in a decorative chintz cover and likewise the heat exchanger could be suitably decorated.
An experiment (I love experiments) would be possible, if you put a large covered pan of water on top of the stove and see how it performs. How hot it gets, over time, how it affects the subjective heating of the room to start with and how warm it still is by morning. My wood burner still feels slightly warm by morning (but the room never gets very cold) and, if I try, I can stoke it up overnight and it will still be smouldering with low heat output.

Perhaps what's been written on this thread has been enough to put you off the idea. The project is non-trivial, if you want it to achieve a significant improvement in the heating of your home. The stove looks too nice to mess with its appearance, I would say. Perhaps the best thing to do would be to use a brick wall (or tall water tank) situated behind and close to the stove. The regulations specify a gap of several cm to the nearest combustible material so there would probably be room.