How do EPA stoves efficiently release heat to room, despite the insulating bricks?

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monteville

Member
Nov 23, 2019
121
Dallas
EPA stoves have insulating bricks around the combustion chamber, theoretically it blocks heat from being released into room, so more heat is kept in flue, but EPA stoves actually have higher fuel efficiency compared to classic wood stoves.

Seems contradictory. Anyone could explain to me?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,667
central pa
EPA stoves have insulating bricks around the combustion chamber, theoretically it blocks heat from being released into room, so more heat is kept in flue, but EPA stoves actually have higher fuel efficiency compared to classic wood stoves.

Seems contradictory. Anyone could explain to me?
The firebrick are used to keep temps in the firebox up until it gets above the baffle where heat transfer takes place.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,667
central pa
"Above the baffle" means the upper facet of firebox and the stove pipes?
In a secondary combustion stove using tubes there is a baffle in the top of the firebox. Pretty much everything under that is insulated to keep temps up in that area for more complete combustion. The exhaust then goes around that baffle and runs back along the stove top. There are usually defectors there to increase dwell time and increase heat transfer there. The flue temps when run correctly can be much lower with a modern stove than with an old one while still burning cleanly. With other combustion technologies the heat transfer is done differently.
 

CharlieTuna

New Member
Jan 10, 2021
52
PA
EPA stoves have insulating bricks around the combustion chamber, theoretically it blocks heat from being released into room, so more heat is kept in flue, but EPA stoves actually have higher fuel efficiency compared to classic wood stoves.

Seems contradictory. Anyone could explain to me?

Presumably, the increased heat gained from more complete combustion of exhaust gasses outweighs the loss in heat transferred at the bottom of the stove. That's my guess.
 
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jetsam

Minister of Fire
Dec 12, 2015
5,283
Long Island, NY
youtu.be
Presumably, the increased heat gained from more complete combustion of exhaust gasses outweighs the loss in heat transferred at the bottom of the stove. That's my guess.

Vastly outweighs it. A whole lot of your heat comes from your secondaries and/or cat.

Old stoves wasted all those BTUs. They could accidentally do some secondary combustion when run in raging inferno mode, but then the newly released BTUs mostly went up the flue anyway. You lost either way.

There's a reason that even crotchety old wood burners wouldn't go back to running a pre-EPA stove unless they had to. (New stoves are MAGIC to us... ;lol )
 
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CharlieTuna

New Member
Jan 10, 2021
52
PA
Vastly outweighs it. A whole lot of your heat comes from your secondaries and/or cat.

Old stoves wasted all those BTUs. They could accidentally do some secondary combustion when run in raging inferno mode, but then the newly released BTUs mostly went up the flue anyway. You lost either way.

There's a reason that even crotchety old wood burners wouldn't go back to running a pre-EPA stove unless they had to. (New stoves are MAGIC to us... ;lol )
By increasing the mass of the stove, it also can store heat, which is then radiated out as the stove cools.
 

jetsam

Minister of Fire
Dec 12, 2015
5,283
Long Island, NY
youtu.be
By increasing the mass of the stove, it also can store heat, which is then radiated out as the stove cools.

Thermal mass is great, but insulation between the fire and the room is bad (and the firebrick is both of those things).

But firebrick bellies pay us back by making secondary combustion work better, and that means more heat from the same fuel!

There's also a whole class of stove that cover the entire firebox with insulation (soapstone), which is a dead loss. People like to talk about them having the same heat output but gentler.... but these are not the kind of people who could explain to you what the difference between thermal capacity and thermal conductivity is. They probably have nice clean chimneys with all the BTUs going up them though!

And then there's heaters where the thermal capacity is the whole point (masonry heaters). You use a small hot fire to warm up a large central mass of stone. It may sound iffy to those of us raised on steel stoves designed to maximize thermal conductivity, but it is still used as primary heat in some very cold places in the world!
 

CharlieTuna

New Member
Jan 10, 2021
52
PA
And then there's heaters where the thermal capacity is the whole point (masonry heaters). You use a small hot fire to warm up a large central mass of stone. It may sound iffy to those of us raised on steel stoves designed to maximize thermal conductivity, but it is still used as primary heat in some very cold places in the world!

Increased mass, as in masonry chimenys, stores heat in winter. So a central chimney would radiate heat out to the rest of the house. It's fine for a usual residence, but not for a weekend cabin in the woods that would first need to heat up and suck up heat.
 

SpaceBus

Minister of Fire
Nov 18, 2018
6,146
Downeast Maine
I think the minimum weight to heat an average American house with a masonry heater is about 10,000 lbs going by anecdotal evidence and examining the various masonry heaters owned by folks that use them regularly. The units designed by the top names in masonry heaters all weigh more than 10,000 lbs. At that point I would be installing a downdraft gasification boiler and thermal storage in the form of water. The point being that enough mass to make your house stay warm requires the house to be built around it from the ground up. Or really beneath the ground because 10,000 lbs takes a heck of a footing to support.
 

monteville

Member
Nov 23, 2019
121
Dallas
In a secondary combustion stove using tubes there is a baffle in the top of the firebox. Pretty much everything under that is insulated to keep temps up in that area for more complete combustion. The exhaust then goes around that baffle and runs back along the stove top. There are usually defectors there to increase dwell time and increase heat transfer there. The flue temps when run correctly can be much lower with a modern stove than with an old one while still burning cleanly. With other combustion technologies the heat transfer is done differently.
Does it mean that we can build an EPA stove out of fire brick, refractory mortar and a little reinforcement steel plates?
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,667
central pa
Does it mean that we can build an EPA stove out of fire brick, refractory mortar and a little reinforcement steel plates?
Not one that will hold up.

You missed a whole lot of r&d engineering and testing as well
 

john26

Minister of Fire
Oct 27, 2008
566
Wildwood MO
Does it mean that we can build an EPA stove out of fire brick, refractory mortar and a little reinforcement steel plates?
You could build an "EPA style" stove or fireplace but it will not have an EPA rating. I don't think it would be worth your time and money to have it rated if even possible. There have been lots of non EPA stoves converted with secondary burn tubes and and a few with catalyst but they will never be a true EPA stove unless tested.
 
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bholler

Chimney sweep
Staff member
Jan 14, 2014
25,667
central pa
You could build an "EPA style" stove or fireplace but it will not have an EPA rating. I don't think it would be worth your time and money to have it rated if even possible. There have been lots of non EPA stoves converted with secondary burn tubes and and a few with catalyst but they will never be a true EPA stove unless tested.
It is also very unlikely any of those modified stoves come anywhere near emissions requirements.
 

Easy Livin’ 3000

Minister of Fire
Dec 23, 2015
2,920
SEPA
In a secondary combustion stove using tubes there is a baffle in the top of the firebox. Pretty much everything under that is insulated to keep temps up in that area for more complete combustion. The exhaust then goes around that baffle and runs back along the stove top. There are usually defectors there to increase dwell time and increase heat transfer there. The flue temps when run correctly can be much lower with a modern stove than with an old one while still burning cleanly. With other combustion technologies the heat transfer is done differently.
I wish my stove had the deflectors you mentioned. In earlier versions of the same model, they had a chunk of ceramic blanket above the baffle weighed down with a steel disk. Now, it just goes right out the chimney. Been meaning to get a chunk of ceramic blanket to recreate the prior models. I guess folks had a tough time dealing with the mess created by the blanket when cleaning the stove and chimney.
 

Highbeam

Minister of Fire
Dec 28, 2006
19,175
Mt. Rainier Foothills, WA
Bricks were placed in stoves long before EPA regulations. I’ve had my head in a few fishers. I think even the primary fire burns better when surrounded by insulative materials.
 

jetsam

Minister of Fire
Dec 12, 2015
5,283
Long Island, NY
youtu.be
Bricks were placed in stoves long before EPA regulations. I’ve had my head in a few fishers. I think even the primary fire burns better when surrounded by insulative materials.

Not every stove had them... For years, when you smelled burning hair in my grandmother's kitchen, you knew that 1) It was winter, and 2) she had cleaned out the ashes that day.

This was because she had an insane cocker spaniel who hid under the stove all seasons of the year, including when the stove was red hot. The bottom didn't get hot enough to make the dog smoke unless it had been cleaned out recently.

Looking back at it after decades of having dogs and heating with wood, it sounds like a crazy story to me too, but I'm not making it up! (That dog had serious mental problems though.)
 

Shrewboy

New Member
Oct 15, 2020
71
Eastern Pennsylvania
In a secondary combustion stove using tubes there is a baffle in the top of the firebox. Pretty much everything under that is insulated to keep temps up in that area for more complete combustion. The exhaust then goes around that baffle and runs back along the stove top. There are usually defectors there to increase dwell time and increase heat transfer there. The flue temps when run correctly can be much lower with a modern stove than with an old one while still burning cleanly. With other combustion technologies the heat transfer is done differently.
Thanks for this information!!
 

Shrewboy

New Member
Oct 15, 2020
71
Eastern Pennsylvania
Not every stove had them... For years, when you smelled burning hair in my grandmother's kitchen, you knew that 1) It was winter, and 2) she had cleaned out the ashes that day.

This was because she had an insane cocker spaniel who hid under the stove all seasons of the year, including when the stove was red hot. The bottom didn't get hot enough to make the dog smoke unless it had been cleaned out recently.

Looking back at it after decades of having dogs and heating with wood, it sounds like a crazy story to me too, but I'm not making it up! (That dog had serious mental problems though.)
LOL!!!!!