Masonry stoves appear to be more effective - why aren't they more popular?

[Prometeo]

I'm sorry, but the *efficiency* given by independent (EPA) standardized tests is precisely that: the amount of heat coming in the home versus what goes out of the chimney (or, equivalently, the amount of heat coming in the home as a fraction of what the BTU content was of the fuel load).

Masonry stoves are indeed very efficient. Maybe even a little more than modern stoves. (My contention is that it's only a little more, and that it's not worth giving up *control* of that heat release rate for that little increase in efficiency - no knob to turn on a masonry stove to "turn it down" as I can on my stove.)

I agree about the EPA tests, but there are no EPA tests of a masonry stove having a greater mass than a metal stove, so this real, superior efficiency of a masonry stove, it is not tested

 

[bholler]

I agree about the EPA tests, but there are no EPA tests of a masonry stove having a greater mass than a metal stove, so this real, superior efficiency of a masonry stove, it is not tested

True. But looking at the efficiency ratings of stoves how are masonry heaters going to gain enough efficiency to cut wood usage very much at all?

 

[rashomon]

EPA Overall efficiency is the percentage of heat that is transferred to the space to be heated when a load of fuel (e.g., firewood, pellets) is burned...

If EPA overall efficiency negatively takes into account wasted heat through the chimney, how do manufacturers make sure they get the scores they want? Meaning, unless I'm running my fan full speed while continuously operating the damper to make sure everything is as efficient as possible I imagine the scores would be much lower as they would be in everyday situations. This makes me feel like the efficiency is kinda hocus-pocus too. right?

You and bholler have done a good job convincing me of a stove, and I do like the BK King recommendation for what I'm looking for. I'm only asking cause I'm curious. It was the exit temps I've heard with masonry stoves which made me think they were more efficient. Same amount of wood + lower exit temps over a similar amount of time would mean the energy is going somewhere else (that's not a very thorough scientific assumption, but I'm hoping to illustrate my point). With flueguard recommending temps between 350-900 F for "optimal operation" that's quite the window for an EPA test to find an efficiency rating--- meaning, is there a target exit temp they use for these EPA tests, otherwise it seems like to large a variable taking into account wasted energy out the chimney.

I have to add masonry heaters need to stay above the condensation point until the exhaust leaves the chimney as well or they will have creosote issues. Believe me I have seen it in poorly designed ones or ones using wet wood. No matter what claims are made they are not achieving complete combustion

Good to know!

 

[Prometeo]

True. But looking at the efficiency ratings of stoves how are masonry heaters going to gain enough efficiency to cut wood usage very much at all?

Exactly, it's impossible, as it is also impossible that the efficiency of these products is higher or equal to normal EPA metal stoves, all that mass also acts as insulation for the fire, and in general, low temperatures will never trigger a minimum of air convection, however, for a small house could have their reasons

 

[rashomon]

I agree about the EPA tests, but there are no EPA tests of a masonry stove having a greater mass than a metal stove, so this real, superior efficiency of a masonry stove, it is not tested

This is kind of what I'm getting at but it still seems we're in disagreement. Less heat wasted out the chimney, it must be stored somewhere, and I'm guessing it's inside the thermal mass (assuming mostly full combustion like they're designed for).

True. But looking at the efficiency ratings of stoves how are masonry heaters going to gain enough efficiency to cut wood usage very much at all?

Now I'm realizing I don't remember seeing any actual efficiency ratings for masonry stoves specifically because they are exempt from EPA testing for being untestable (as stated here).

My head is spinning. All I want to know is if exit gas temps negatively impact an EPA efficiency rating, then how does one insure heat isn't wasted during the test? Because it seems exit gases should be as low as possible without compromising the actual efficiency of combustion which I thought is exactly what masonry stoves do by design.

Exactly, it's impossible, as it is also impossible that the efficiency of these products is higher or equal to normal EPA metal stoves, all that mass also acts as insulation for the fire, and in general, low temperatures will never trigger a minimum of air convection, however, for a small house could have their reasons

The mass acting as insulation is exactly the point. The firebox is hotter making sure all the gas is burned. Low temperature only exists at the beginning or end of the burn. The mass spreads that heat and releases it much slower than a metal stove. To me it makes more sense that a masonry stove stores more heat, and that's what everyone claims.

 

[rashomon]

Actually, considering a catalytic burner on a metal wood stove being so close to the flue, wouldn't a considerable amount of heat be wasted? (If exit temps negatively affect EPA ratings?)

Forgive me for taking this off topic, though it's the last hump I have in holding onto why I believed masonry stoves may be superior at using more of the available heat output.

 

[stoveliker]

to know is if exit gas temps negatively impact an EPA efficiency rating, then how does one insure heat isn't wasted during the test?

The EPA efficiency rating is *based on* measuring the heat. So it takes into account the loss/waste through the flue. The number is the result of measuring (taking the waste into account).
You *know* how many BTUs go into the firebox.
You *measure* how many BTUs go into your room.
That fraction is the efficiency.

Consider this: look at the heat as a bucket of water.
The EPA (accredited labs) test this by putting the bucket of water in the stove, and measuring how much water comes into the room. If out of the 10 gallons (large bucket ;p ), 8 gallons come into the room, that means 2 gallons disappeared out of the flue.
That's the 80% efficiency of the overall efficiency of the EPA list.

Make no mistake: Masonry stoves are efficient (if operated properly with proper fuel) - no doubt about that.
But as I said, I often run my stove *as efficient as can be* - with flue gases skirting the minimum temperature. You can design any 60,000 ton stove to try to get more out of it, but as bholler says, it's not going to work well if you're below the condensation temperature. There's no way to make that more efficient with my stove. If the gases at the chimney cap are above 230 F or so (strictly 212 F), that's the max one can do for a low burn.

The point is that other stoves are nearly as efficient. And allow control during the burn that masonry stoves don't. That trade of is huge imo. If the sun comes out and heats your home a bit more on a day, you're stuck with the masonry stove radiating and over heating your home. I just dial down my stove.

You said:
" The mass spreads that heat and releases it much slower than a metal stove. To me it makes more sense that a masonry stove stores more heat, and that's what everyone claims."

That is true.
But I don't have to store it as I release it at a lower rate (rather than all in an explosively hot, quick fire twice a day as in a masonry stove). Just another way: either get the heat out of the wood immediately and store it for slow release, or just slowly get the heat out of the wood.
That does not affect how much of the available output is being used. It's just another route, with a "parking spot" for the heat in a masonry stove. That in itself has no consequences for the efficiency.


Finally, note that this is all true for burning low.
When one burns high/hot, the efficiency (of my stove) does not change. But the chimney is hotter. How?
Because the fraction of BTUs produced that go into the room is still the same: a lot more heat coming off into the room, and the same percentage increase in heat going up the flue. One might think "that's more heat going out of the flue with the same load" - but that's not necessarily true, because the burn time is shorter: the hot gases go out the flue for a short time (wood quickly gone), whereas burning low the "warm" gases keep going out for a long time. That equates still about the same heat low through the flue over a full load burned. (As reflected by the similar efficiency of my stove for low and high burns, according to the mfg who quoted independent tests, i.e. not their own data.)

 

[Prometeo]

there is no heat wasted on the flue, heat is needed to avoid creosote, and generally every EPA stove user is able to manage this minimum necessary temperature, without excess, so in consideration of this, a greater mass will only be an obstacle between you and the heat, Furthermore, Have you considered the maximum power of a masonry stove? it is much lower, so if your home needs more btu, masonry stove it won't be enough. Furthermore, It is NOT true that a higher temperature inside the combustion chamber necessarily produces greater efficiency without specific sensors that wood stoves don't have

 

[stoveliker]

Anyway, the good news is that you're thinking about things.

Don't get hung up on a BK.
If you want heat based on twelve hour reloads, there are many stoves that can do that.
You may need more heat than a BK burning at low output. If so, the advantage of a BK is none.Other stoves with similar efficiency (i.e. similar BTU input into the home from a x cubic ft wood load) exist to meet your needs.


The nice thing about a BK is the thermostat, in my experience. Especially if the stove is in a basement where you don't walk by often.
It keeps the heat ouput constant.

 

[EatenByLimestone]

I noticed you mentioned that when you go to reload, you can’t leave the door open long without cooking your hands from the heat. This seems a bit hot. Maybe you still have a lot of heat/burn left in the stove before you reload.

 

[KDubU]

I had a custom masonry heater built in a new build back in 2009 in Ontario, Canada. It was planned as part of the build with 20” of extra concrete poured in the basement where the base would be built on. The base was all concrete block up the first floor and then a concrete floor poured where the masonry heater would be built on. The mason who was certified in masonry heaters stated the weight once complete was thousands of pounds. It was 6’ wide, 4’ deep and just under 8’ tall as it was a single story craftsman build.

It is the best way to heat if you are using wood but the above is why they are not more popular. The majority of them need to be planned as part of a house build. Yes there are some smaller soapstone as indicated above and they are very nice but think mass when it comes to these as they will heat your whole house if built and sized right.

Edited to add they cost some dollars. Mine was around $15-16k in 2009.

 

[peakbagger]

I had a custom masonry heater built in a new build back in 2009 in Ontario, Canada. It was planned as part of the build with 20” of extra concrete poured in the basement where the base would be built on. The base was all concrete block up the first floor and then a concrete floor poured where the masonry heater would be built on. The mason who was certified in masonry heaters stated the weight once complete was thousands of pounds. It was 6’ wide, 4’ deep and just under 8’ tall as it was a single story craftsman build.

It is the best way to heat if you are using wood but the above is why they are not more popular. The majority of them need to be planned as part of a house build. Yes there are some smaller soapstone as indicated above and they are very nice but think mass when it comes to these as they will heat your whole house if built and sized right.

Edited to add they cost some dollars. Mine was around $15-16k in 2009.

I would argue the point that a masonry heater is the "best" way to heat. A well designed masonry heater probably qualifies as a better way to heat, but its still a space heater subject to the limitations that every space heater has. IMO hard to beat a gasifier with thermal storage and low temperature hydronic distribution. Masonry heaters still have the inherent problem that heat distribution inside the space is dependent on convection. Heat loss tends to occur at the boundaries of the building and the outdoors. The best way to offset that is to put heat at the boundary using low temp hydronic radiant emitters with local thermostats for each space. If the space is not being used its heat can be turned down and its quite easy to automate that so it can happen automatically or remotely.

Anytime there is a central heat source, the only way to heat the remote boundaries of the house is to overheat the central core and hope convection moves enough heat to the boundaries. Sure ductwork and fans can be added but they take up a lot of room. Moving a small amount of water moves a lot of heat and it can be directed to the coldest spot. Plenty of studies out there that a radiantly heated space can be maintained at a lower temperature than a hot air heated space.

The other aspect with a masonry heater or any space heater is backup for when an owner is not home or able to feed the unit. With hydronic with low temp emitters, cold climate air to water heat pumps (or small scale geothermal) can act as a near seamless backup utilizing the same distribution system. Also a lot easier to manage in shoulder seasons where heating may only be required for a few hours at night, while no heat is needed or wanted once the sun is up.

Of course the most efficient heat is the heat you dont need to generate. Using proven building techniques almost any home can be built to require little or no supplemental heating. The supplemental heating needed can be obtained from solar panels using net metered solar to run heat pump technology for heat (and cooling).

 

[Prometeo]

I would like to specify that this type of stove does not exceed approximately 10,000 btu per hour so it is necessary to evaluate the heat requirement per hour, of the house.

 

[KDubU]

I would argue the point that a masonry heater is the "best" way to heat. A well designed masonry heater probably qualifies as a better way to heat, but its still a space heater subject to the limitations that every space heater has. IMO hard to beat a gasifier with thermal storage and low temperature hydronic distribution. Masonry heaters still have the inherent problem that heat distribution inside the space is dependent on convection. Heat loss tends to occur at the boundaries of the building and the outdoors. The best way to offset that is to put heat at the boundary using low temp hydronic radiant emitters with local thermostats for each space. If the space is not being used its heat can be turned down and its quite easy to automate that so it can happen automatically or remotely.

Anytime there is a central heat source, the only way to heat the remote boundaries of the house is to overheat the central core and hope convection moves enough heat to the boundaries. Sure ductwork and fans can be added but they take up a lot of room. Moving a small amount of water moves a lot of heat and it can be directed to the coldest spot. Plenty of studies out there that a radiantly heated space can be maintained at a lower temperature than a hot air heated space.

The other aspect with a masonry heater or any space heater is backup for when an owner is not home or able to feed the unit. With hydronic with low temp emitters, cold climate air to water heat pumps (or small scale geothermal) can act as a near seamless backup utilizing the same distribution system. Also a lot easier to manage in shoulder seasons where heating may only be required for a few hours at night, while no heat is needed or wanted once the sun is up.

Of course the most efficient heat is the heat you dont need to generate. Using proven building techniques almost any home can be built to require little or no supplemental heating. The supplemental heating needed can be obtained from solar panels using net metered solar to run heat pump technology for heat (and cooling).

I believe I said with wood, not the best way to heat overall.

 

[stoveliker]

Lol that is only two 1.5 kW plug in electric space heaters...

I think the "real" masonry heaters (tons of weight, see Finland and Russia) may be able to do more - though with generally small fireboxes and thus small fuel loads, BTU outputs indeed seems limited.

My BK can do 12000 BTU per hour - for 30+ hrs straight...
And a lot more BTUs per hour for a lot shorter duration.

 

[stoveliker]

I believe I said with wood, not the best way to heat overall.

Gasifier is a wood furnace.

 

[bholler]

I believe I said with wood, not the best way to heat overall.

He was talking about wood as well

 

[Prometeo]

Lol that is only two 1.5 kW plug in electric space heaters...

I think the "real" masonry heaters (tons of weight, see Finland and Russia) may be able to do more - though with generally small fireboxes and thus small fuel loads, BTU outputs indeed seems limited.

My BK can do 12000 BTU per hour - for 30+ hrs straight...
And a lot more BTUs per hour for a lot shorter duration.

I haven't tried the plug ins but the 2kw ones with wire and fan and they're damn efficient as long as they're on

 

[stoveliker]

Indeed, 100 percent efficient. Half that of a poorly performing mini split heat.pump...

 

[rashomon]

Wow! I had an event last night so I wasn't able to get to this until today. It was wonderful to see all these great responses. Thank you all so much for all your help.

The EPA efficiency rating....takes into account the loss/waste through the flue.....

....(Masonry stoves)....it's not going to work well if you're below the condensation temperature. There's no way to make that more efficient with my stove. If the gases at the chimney cap are above 230 F or so (strictly 212 F), that's the max one can do for a low burn.
....The point is that other stoves are nearly as efficient. And allow control during the burn that masonry stoves don't. That trade of is huge imo. ...
...I don't have to store [heat] as I release it at a lower rate (rather than all in an explosively hot, quick fire twice a day as in a masonry stove). Just another way: either get the heat out of the wood immediately and store it for slow release, or just slowly get the heat out of the wood....That in itself has no consequences for the efficiency.

Finally, note that this is all true for burning low.
When one burns high/hot, the efficiency (of my stove) does not change. But the chimney is hotter. How?
Because the fraction of BTUs produced that go into the room is still the same: a lot more heat coming off into the room, and the same percentage increase in heat going up the flue. ..... That equates still about the same heat low through the flue over a full load burned. (As reflected by the similar efficiency of my stove for low and high burns, according to the mfg who quoted independent tests, i.e. not their own data.)

Sooo much great info. I have certainly gotten sidetracked by this EPA efficiency thing. I want to drop it if I can get some confirmation that indeed the when EPA stoves are tested they are done so to maximize their efficiency in order to get the highest scores they can. I know that's obvious. But as I mentioned, and in reference to your last paragraph about a stove being just as efficient when run high versus run low, that this does make sense, EXCEPT when I run my stove high I have the ability to run my blower fan also on high which does an insanely good job at lowering my flue temps. It only makes sense to me that the flue temps are lower because my fan is pushing that heat into my space (obvious, right?).

For one, do EPA efficiencies take into account this variable for additional efficiency? Is this not even good practise and I shouldn't be doing this? Am I correct that 350 (going off the flueguard markings) should be the lowest, or do you go as low as 230 like you teased?

The idea that my stove did not come with a blower and I had to buy it separately, and yet my stove claimed the efficiency it was rated gives me suspicion about the wasted heat that goes up the chimney during the EPA testing. If stoves run just as efficient low or high I can understand this heat wouldn't be wasted, so I guess my last question is about the fan. Do I have an even greater efficiency with the fan keeping my flue temps low?!

All those points about Masonry heaters make sense. Your trading controlling the burn vs controlling the release of heat. I need more evidence flue temps on masonry heaters are less than the 350 I'm assuming for traditional stoves, yet more than the condensation temps) before I presume they are any more efficient.

there is no heat wasted on the flue, heat is needed to avoid creosote, and generally every EPA stove user is able to manage this minimum necessary temperature, without excess, so in consideration of this, a greater mass will only be an obstacle between you and the heat, Furthermore, Have you considered the maximum power of a masonry stove? it is much lower, so if your home needs more btu, masonry stove it won't be enough..

So am I correct to be 'managing' my necessary temperature by running my current EPA stove using a fan to keep my exit temps at a minimum? Otherwise I feel I would be 'wasting heat on the flue'. Do you have a target exit temp for capturing as much heat as possible?

It's possible a masonry stove wouldn't have the heat output I need. I will talk to my cousin who does energy audits and see if I can calculate that.

Anyway, the good news is that you're thinking about things.

Don't get hung up on a BK.
If you want heat based on twelve hour reloads, there are many stoves that can do that.
You may need more heat than a BK burning at low output. If so, the advantage of a BK is none.Other stoves with similar efficiency (i.e. similar BTU input into the home from a x cubic ft wood load) exist to meet your needs.

The nice thing about a BK is the thermostat, in my experience. Especially if the stove is in a basement where you don't walk by often.
It keeps the heat ouput constant.

I really appreciate this. I will take all this advice. Though I do like the thermostat for exactly the reason you're saying.

I noticed you mentioned that when you go to reload, you can’t leave the door open long without cooking your hands from the heat. This seems a bit hot. Maybe you still have a lot of heat/burn left in the stove before you reload.

Yes, I think I'm overworking my stove. I'm certain I am. When it's really cold I'll have my blower on full and the sucks a lot of heat before it exits the flue. So when my flue temps drop below 400 or 350 I end up adding wood even if it's not empyt (and just coals). That box is scorching hot, but the fan must really does a good job pulling the heat off the back (not sure if that's a bad thing).

I had a custom masonry heater built in a new build back in 2009 in Ontario, Canada...

Thanks for sharing that.

...IMO hard to beat a gasifier with thermal storage and low temperature hydronic distribution....

Of course the most efficient heat is the heat you dont need to generate. Using proven building techniques almost any home can be built to require little or no supplemental heating. The supplemental heating needed can be obtained from solar panels using net metered solar to run heat pump technology for heat (and cooling).

Yes, I've only recently learned about gasifiers. Does one simply not use them in the warmer months? I know they can provide your hot water during summer, but does that make sense? The units themselves seem affordable but I assume that doesn't include the costs of the distribution. Could one have a branches that can be turned on for other uses like a greenhouse, to melt a driveway, or heat a shed do they have to constantly be going so they don't freeze? I imagine you can have a heat exchange to glycol or something but that adds cost.

Indeed, 100 percent efficient. Half that of a poorly performing mini split heat.pump...

Are you saying mini splits are 200% efficient?

 

[stoveliker]

Wow! I had an event last night so I wasn't able to get to this until today. It was wonderful to see all these great responses. Thank you all so much for all your help.

Sooo much great info. I have certainly gotten sidetracked by this EPA efficiency thing. I want to drop it if I can get some confirmation that indeed the when EPA stoves are tested they are done so to maximize their efficiency in order to get the highest scores they can.

No, the tests are prescribed, a fixed procedure with a fixed amount of wood etc. NOT to maximize efficiency (which would be a different burning procedure for each different stove!), but the same, fixed procedure for all stoves so that the numbers can be compared - even if they are not that representative of "real world burning in a home".
As I said, wood load, moisture content, flue height etc etc etc will anyway vary the efficiency one gets while burning at home.
Don't get hung up on the value (except for it being high enough to get the tax credit ).

I know that's obvious. But as I mentioned, and in reference to your last paragraph about a stove being just as efficient when run high versus run low, that this does make sense, EXCEPT when I run my stove high I have the ability to run my blower fan also on high which does an insanely good job at lowering my flue temps. It only makes sense to me that the flue temps are lower because my fan is pushing that heat into my space (obvious, right?).

For one, do EPA efficiencies take into account this variable for additional efficiency? Is this not even good practise and I shouldn't be doing this? Am I correct that 350 (going off the flueguard markings) should be the lowest, or do you go as low as 230 like you teased?

I am not sure whether the EPA test procedures use the fan or not.
Yes, you using the fan will increase the efficiency, especially when you burn "high".
I have burned at 230 yes. Accidentally (my stove is in the basement and I don't keep looking at it...); not ideal, as it would be below condensation at the top of my flue.

(This is the reason that when it's 45 or higher or so for 24 hrs or more, I use my minisplit - which is 400+% efficient then, see below).

The idea that my stove did not come with a blower and I had to buy it separately, and yet my stove claimed the efficiency it was rated gives me suspicion about the wasted heat that goes up the chimney during the EPA testing. If stoves run just as efficient low or high I can understand this heat wouldn't be wasted, so I guess my last question is about the fan. Do I have an even greater efficiency with the fan keeping my flue temps low?!

All those points about Masonry heaters make sense. Your trading controlling the burn vs controlling the release of heat. I need more evidence flue temps on masonry heaters are less than the 350 I'm assuming for traditional stoves, yet more than the condensation temps) before I presume they are any more efficient.

So am I correct to be 'managing' my necessary temperature by running my current EPA stove using a fan to keep my exit temps at a minimum? Otherwise I feel I would be 'wasting heat on the flue'. Do you have a target exit temp for capturing as much heat as possible?

As low as safe - and that depends on how much the flue gases cool down on their way to the top.
I also note that I don't set the stove primary air to do this, I set the air *to get the amount of heat I want*... That's after all the goal.

It's possible a masonry stove wouldn't have the heat output I need. I will talk to my cousin who does energy audits and see if I can calculate that.

If you're pushing your current stove to its max, 10,000 BTU/hr masonry output would not (even close) cut it.
On the other hand an additional BTU/hr release in your home is helpful at any level... !

Are you saying mini splits are 200% efficient?

For a *poorly performing* one yes. A minisplit does not generate heat, it transports it. That costs energy. But the energy in kWh it takes to deliver 1 kWh in heat inside is less than 1 kWh.
A resistance plug in heater uses 1 kWh to produce 1 kWh of heat. 100% efficient (A stove uses wood the equivalent of 1 kWh chemical energy content to produce say 0.75 kWh of heat in the room - hence 0.75 kWh efficiency - some caveats here, but that's the basics).

My minisplit has a COP of 2.67 when it is 17 F outside.
That means it uses 1 kWh of power to deliver 2.67 kWh of heat in my home. *that it extracts from 17 F air outside!!* That is an efficiency of 267%
At 47 F, the COP is 3.5 or so. So it uses 1 kWh of electrical power to deliver 3.5 kWh of heat - an efficiency of 350% !!

 

[stoveliker]

Bottomline: Yes, burning at home, it is best if you can design the system in such a way that you can have the least heat go up the flue.
For some that means adding a key damper in the flue to decrease the draft ("suction")that, if too much, would suck out the hot gases before they can release their heat to the room.

But, care should be taken to not decrease below what is safe. As no combustion is 100% complete at all points in the cycle, gases/smoke/particles will go up the flue. Those should go out, not get stuck in condensed water in the pipe. Hence the need to keep some temps above 212 F until the top of the flue (also for masonry stoves that's best).
Moreover, flue temps higher than the surroundings is what creates draft - which is what drives your stove. No draft (because too low flue temps), and no air gets sucked into the stove, meaning not enough oxygen, a starving fire, smoke, and all consequences of that.

So don't go too low in flue temps.

A fan is fantastic to increase the efficiency of a stove when you're burning high, but if you're burning low, and your exhaust gases are skirting the safe temp range, i'd not use the fan.

The beauty is that you can easily switch off the fan...

 

[EatenByLimestone]

It sounds like you need a bigger stove, or an additional one. Maybe the stone is stopping the radiant heat you are looking for?

 

[Prometeo]

@rashomon
absolutely, the first thing is to establish, in the cold season, how many btu your house needs per hour to be heated, if over 10-15000 forget the masonry stove

 

[stoveliker]

I looked this up, and there are masonry stoves with more appropriate output:

Types of Masonry Heaters - stovemason

masonry heaters - uk - scotland - england

www.stovemason.com

Max load is about 50 lbs. Assume 7000 BTUs per lbs, and a (generous...) efficiency of 90%, that means 315,000 BTUs going into the room. If (...) that's spread over 12 hrs, that averages to 26,000 BTUs per hour.

Of course there's no control, and I don't know how even the heat release into the room is within the burn+cool cycle.

But there are masonry stoves that can do decent BTU production.

These are very different from the lighter build it yourself package kits one finds online. I think the big ones (as in half a small European room-sized, with bed alcoves in/on it, different rooms on different sides of the stove), can do this, i.e. more than the 15,000 BTU per hour.