Getting heat out of the fireplace

[Fried Chicken]

I've been told adding an air source can make a significant difference. A friend of mine did this by literally boring holes through the floor in front of his fireplace & he swears by it

This works: the fireplace pulls outside air instead of inside air and radiates heat out.

I built a Rumford designed fireplace from I think about 1790. It has a shallow back wall revealing a large area to reflect heat out. It also has a full width throat with a smoke shelf behind and an adjustable flap to help prevent downdraught and also to stop heat escaping if fully closed when perhaps you are using other heat sources
My old fashioned builder gave me an earful saying he had never seen anything like it and it would never work, but in the Winter I caught him and his mate warming themselves when they should have been on the roof tiling! “ Well I never would have believed it!” They said!

He clearly wasn't old fashioned enough!


I did some reading about fireplace inserts, and maybe I need to overcome my biases. They achieve efficiencies of up to 80% where an open fireplace is happy to get 20%

https://forestry.com/best/best-wood-burning-fireplace-inserts-of-2024/

That said, I don't live somewhere cold so heating needs are minimal. I wish I could burn firewood inlieu of an air conditioner.

 

[Hermio]

I've done a bit more research and slept on the thought.

I don't like the idea of an insert when there's already a perfectly good fireplace. It's a very space inefficient solution.

I do like the idea of glass doors with a blower/heat exchanger of some sort. Unfortunately the only solutions I've find (like this one) similar to the inserts throw a lot of otherwise usable space away. It also looks like a nightmare to clean.

I found a product that kind of looks like it might achieve this

That is similar to what we had. I am a bit shocked by the price, though.

 

[Hermio]

I've been told adding an air source can make a significant difference. A friend of mine did this by literally boring holes through the floor in front of his fireplace & he swears by it

No need to do that. The cold air still needs to be heated up to the firebox temperature whether it comes from room air or a special duct. It cancels out.

 

[Hermio]

I built a Rumford designed fireplace from I think about 1790. It has a shallow back wall revealing a large area to reflect heat out. It also has a full width throat with a smoke shelf behind and an adjustable flap to help prevent downdraught and also to stop heat escaping if fully closed when perhaps you are using other heat sources
My old fashioned builder gave me an earful saying he had never seen anything like it and it would never work, but in the Winter I caught him and his mate warming themselves when they should have been on the roof tiling! “ Well I never would have believed it!” They said!

I also put two four inch diameter pipes feeding air from outside to the sides of the fireplace so the air was not pulled under the doors or through gaps in windows
I put a length of bar from the bottom of the flap over the throat so that I could adjust it from outside with a screw thread.
A pair of mesh doors finished the job over 40 years ago so that it could be safely left unattended or the room shielded if it became too hot

https://images.app.goo.gl/2pxecQnbbyEogX7v6

The Rumford style is much more efficient than most modern designs. The outside air makes no difference though; it reduces the flame temperature a bit.

 

[Hermio]

It is a hot as my wood stoves, an Aarrow and a Danish Aduro and has never smoked even when lighting. Behind the smoke shelf I installed a hinged metal door which can be opened and the chimney swept with the throat flap firmly shut and the soot going outside

With a masonry heat exchanging fireplace such as Tulikivi, you would not get any soot. But these days they usually go for more than $20,000 installed cost.

 

[Hermio]

This works: the fireplace pulls outside air instead of inside air and radiates heat out.

He clearly wasn't old fashioned enough!


I did some reading about fireplace inserts, and maybe I need to overcome my biases. They achieve efficiencies of up to 80% where an open fireplace is happy to get 20%

https://forestry.com/best/best-wood-burning-fireplace-inserts-of-2024/

That said, I don't live somewhere cold so heating needs are minimal. I wish I could burn firewood inlieu of an air conditioner.

Woodstoves and inserts can be efficient if they are allowed to burn the wood completely with no CO or soot or creosote formation. That means burning hot with a small excess of oxygen. But very few people would operate them that way, as under such conditions, the metal will actually glow red hot and the heat output would drive people out of the room. In the real world, people operate such stove with a slow, smoldering burn, which creates a lot of crap in the chimney and reduces efficiency to maybe 30-50%. The Euro style heat storage fireplaces get maybe 85% efficiency by always burning hot and fast but storing the heat in the stone. I have had one in my house for 29 years, burning about 6 cords per year, and there is no creosote in my chimney. I have never had to clean it. I just have to remove the ash from the pit and the internal channels of the unit once per year.

 

[Fried Chicken]

Woodstoves and inserts can be efficient if they are allowed to burn the wood completely with no CO or soot or creosote formation. That means burning hot with a small excess of oxygen. But very few people would operate them that way, as under such conditions, the metal will actually glow red hot and the heat output would drive people out of the room. In the real world, people operate such stove with a slow, smoldering burn, which creates a lot of crap in the chimney and reduces efficiency to maybe 30-50%. The Euro style heat storage fireplaces get maybe 85% efficiency by always burning hot and fast but storing the heat in the stone. I have had one in my house for 29 years, burning about 6 cords per year, and there is no creosote in my chimney. I have never had to clean it. I just have to remove the ash from the pit and the internal channels of the unit once per year.

I'm looking at an insert right now, used, but I could have it installed.
Seller says the blower is broken, so I'd have to look into fixing it. Unfortunately parts are no longer available (Country Flame brand). Are these any good?

 

[Fried Chicken]

No need to do that. The cold air still needs to be heated up to the firebox temperature whether it comes from room air or a special duct. It cancels out.

It's not a question of the temperature of the air being burned. If you pull air in from the outside with a dedicated duct, you're not pulling air into the house via a vacuum

 

[sean donato]

Woodstoves and inserts can be efficient if they are allowed to burn the wood completely with no CO or soot or creosote formation. That means burning hot with a small excess of oxygen. But very few people would operate them that way, as under such conditions, the metal will actually glow red hot and the heat output would drive people out of the room. In the real world, people operate such stove with a slow, smoldering burn, which creates a lot of crap in the chimney and reduces efficiency to maybe 30-50%. The Euro style heat storage fireplaces get maybe 85% efficiency by always burning hot and fast but storing the heat in the stone. I have had one in my house for 29 years, burning about 6 cords per year, and there is no creosote in my chimney. I have never had to clean it. I just have to remove the ash from the pit and the internal channels of the unit once per year.

your cart blanc statement about efficiency and burn rate is bs. Any even semi modern listed furnace will be very efficient even when not burning wide open, and not produce creosote in the chimney. Reburn chambers, secondary air injection, and cats have improved output and efficiency greatly over smoke dragons of yesteryear.

 

[Hermio]

I'm looking at an insert right now, used, but I could have it installed.
Seller says the blower is broken, so I'd have to look into fixing it. Unfortunately parts are no longer available (Country Flame brand). Are these any good?

I don't know.

 

[Hermio]

It's not a question of the temperature of the air being burned. If you pull air in from the outside with a dedicated duct, you're not pulling air into the house via a vacuum

One way or another, the combustion air has to be heated. It will take the same amount of heat away from the fire if added directly as it will if it goes through the house, so that theoretically cancels the benefit. The folks at Tulikivi researched this and found that letting cold air go directly to the fire actually reduced combustion efficiency, so the end result was actually a slight heat loss. They do not recommend an outside air duct going directly into their fireplaces.

 

[Hermio]

your cart blanc statement about efficiency and burn rate is bs. Any even semi modern listed furnace will be very efficient even when not burning wide open, and not produce creosote in the chimney. Reburn chambers, secondary air injection, and cats have improved output and efficiency greatly over smoke dragons of yesteryear.

I am aware that the features you mention improve efficiency. But it is simply a fact that a smoldering fire is less efficient than one which achieves complete combustion. The catalyst may prevent creosote but it will likely get clogged at some point. Would you trust such a system to go 20 years or more with no chimney cleaning?

 

[sean donato]

I am aware that the features you mention improve efficiency. But it is simply a fact that a smoldering fire is less efficient than one which achieves complete combustion. The catalyst may prevent creosote but it will likely get clogged at some point. Would you trust such a system to go 20 years or more with no chimney cleaning?

Yes, with proper care, use and seasoned wood there's zero reason a secondary burn system or cat stove will give issues in a 20 year period. The cats will eventually use up the precious metals used so naturally they will eventually need replaced. Secondary burn systems are self contained and have no real wear parts. I have the latter. So long as I keep pipe outlet temps above 300* there's virtually zero build up in a years time. Heated secondary air combustion burns off any soot/ unburned gassed in the case of my stove. Start up smoke is minimal, and not any worse then a rocket mass stove till it gets some heat in it.
Most people's issues are lack of knowledge of how to use the systems, improper care and wet wood. Everyone want to bag on the systems as junk, but when pointed out they were using wet wood, took a wire brush to the cat or choked it down to the point secondary combustion can't take place, they get all butt sore and defensive. Doesn't make it any less true. Certainly there are old stoves, fireplaces and furnaces that will burn like crap not running wide open, but that's been a load of bs for many years now.

 

[muddy42]

Ok Im just going to say it. I get it that stoves are more efficient than open fires but I am still not convinced by this whole negative efficiency theory for open fires. How do you test it?

Ive certainly never thought an open fire cools the rest of the house and I light one every night in our old house.

Could this not just be a myth peddled by stove sellers wanting to sell stoves, flue liners etc.?

Have todays builders lost the skill to design fireplaces?

 

[Hermio]

Yes, with proper care, use and seasoned wood there's zero reason a secondary burn system or cat stove will give issues in a 20 year period. The cats will eventually use up the precious metals used so naturally they will eventually need replaced. Secondary burn systems are self contained and have no real wear parts. I have the latter. So long as I keep pipe outlet temps above 300* there's virtually zero build up in a years time. Heated secondary air combustion burns off any soot/ unburned gassed in the case of my stove. Start up smoke is minimal, and not any worse then a rocket mass stove till it gets some heat in it.
Most people's issues are lack of knowledge of how to use the systems, improper care and wet wood. Everyone want to bag on the systems as junk, but when pointed out they were using wet wood, took a wire brush to the cat or choked it down to the point secondary combustion can't take place, they get all butt sore and defensive. Doesn't make it any less true. Certainly there are old stoves, fireplaces and furnaces that will burn like crap not running wide open, but that's been a load of bs for many years now.

Don't worry, I am not but sore or defensive. But I am skeptical about efficiency claims. I have tried to research how efficiency is measured, and the methods are clear enough, but they do not explicitly state the conditions. It is my belief that even with secondary combustion, burning slow is less efficient overall than a fast, hot fire. Therefore, I suspect the efficiencies were measured under ideal conditions (fast burn) rather than the actual conditions most people use. As a side note, there are home gas furnaces and water heaters that go above 90%, but to do that, they actually have to extract heat from the combustion water vapor and condense it, resulting in flu gas temperatures below 150F, so they need a blower to drive the flu gas up the chimney, which is often made of PVC. No good in a power failure, though. I get what you say about wet wood. That is why I stack wood in the round for 2 years before I split it, and then I split it between March and the end of May each year, putting the split wood under an 8' overhang on the back of my garage, where it gets plenty of air and sunshine but no direct rain. My moisture meter typically reads between 8 and 15% moisture when I burn the wood. I have not measured the flu gas temperature on my Tulikivi, but the manufacturer says 250-350F is typical even though it is a short, hot fire because the soapstone absorbs 80-85% of the heat. THe masonry heater stays warm or hot to the touch for at least 24 hours after the fire goes out.

 

[Hermio]

Ok Im just going to say it. I get it that stoves are more efficient than open fires but I am still not convinced by this whole negative efficiency theory for open fires. How do you test it?

Ive certainly never thought an open fire cools the rest of the house and I light one every night in our old house.

Could this not just be a myth peddled by stove sellers wanting to sell stoves, flue liners etc.?

Have todays builders lost the skill to design fireplaces?

To answer your last question, most modern fireplaces are decorations rather than true heaters. The old Rumford-style units were more efficient. But it is possible that more fireplace builders are re-learning how to build a fireplace that actually heats. I have not researched this for a number of years. I did extensive research on it before we got our current house built in 1995.

 

[Mad Professor]

Woodstoves and inserts can be efficient if they are allowed to burn the wood completely with no CO or soot or creosote formation. That means burning hot with a small excess of oxygen. But very few people would operate them that way, as under such conditions, the metal will actually glow red hot and the heat output would drive people out of the room. In the real world, people operate such stove with a slow, smoldering burn, which creates a lot of crap in the chimney and reduces efficiency to maybe 30-50%. The Euro style heat storage fireplaces get maybe 85% efficiency by always burning hot and fast but storing the heat in the stone. I have had one in my house for 29 years, burning about 6 cords per year, and there is no creosote in my chimney. I have never had to clean it. I just have to remove the ash from the pit and the internal channels of the unit once per year.

I beg to differ. If you have dry hardwood and an insulated flue there is very little buildup.

I installed a boilerplate Warner stove in my fireplace many years ago. Some people call them smoke dragons, but it has an efficient baffle system of thick boilerplate and reburns the gases before they leave the stove; will take 30" logs. It also has internal air passages that allows room air to flow within the stove then exit the sides via convection. This system also has an electric blower that never gets used unless I come home and the house is nearly freezing.





The traditional fireplace, that has a damper and smoke shelf, got a complete insulated SS liner with a cap and T-cleanout at the stove.



I extended the hearth to meet fire code using fireboard overlaid with 2" granite slabs. Made a heat deflector to keep the fireplace mantle cool.



In reality you get a hot fire going to start things up and warm the flue then you can feed it and get the stove hot. Once at that point you can slow the draft quite a bit and just feed the stove as much wood as you need heat.

I've heated my home (~1200 cu ft) in Vermont (zone 4) almost totally on wood unless I'm gone for > 2 days and the furnace kicks on. I clean the flue once a year and get ~ a gallon of crud. The worst buildup is on the chimney screen on top that cools off, that needs a wire brush treatment.



This would be way overkill for heating a home in Texas. But a smaller efficient stove inserted in a similar way would work well.

For ambiance a stove with glass doors. Or with a little less efficient Franklin stove that can be operated with doors open or closed. My parents home had a Franklin stove that had a fixture to attach either a grill or hang a Dutch oven. That was fine ambiance of an open fire, and also great cooking steaks on an open fire inside the kitchen.

 

[Fried Chicken]

I am looking at a country flame insert tomorrow. Buyer claims the blower unit is broken, but the price sort-of reflects the condition. I'm also in talks with an installer, but i'm also tempted to just do it myself.

Why do fireplace inserts need a separate flue from (but inside) the chimney?