I have a client who I am working through the design phase on what is shaping up to be a very neat house project. She is pretty set on wanting a masonry stove which would be capable of heating the whole house (less than 1500 sq. ft.). However, she wants the ability to travel etc. for periods during the winter and so we will need a secondary heat source. I was thinking grumpily about the redundancy and cost inefficiencies that come with installing two fully functional heating systems in a house when I had what seemed to me a potentially Very Interesting Idea.
Would it be possible to install a thermostat controlled propane burner in a masonry heater, thus making one heat delivery method (the masonry mass) duel fuel and capable of automatic control? Naturally the propane burner would have to have it's own burn chamber, separate from the wood burn chamber, and would want it's own small flue running up the chimney.
What are the problems with this idea? Has this ever been done before that anyone knows of?
I imagine your idea would work. Not sure if the authorities would be open to it?
I cat and chicken sat for some friends last Easter for 10 days and it got fairly cold (5F lows). Their house is only heated with a huge masonry heater. It has an earthen roof, is buried on 2.5 sides and is modest in size. They normally light a fire every 2-3 days in the winter. We weren't expecting this level of cold so I was worried how warm the house would stay. On the last day it had cooled down to 55F.
I'm pretty sure that was due to the partially buried aspect of the house. The heater was probably doing its job for the first 3 days but then the thermal inertia of the surrounding soil kicked in.
You put the propane burner in a pocket, opposite the heat riser, and i think you should be fine. May be doing a sort of door on that narrow side too, onto which the burner is attached, and be moved away when the stove is used with wood. And replaced by a plain metallic door. You know, a bit like the door on oil burning boilers.
I think the issue here is safety. Your Masonry stove would need to be built to accept a propane burner without compromising the wood portion. Do you know a mason that is confident doing this ? I don't know if they make a stand alone burner assembly complete with thermostat and safety shutoffs, that is meant to be installed in a remote location. I suspect that the liability for the burner manufacturer would be horrendous. As a contractor anything short of a factory unit and a competent mason you might find your liability in question.
I suggest investigating this idea further with a propane guy and a mason to see if it has been or can be safely done. Otherwise a couple of strategically placed wall mounted ,vented propane units would keep pipes from freezing and save you any liability from a fire.
If there is a burner assembly available, then Max's idea of going into the bell is a good one.
EDIT) In retrospect a safe way to seal that bell during wood burning would be an issue. I'm sure there is some way it could be done. But for a customer rather than yourself probably not a good idea.
Thanks all for the input and suggestions. Many good thoughts there.
I'm thinking along the lines of having a totally separate firebox for the propane section with it's own fresh air intake and exhaust. Yes, the stove would be freestanding near the center of the house. I like the idea of incorporating the burner at floor leve, below the wood firebox.
I really haven't gotten as far as discussing it with any masons, and had just had the initial brainwave.
I was sort of half expecting that someone somewhere has done it already. (Nothing new under the sun and all that.)
I'm on the same page with some of you on the liability issue. This would have to be a burner which was in some way "legit" for the application seeing as it's for a customer and not for myself. Also, installed and designed with the blessing of the mason executing the work. Nothing janky for customers! Myself, that's another matter...
Caveats aside, the difficulties really don't seem to be insurmountable and seems to have the possibility to be a worthwhile idea to pursue further.
Maybe this will go somewhere in the short term, or it may end up shelved in the mental warehouse for later...
Sorry but it seems like you're missing some basics here. Or maybe I'm missing something.
You use a masonry heater because you're trying to capture the excess heat you produce that is lost up the flue. Wood fires can't make just a little heat, nor can they cycle on and off to keep the temperature relatively stable.
This isn't the case with a modern propane burner. They make just enough heat by design. Very little is wasted.
All you're going to accomplish is to burn a lot more propane than you need to if you use it to heat a thermal mass.
It's perfectly plausible that I am totally missing the basics of heat transfer here. I'm a timber framer by trade, so this is not exactly my specialty.
I'm not sure I understand why it would take more propane to heat the mass than heating the air directly? I understand that it would take longer to transfer through the thermal mass, but where would the loss occur?
I'm no engineer either but I'm trying to think of the practical aspects of the installation. Good luck getting any manufacturer to sign off on it.
A masonry stove is great. You can't really burn a wood fire cool enough (or short enough duration) for heating our homes so like a RMH you're trying to capture some of the heat lost up the flue. Again, it's to capture heat that would otherwise just be lost up the chimney. Modern gas burners are pretty darn efficient and there isn't that much heat lost. To make the thing work like a wood stove the burner will run steady for .. an hour? 2? So you'll use more propane than you need.
How will you regulate temperature? Think where your thermostat is. Sensing room temp as a normal thermostat? The burner will run for a very long time before the room warms. Probably now the lag time has been so long you'll get a large overshoot on the temperature so it's too hot. So you'll use more propane than you need.
What if you just have a sensor on the stove itself? Might work but doesn't take into account the variations you get from a warm day or a cold one. Windy day or not. So you'll use more propane than you need. Or worse, some cold windy night a water pipe in the corner will freeze even though the mass is at your chosen temperature.
A separate install with a normal room thermostat will heat the air just enough and then cycle off. It will take into account whether it's daytime and the sun is beaming in or a windy night, only using just enough gas to maintain room temp. The thermal mass of the stove will still do it's job, toning down those fluctuations in temperature.
What might be a good idea though is to have the stove close enough to a window that the sun can shine in and passively store some of that heat on a sunny day.
Here's the real bonus as I see it. Those long winter nights would be so much nicer if when the wood fire burned down in the middle of the night and you're faced with getting up in the dark to toss another log on you heard the propane burner kick on ... just roll over for another hour or two. It would heat the air just enough till you get up and stoke the fire again.
Could you run the exhaust pipe from a propane burner through the mass? Maybe but if it has to be inspected and approved.... good luck. Too much moisture in the gases to use the same flue as the wood burner I think.
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