Scavenging Heat from Gas Fireplace Chase
I’m installing a new gas fireplace. DirectVent 4/6 with 14′ of near straight vertical pipe. Primary reason is design considerations and aesthetics. Secondary is my wife wants lots of heat on a switch to heat up our main area. The our hydronic heat doesn’t react quick enough for big temp swings. I also just like a big, centrally located mantel.
Anyway, I’ve got a little 18,000btu unit that at elevation is probably significantly less than that, but man does it crank out some heat.
I’ve got a blower for the firebox that helps bring that heat out into the room.
But the fireplace chase itself will get fairly warm. Despite the loose fit firestop at the ceiling, there is just not enough airflow to cool that space (12″ deep, by 60″ wide, by 12′ tall) and the pipe get pretty warm.
I don’t mind the warmth of the pipe – I trust my distances to combustibles – but I’m also burning gas that’s just going to warm up my fireplace chase – not my room.
Could I vent the chase into the conditioned space some way to scavenge heat from that area and improve vent cooling?
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This is basically what a condensing furnace does, which is why it's able to be vented using PVC pipe. There are two concerns. First, the exhaust from burning gas has a lot of water vapor in it. If you cool that exhaust stream that water vapor condenses out and you need a way of handling that condensate. Second, the other exhaust products that condense out with the water tend to make it highly acidic. Anything that touches those products has to be resistant to acid.
To be clear, I'm talking about a rather large fireplace chase that I want to take from 140º when sealed to just slightly higher than room temp.
Not planning on actively trying to cool the vent.
Rather - I'm trying to make the fireplace chase more apart of the conditioned space.
If you cool it that much you'll definitely have to deal with condensate.
Yes, that's right in the range where you could start creating condensation. Water vapor is a major component of the combustion products.
Is this worth the risk and cost? You’re saving maybe 1,000 btu/h on an auxiliary heat source? So about $.01-.02 per hour?
Risk and Cost of what?
Do you mean the fireplace as a whole? Yes. It's worth it to us. Some people (like my wife) will sit all day during the winter in front of an electric heater. Right next to it. She'll crank a heating blanket to the max and spend hours under it. Our radiant heat is never hot enough for her. The whole house will be 70 degrees or more and she'll still feel cold. For her, she likes being hot - the ideal temp is 85º+. She is a literal lizard - she creates no body heat unless she's actively exercising - so the world is a cold place to her (unless she's in the desert).
We wanted a fireplace mantle. I didn't mention it - but we actually had a gas fireplace in another location in the room and it was terrible. Took up far too much room, looked out of place, barely worked, and being a Type-B vent it had its own risks. The old fireplace didn't have a proper mantle and it bugged us.
We decided we wanted to move the fireplace to a better location, give it a proper mantle fit for the holidays, and make it more of a centerpiece of the room instead of an afterthought.
The question was what we wanted under the mantel - nothing - a electric fireplace - gas - wood? We went with gas.
As far as the venting of the chase? That's what I'm trying to determine.
The risk is you cool the flue to below the condensation point. If the flue wasn't designed to operate that way it could quickly corrode through and start leaking exhaust gases into your house. If the condensate isn't captured and routed away there is also a risk that it drips into your framing causing rot and mold.
Those are the risks.
If you want more heat, get a bigger burner.
It's interesting that building performance people spend a lot of time and effort aiming to get temperatures uniform throughout a space through excellent insulation and good heat distribution, and worry about having enough heat sources to ensure that no rooms are colder than others. But in fact, I've often observed and heard that people with wood stoves really enjoy having one room much warmer than the rest, and having spots you can sit in near the wood stove that are super cozy and warm.
I think it would be good if heating systems were more often designed to work well for people like your wife.
This is a really good point.
From time to time someone will tell you something that changes the whole way you think about a subject. I wish I could remember who, but someone once said to me, "the whole point of HVAC is comfort." In that if your house is warmer than about 40F and cooler than about 110F you're not going to die, you'll just be uncomfortable. How uncomfortable or comfortable you are is your choice.
No, I don’t mean the fireplace. I mean the scavenging. Like even if you can do it without condensing, why?
If the fireplace vent pipe is insulated, B-vent, triple-wall, or one of the various other options, then scavenging that heat won't cause condensation. If it is single wall pipe, condensation is a possibility, though not guaranteed. After all, single-wall metal pipe has been used for years in appliance connections to the chimney. OTOH, I have seen serious corrosion issues in these single-wall vent connectors, as well as single-wall pipes installed inside chimneys for sizing.
If you are talking about passively venting the chimney chase space within a single room or floor, I'd say that your risk is low. The IRC does not allow this chase to have openings on different floors because of the risk of fire, with the chase suddenly becoming an unintentional chimney. I wouldn't even try to use a fan to boost the heat production because the electricity used by the fan will probably exceed the value of the heat recovered.