Using a fireplace heat exchanger as a part of the central heating system.
Jim left the following question in a comment on another article (formatted by Paulie):
“We have a wood-burning fireplace in our home. We also have an oil-fired hot water heating system. What is the possibility that we could put into our fireplace a heat exchanger like the one in our furnace, and connect it to the hot water heat system? The room where the fireplace is located has hot water heating pipes going through it in a loop around the room, past the fireplace, and back to the furnace. This is the room we use most, and it is also the hardest to heat because of the high vaulted ceiling. Could we put a heat exchanger into the fireplace and attach it to the heat pipes in the room? Our thought is that this heat exchanger would effectively keep the furnace from running to heat that room, and of course, the heat from that room could also move into adjoining rooms. What do you think of this idea?”
Thanks for the brilliant question, Jim! In fact, you got me so excited it’s prompted me to write an entire post in a whole new “Q&A” category I’ve just created, rather than merely respond briefly in the comments section.
It’s an interesting idea and it’s something I’ve looked into for my own use a while back, albeit briefly. The first floor of my house has a hot water heating system in some rooms and the water just never seems to get hot enough to get radiators to yield enough heat, especially in the kitchen area which has a triple-height ceiling. It had occurred to me that using a fireplace heat exchanger to supplement the furnace could help with that issue. Unfortunately, the radiator lines are too far from the fireplace to make that practical so I never got very far with that plan, but thankfully that’s not a problem in your case.
The first thing you need to know is that heating systems live and die by their efficiency, which in layman’s terms boils down to how much of the thermal energy makes it from the source (the fire in the fireplace or oil furnace, in your case) to the space you’re attempting to heat. Each step of heat exchange can reduce efficiency considerably, with the efficiency of the heat exchanger itself and the properties of the heat retention medium (water in your case) being the most important factors. Beyond that, anything you connect to a closed loop heating system – which your hot water oil-fueled heating system currently is – can suck a substantial amount of heat out of that system in certain circumstances, thereby greatly reducing its efficiency.
How does all that apply to your situation, you might ask? Consider how you might connect a fireplace heat exchanger to your hot-water radiator line. The most straightforward way to do that would be to use one of the common types of fireplace heat exchangers, which heat air, and augmenting it with an air-to-water heat exchanger that would then be spliced into your hot-water line. While this is straightforward and could be accomplished with products and parts which are easily available, the efficiency of the setup wouldn’t be too good because of the extra heat exchange step.
Your idea, from what I understand, is installing a heat exchanger in the fireplace which would heat water directly and connect to your hot water pipe. While this would certainly get around the problem of reduced efficiency due to an extra heat exchange step, it creates another: if your fireplace were not to be lit, the heat exchanger would be cold and would surely drain heat from the hot water system. Such heat exchangers are designed to be used in conjunction with always-on heat sources such as your oil furnace, where this wouldn’t be an issue at all. It is very uncommon for residential heating systems to have multiple heat sources, especially when one of them could be off while the other one is on, so there isn’t a wealth of solutions available to solve an issue like this. Is this the end of it, then? Not necessarily. Find a connector pipe and valve which do not conduct heat (metal is out, heat-resistant plastics are in) and install it between the fireplace heat exchanger and the hot water line. You could then close the valve when your fireplace isn’t lit, thereby breaking the connection between the main closed loop and the cold fireplace heat exchanger – you would still be losing heat radiated into the air, but that also happens to be your heating system’s method of action, so you’re not losing anything. Problem (at least this problem) solved!
The trouble is that the vast majority of fireplace heat exchangers are meant to heat air rather than water and a model which can do that is going to be a hard to find specialty item, if you can find one at all. Since fireplace heat exchangers are typically designed to fit fireplaces with specific measurements for safety and efficiency reasons, even the models you do find might not fit your fireplace. Using a general purpose or furnace heat exchanger in a fireplace might be unsafe and ineffective (especially if they don’t fit well) and is very likely to be illegal in some way – against building codes, fire safety regulations and/or in violation of the terms of your homeowner’s or renter’s insurance. Your best bet is trying to find a wood-burning fireplace heat exchanger insert that does what you need and fits into your fireplace. A wood stove-type fireplace insert might also be an option. I’ll do some digging through my notes and update this post with whatever I can dig up. Aside from all this, sticking something like that into your beautiful fireplace isn’t likely to lead to the most aesthetically pleasing results, which may or may not be something you care about.
The last remaining part of the question is whether hooking up this extra heat source would somehow lighten the load on your oil furnace. This depends entirely on how the operation of your oil furnace is regulated. If your oil furnace is switches off based on thermostat temperature readings, it may very well stay off for a bit longer depending on how much help the fireplace heat exchanger is. My guess is that it wouldn’t be significant enough for you to notice much difference in your oil bill. If, on the other hand, the oil furnace varied the amount of heat it was putting out by, say, altering the intake of fuel based on water temperature, things might look a bit better. If your primary concern is saving oil, this might not be the silver bullet you’re looking for. However, if you care more about heating the space the fireplace is in better, it might well go a long way toward achieving that.
While this is a very interesting home improvement engineering problem to ponder and despite me being a big fan of brilliantly inventive solutions to quirky problems, as I’ve been writing this post, the feeling that we’re making things more complicated than they need to be has grown stronger and stronger. If I’m understanding your situation correctly, what we have is the following:
- A hot water heating system powered by an oil furnace.
- This heating system has trouble heating a room with high ceilings.
- A wood-burning fireplace is located in that same room.
- We want to heat this room better. (Objective #1)
- If possible, we’d like to heat other, adjacent spaces better. (Objective #2)
- If we could save some oil by saving the oil furnace some work, it would be a cherry on top. (Objective #3)
What you have here is a textbook example of what a zone heating system is meant to help with. As it happens, a fireplace heat exchanger is great at being a part of a zone heating system! If you were to install a run of the mill, typical fireplace heat exchanger which puts out enough heat (look for BTU on the specs) for the space the fireplace is in, you would achieve your first objective. Assuming you can extract enough heat from your fireplace and ensure somewhat decent air circulation between the room with the fireplace and adjacent rooms, you would also go a long way towards achieving objective #2. At the heart of the zone heating system concept is the fact that the room you are in most of the time, e.g. a living room with a fireplace, needs to be heated to a temperature which would make lounging and generally hanging out comfortable, but the rest of the house does not. As you move around the house, even a lazy walking pace causes your body to produce enough heat to make a non-negligibly lower ambient air temperature sufficiently comfortable. This means that you can use your fireplace in conjunction with a regular fireplace heat exchanger to heat your living room as much as the fireplace output will allow and turn down the thermostat considerably (which would now only affect the rest of the house), thereby allowing the central heating system to work less intensively and slashing your fuel costs.
I hope I’ve answered your question – if you’ve got any more, please send them in! This goes for the rest of you readers out there as well; this site is about helping you research your options and there’s no better way to do that than getting a well-researched, exhaustive answer to your question from Paulie.