What Is A Fireplace Insert

A fireplace insert is a device that you can, unsurprisingly, insert into your fireplace; it will usually incorporate a heat exchanger, fans and/or blowers as well as additional exhaust mechanisms if required. If you have an existing masonry, metal or similar fireplace, a fireplace insert is an ideal way to give it a new lease on life, improve its efficiency, make it practical as well as beautiful to look at and make it a conversation-starting centerpiece of whatever space it is in.


Types Of Fireplace Inserts

While an electric fireplace insert is often the best and easiest choice, which makes it very common, gas fireplace inserts and wood-burning fireplace inserts are quite popular as well. Other types of fuel include propane, pellets and coal in decreasing order of popularity in the US market. See our articles below for more information on each type, from introductory material to buying guides and reviews:

Why would you need a wood fireplace insert for your wood-burning fireplace?

Each fuel type has an optimal combustion temperature at which it is burned almost completely. If it is burned at a temperature lower than the optimum, not only are you wasting heat, fuel and efficiency, you’re also emitting possibly combustible exhaust gases. In the case of wood, the optimal combustion temperature is quite high and is rarely achieved in an open fireplace. Creosote (also known as ‘wood gas’) is emitted in the exhaust and is highly combustible. In fact, during World War 2, the Germans modified a lot of their cars to run on wood gas due to the severe shortages of oil they were experiencing and the consequent unavailability of petroleum gasoline for civilian use. This is both a major safety issue and an efficiency issue – if wood were burned completely, the energy in creosote would have been yielded for your use. A wood fireplace insert, which must be certified by the EPA to burn wood cleanly and thoroughly, will extract all the thermal energy it can from your fuel and make your fireplace more environmentally friendly in the process. It does this by creating a closed combustion system and achieving a much higher burn temperature than an open fireplace would, thereby getting closer to optimal burn.

Open fireplaces suck in air from the space they are in to fuel combustion. You may very well be warm near the fire, but the rest of the house is going to get colder as outside air is sucked in to replace the air used in your fireplace. A wood fireplace insert sidesteps this issue by having doors and drawing the oxygen it needs for combustion from elsewhere.


Fireplace Heat Exchanger Or Chimney Heat Exchanger?

I have received a couple questions about chimney heat exchangers recently, so I’ve decided to interrupt the fireplace insert post series to write this quick post.

What is a chimney heat exchanger (or flue heat exchanger)?

In common usage, a chimney heat exchanger refers to a device installed in the flue pipe of a wood, coal or pellet burning stove. The idea is simple – a lot of heat is lost up the flue, so if you could recover at least some of it, you’d be sitting pretty. To that end, a section of flue pipe is replaced with a flue heat exchanger which usually contains a number of tubes for heat retention and a fan or a blower for air circulation. Successful installations with a quality heat exchanger model can routinely reclaim up to 30% of heat and put out anywhere between 5,000 and 30,000 BTU, depending on the quality of the heat exchanger, quality of the install, insulation of the flue and most importantly the actual heat output of the stove.

Safety considerations when using a chimney heat exchanger.

There are a number of safety considerations with chimney heat exchangers which everyone contemplating using one should not only be aware of but thoroughly educated about. I’ll be writing a separate post about creosote buildup and its safety implications, because it’s a topic deserving of a thorough examination, but I’ll provide a brief summary here. When fuel is not burned thoroughly in a fireplace or a stove – and this is almost always the case since wood, for example, has a fairly high optimal combustion temperature – combustible gases will be present in the exhaust and need to be vented quickly and efficiently to prevent buildup. Sucking too much heat out of the flue can impair the strength of the egress draft and improper installation of (or use of inappropriate materials in) devices in the flue can create conditions where the device causes creosote condensation. All of this can add up to highly flammable gases (or in the case of condensation, liquids) building up in your flue, which will inevitably lead to a very nasty chimney fire sooner or later. Although I haven’t witnessed one in person (thankfully), I’ve spoken to people who have and they assure me it’s not something to be trifled with. I am inclined to agree. Consider this: wood gas (basically creosote) was used as fuel for cars in World War 2 Germany because of severe oil shortages – if it’s combustible enough to fuel a BMW, I’d say it’s combustible enough to burn down your house.

The same type of issues – improper installation, installation of inappropriate devices, removing too much heat from the flue thereby impairing the draft – can also lead to other, non-flammable but highly toxic exhaust gases not being vented properly. Can you say monoxide poisoning?

Last, but definitely not least, is making sure your flue is protected from the elements and wildlife. It is imperative that you install a flue cap to prevent rain and birds from coming down the flue and into your heat exchanger. They don’t cost much and a good quality cap will last for a very long time, so investing in one is a no-brainer.

An actual chimney heat exchanger.

As you may have noticed, the device we’ve been talking about so far is actually a flue heat exchanger. Although people commonly call it a chimney heat exchanger, it isn’t and is meant for stoves. What if you don’t have a stove? Can something be done about heat being lost up the chimney of a fireplace?

If you can get to the flue of your fireplace, you could install a flue heat exchanger and attach a duct to it which would vent into a room of your choice. While this is theoretically doable, I haven’t heard of it being done and haven’t seen a commercial product that offers this. My guess is that requiring you to make holes in your walls and the highly individual circumstances of each build makes for a small market that’s not worth pursuing commercially for large production run manufacturers.

An actual chimney heat exchanger almost by definition has to involve a heat exchanger attached to the top of the chimney. There’s a patent here for a device that does that and there are a number of more modern referring patents listed in the references section. While I’ve come across products which reclaim heat in this manner, they are all meant for large, industrial chimney use. I’d like to be proven wrong on this, so feel free to leave a comment with a link and I’ll amend this post.

The verdict: fireplace heat exchanger or chimney heat exchanger?

If you have a fireplace, rather than a stove and want something you can buy and install (or have installed), a chimney heat exchanger is just not an option and you’re stuck with a fireplace heat exchanger. They’re quite good though, so it’s not that bad being ‘stuck’ with them.

Gas Fireplace Heat Exchanger Insert

Gas fireplace inserts are the most popular choice for those who are willing to forgo the niceties of a traditional wood fire in exchange for a drastically increased ease of use. I call this group “the silent majority of fireplace owners.” A gas fireplace insert is, as are fireplace inserts in general, a self-contained combustion system. It is fueled by natural gas, which is fed in through a gas line; needless to say, this gas line needs to be installed professionally and this can be both time consuming and somewhat expensive depending on how far your fireplace is from the splice point with the main gas line. Another line supplies air to the gas fireplace insert (you need oxygen for combustion unless the fuel provides its own) and is typically connected to the top of the unit, stretching to it down the chimney. The last line stretches up the chimney from the insert and serves as a means to vent the exhaust gases. While the fire in a gas fireplace insert doesn’t look or behave like a real wood fire, some models get the look close enough for willing suspension of disbelief to take you the rest of the way; a set of ceramic logs is usually supplied with the unit to be placed over the burner to further the illusion. Other models don’t even try to approximate the wood-burning fireplace look, concentrate instead on being beautiful in their own right and quite often succeed – there are even those who prefer the cool crispness of a gas flame. Most models use a blower 0r a fan to circulate the air that’s been heated and as such require an electrical outlet.

Gas fireplace inserts have a number of advantages:

  • You don’t have to refill them with fuel, unlike with a propane fireplace insert or a wood-burning fireplace insert.
  • Natural gas burns very cleanly, so you don’t have to keep cleaning the chimney or inside the fireplace insert itself.
  • The only maintenance you would likely have to perform is wiping the fireplace insert doors.
  • A gas fireplace heat exchanger insert is an excellent choice for zone heating.
  • Gas fireplace inserts can produce up to 40,000 BTU (and sometimes even more), which is enough to heat a moderately large space well.

The disadvantages can broadly be said to be minor:

  • You need a carbon monoxide detector to monitor potential leaks – this is minor in the sense that it’s easy to get one and not in the sense that you don’t need it, because you do (if you’re fond of being alive, that is).
  • Running a natural gas line to your fireplace can be both time-consuming and somewhat expensive, depending on how far away your main gas line is from your fireplace.

Don’t want a gas fireplace insert because you insist on a real wood flame? Stay tuned for the next post in the series…

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.

Electric Fireplace Heat Exchanger Insert

Electric fireplace inserts are by far the most simple of all the insert types available. There is no fire involved per se – flames are emulated by a number of reflective lights with the size and behavior of the “flames” often capable of being controlled by the user through a dial or even by remote control. While people generally purchase them for aesthetic pleasure, most electric fireplace inserts include a heating element which can produce a small to medium amount of heat (1000 to 10000 BTU). The absence of actual combustion gives electric fireplace inserts a number of advantages:

  • You don’t need a permit to install an electric fireplace insert.
  • You don’t have to be a licensed technician to install an electric fireplace insert regardless of what state or jurisdiction you are in.
  • While the electric fireplace insert obviously still needs to fit into the fireplace opening, you don’t have to worry about the clearances to the fireplace walls or the mantel.
  • An electric fireplace insert never requires you to re-line your fireplace’s firebox.
  • You only need access to one electrical outlet to power the heating element (if present) and the flame emulator – this is substantially easier to come by than a gas line, safer than a propane bottle and beats having to gather firewood by a mile.

An electric fireplace insert is a great choice in many situations when using other types of fireplace heat exchanger inserts is impractical or even impossible:

  • In non-functioning fireplaces in city apartments.
  • Where natural gas is unavailable.
  • Where propane bottles or tanks are either undesirable or prohibited.
  • In fireplaces the chimneys and/or flues of which have not been cleaned or properly maintained and which are consequently inoperable or incapable of being operated safely.

There are, of course, a few disadvantages as well and they are very significant – in fact, significant enough that most people not constrained by the situations listed above decide against an electric insert:

  • Even in the most expensive models, the flame emulator is only good enough to fool you from a distance and even then only if you don’t already know the flame to be illusory. Needless to say, the owners who install the electric insert are unlikely to forget they’ve put one in in the absence of severe neural sclerosis.
  • While most electric inserts do come with a heater, the heater is never massive – if the space the fireplace in is large, you shouldn’t expect to be able to heat it solely with the heating element in the electric fireplace insert.

Don’t have a fireplace? Have no fear! There is a whole class of electric inserts available which are meant to be permanently installed in a wall – just stick a mantel above it et voila – you’ve got a fireplace where there was none.

Fireplace Heat Exchanger Insert: What Is It?

Fireplace heat exchanger inserts are surely the most popular option when it comes to fireplace heat exchangers. However, although mere popularity is often taken to be an indisputable indicator of worth, one must never forget the popularity Vanilla Ice once enjoyed. In this article, I’ll examine what a fireplace insert is and what kinds are available. In future articles, I’ll describe each type in detail.

Fireplace Heat Exchanger InsertQu’est-ce que c’est?

Perhaps unsurprisingly, it’s something inserted into a fireplace. More precisely, it’s a firebox made of metal (steel or cast iron) with a fireproof glass at the front. This design creates a closed combustion system. Most inserts, especially those created for efficiency, also have fans or blowers. The metal retains the heat and warms up air, which is subsequently circulated out by the fans or blowers. There are some inserts which achieve air circulation through differences in air pressure, but those are rare because it’s exceedingly difficult to design them so they would work under a large range of conditions.

Essentially, an insert is a self-contained fireplace in itself and some even have nothing to do with fire! As such, it’s perfectly possible to have a gas fireplace heat exchanger insert in a wood-burning fireplace.

Types of fireplace heat exchanger inserts.

Fireplace inserts are typically classified according to the type of fuel they burn:

  • Wood fireplace heat exchanger inserts
  • Gas fireplace heat exchanger inserts
  • Pellet fireplace heat exchanger inserts
  • Coal fireplace heat exchanger inserts

Additionally, electric fireplace inserts are quite popular as well, although strictly speaking they don’t “burn” anything and don’t involve fire in any way, shape or form. The latter two aren’t all that common, so we aren’t currently planning to write about them further unless our readers chime in and request that we do; as for the rest, stay tuned for an article describing the advantages and disadvantages of each over the next week!

Why Is a Fireplace Heat Exchanger Necessary?

It is reasonable to assume that people reading a website about fireplace heat exchangers already know that it’s a good idea to have one to get more out of their fireplaces. However, it has just occurred to me that not everyone might know exactly, in detail, why a fireplace is so bad at heating the room it’s in.

Why do fireplaces suck?

Fireplaces are not particularly good at being heating systems. How can this be, you ask, when a fireplace is just the modern descendant of the camp fires our ancestors enjoyed on the steppes or in the woods of the Old Country? Whether your fireplace is masonry or factory-built, it’s got a major problem – heat loss. In fact, traditional fireplaces are usually only 5 % to 15% efficient. As it operates, a fireplace actually pulls warm air out of the room it’s in into the fire and that thermal energy is then either vented out of the chimney or dissipates into the building material around it. When the containing room’s temperature drops below zero degrees Celsius (ie below freezing), most fireplaces actually start losing more thermal energy than they deliver. So you see – fireplaces literally suck warmth out of the room.

Enter the fireplace heat exchanger!

A fireplace heat exchanger can help solve this problem, but different types do it in different ways. A fireplace grate heat exchanger, for example, simply tries to retain some of the heat produced by the fire, warm up some air and spit it out into the adjoining space. A fireplace heat exchanger insert, on the other hand, creates a closed combustion system, typically taking air from elsewhere, heating it up and venting it into the room, while venting exhaust up the chimney.

This should do for now, stay tuned for articles which explain the characteristics, pros and cons of each type of fireplace heat exchangers in more detail.

We’re starting a somewhat regular (depends on you guys sending stuff in) column here at FireplaceHeatExchanger.net based on stories you, our readers, share with us. Kicking things off is Kathy K. from Philly:


How a Fireplace Heat Exchanger Saved My Dream

When I was a little girl, my mother took me to see some family friends in Maine. I distinctly recall it being absolutely freezing and the whole world seeming an utterly inhospitable place as we drove towards their house. When we finally arrived, the greetings were, it seemed to me, torturously drawn out and it felt like ages before we finally went inside. Hurrah! There was a giant fireplace with a roaring fire right in the living room – salvation at last. I don’t think I left the small semi-circle around the fireplace for more than a few minutes at a time for the entire stay – I even slept there most nights. There’s nothing more hypnotic than watching a wood fire – you can stare into it for hours, imagining far-off, hot places you might never see – the Sahara with its barren landscape, the steppes of Kenya with its herds of gazelle or the banks of the Nile which gave rise to a whole civilization.  This was the best time of my young life and ever since then, I’ve dreamed of one day having a fireplace of my own.

Last year, it looked like my dream was within reach. My husband took a job in Philadelphia and we put an offer on a beautiful Great Depression era house on the Main Line. The house was all stone and had all the elegant yet quaintly rustic niceties characteristic of that period’s architecture, but most importantly it had a wood fireplace! I think I may have freaked out my husband a little bit with all the attention I lavished on the fireplace during the house showing – was there a sensual preoccupation I had that I’d neglected to share with him? buy the house and find out, I said – and the realtor assured us the fireplace was functional, but categorically refused to demonstrate it. I was so excited that we agreed to a higher price in exchange for a faster closing. I realize that this is all quite silly, but as far as irrational love of inanimate objects, this at least isn’t in Imelda Marcos’ class.

The first night after we moved in, even before the first of our things were unpacked, our little family – my husband and I, our daughter Kelly and our dog Dzerzhinsky (don’t ask) gathered in the living room for the christening of the fireplace. I was almost shivering with anticipation as my husband loaded the wood inside.. when Kelly started sobbing uncontrollably only to break out into full scale weeping. I had made sure to instill my love of nature and our planet in my daughter, always reminding her to take care of the environment and not using resources frivolously in the only way you can with small children – by anthropomorphizing. Giving trees names has come to bite me in the behind just in time to shatter my childhood dream. No amount of cajoling could save the situation – Kelly insisted that it wasn’t right to set parts of Mr. Tree on fire just to look at the pretty flames.

I gave up. As much as I wanted to rationalize around the problem, I had to acknowledge that Kelly was right and drop the fireplace idea like a hot potato (pun intended). For a few weeks, I wandered around the house as if in a haze – if you didn’t know what had actually happened, you could have thought that I’d lost a loved one, and frankly you wouldn’t have been too far off.

A month after our move-in, we had some neighbors over for a housewarming party. At the end of the night, after my husband had had a few, he confided in our neighbor to the right about our predicament. Lo and behold, it turns out he had a solution! They had installed a fireplace heat exchanger the previous winter to get the first floor of their house to warm up quicker and he thought such a practical use might soften my inconveniently conscientious child’s opposition to the fireplace.

After much thoughtful consideration – 10 seconds is about what it takes for that with a 9 year old – the fireplace heat exchanger was given the Kelly Seal of Approval. My husband took on the responsibility of doing research on the internet – how do you know if you need a fireplace grate heater or a fireplace heat exchanger insert, for example? – and after reading everything on your site (and some others) we settled on a grate heater with a blower and called in a guy to install one the very next day. That night, my dream which I had just recently thought destined to remain forever frustrated, came true and I was able to share it with my entire family, including Kelly.

(slightly edited By Paulie)


Thank you very much for your heart(h)-warming story, Kathy. We’re glad fireplaceheatexchanger.net could be of some small use.

A question came in from a reader:

Q: “I’ve got a gas furnace at my house and I’ve been told the heat exchanger needs to be replaced. The trouble is that my furnace is quite old, so it may not even be possible to find a replacement furnace heat exchanger for it. In any case, the price I was quoted is beyond my means at the moment (both for the heat exchanger and for a whole new furnace). I started doing research on the ‘net and happened across your site. My question now is can I use a fireplace heat exchanger as a cheaper way out?” – Bob from St. Louis

A : Bob, my condolences on the premature passing of your heat exchanger. To be honest, I’m not absolutely sure what you’re asking here. The two possible interpretations are:

  1. “Can I use a fireplace heat exchanger with my gas furnace instead of a heat exchanger made specifically for furnaces?”
  2. “Can I use my fireplace with a fireplace heat exchanger instead of a gas furnace?”

I’ll address both questions below – this should satisfy you whatever you may have meant.

Using a fireplace heat exchanger in a gas furnace.

Regrettably, this is not an option at all.  The way a gas furnace works is, in brief:

  1. The flame comes from the burners.
  2. The hot gases head through the heat exchanger (usually due to negative pressure).
  3. The heat exchanger predictably heats up as hot gases flow through it.
  4. The gases exit the heat exchanger and are vented.

The requirement that negative pressure be maintained to ensure the correct flow of combustion byproducts means that not only can’t you use a heat exchanger meant for a different purpose, you can’t even use a heat exchanger meant for a different furnace! Maintaining the correct pressure in the different parts of a machine requires that machine to be airtight, so each part has to be made specifically for such a machine (unless each machine model’s measurements are the same). By the same token, you wouldn’t have any luck with a heat exchanger for, say, a datacenter HVAC system.

Using a fireplace heat exchanger (in a fireplace) instead of a gas furnace.

While I can’t give a categorically negative answer to this one, I’d still have to say ‘probably not’. A gas furnace (with a working heat exchanger, that is) is a high volume, always-on whole house heating apparatus designed and manufactured with a view to being very efficient, doing a lot of work and staying on without pause, if necessary, for an extended period of time. A fireplace heat exchanger, on the other hand, is a neat way to get a bit more warmth from your fireplace. Sure, you can get a really efficient one and keep the fireplace going all the time, but it’s unlikely to provide enough heat and circulation for your entire house, unless it’s a log cabin. Trying to hunt an elephant with a BB gun is doomed to fail even if it’s the best, most expensive BB gun in the world.

While I am not a specialist, my layman’s advice to you is to bite the bullet and maintain your gas furnace in a working condition. There may be other alternatives, mind you, but heating your entire home with a fireplace isn’t a viable one, I’m afraid.

- Paul in Philly

How different are fireplace heat exchangers for fireplaces burning different kinds of fuel?

Quite a bit and not so much at the same time. I know that this isn’t the clear-cut, simple answer one always wishes to hear, but it’s accurate. The principles of heat exchange don’t change regardless of the source of the heat, so the designs are broadly similar and some can even be used interchangeably. However, materials can and do differ and how the heat exchanger fits into the fireplace is typically different as well.

Fireplace types by fuel.

While there are a number of fuel types which can be used in fireplaces, some have fallen into disuse and are of historic interest only. For example, coke (a fine particle type of coal) was used in poor Appalachian households together with some of the first fireplace heat exchanger inserts because the coke could usually simply be dug up in the back yard and the fireplace insert made the whole thing efficient enough to make using coke practical. The two most common types of fireplace by fuel today are:

  • Gas-burning fireplace
  • Wood-burning fireplace

Gas-burning fireplaces are more common in houses of modern construction (or at least those renovated in the last couple of decades) where efficiency, safety and ease of use both with respect to maintenance and cleanup are considered to be more important than historical correctness and purity of sensory experience. Additionally, where a chimney or other flue are simply not practical or indeed possible, a ventless gas burning fireplace may be the only option. While the former typically use the house gas supply, the latter type will normally rely on lp or propane gas tanks.

Wood-burning fireplaces are usually found in older homes and/or areas lacking a utility gas supply while having wood in abundance. Beyond that, many people who want the traditional fireplace experience complete with wood crackling and a fire which looks more ‘alive’, are willing to pay extra to have a wood-burning fireplace incorporated into their new home’s design.

Regardless of the fuel type, most choose to have a fireplace heat exchanger installed as part of the initial build or fitted to an existing fireplace.

Your options when choosing a wood-burning fireplace heat exchanger.

Very few preexisting wood-burning fireplaces have heat exchangers built into their design. The most common ways to incorporate one into your fireplace in this case are:

  • A fireplace heat exchanger insert
  • A fireplace grate heat exchanger
  • A chimney or flue heat exchanger

A fireplace heat exchanger insert is the most common, since – with appropriate forward planning and savvy selection – one can be installed without modifying the fireplace or altering its operation in a significant way and in some cases without the assistance of a professional. A grate heat exchanger is the second easiest option, especially if your original grate is removable and the size of your fireplace is not unusual. However, both of these are typically quite visible and obvious, which is often less than desirable if a wood-burning fireplace was first and foremost an aesthetic choice in your case. A fireplace chimney heat exchanger, on the other hand, is – when installed properly – entirely invisible, allowing you to preserve the historically correct look of your fireplace. It is also typically the most expensive and the most difficult to install among the three. We will be writing about these as well as the less common options in more detail in the near future, so stay tuned.


Gas-burning fireplace heat exchanger options.

Some modern gas-burning fireplaces already come with a heat exchanger built-in. Indeed, a ventless gas-burning fireplace is itself a heat exchanger almost by definition. Most, if not all, offer an option to install factory-fitted doors which incorporate an active fireplace heat exchanger. Inserts are also quite common and look far more appropriate in a gas fireplace than in a wood-burning one. The tubular fireplace heat exchanger design with blower(s) providing circulation is ubiquitous in this setting, sometimes to the exclusion of other, perhaps more interesting designs. This is testament to how well it works, rather than some lack of imagination or expertise on the part of the manufacturers. Look out for a series of articles examining these options in far more detail as well.


Is a homemade fireplace heat exchanger viable?

Indeed it is – not only have folks been making them successfully for the common varieties of fireplaces, but they’ve also made them for wood-burning stoves, barbeque pits and more! We’ll be running a series of articles pertaining to this topic, so if you’re the kind of guy or gal who likes to fabricate his/her own metal pieces so they look and work just right, check back soon and check back often.

Types of Fireplace Heat Exchangers

Fireplace heat exchanger with a passive unit

Beautiful fireplace with a passive fireplace heat exchanger

Fireplace heat exchangers can roughly be broken down into two types:

  1. Passive fireplace heat exchanger.

    In general, a device is considered passive if it has no moving parts powered by an external power source. For fireplace heat exchangers specifically, this essentially means that there is no electrically-powered air circulation device involved. As such, a passive fireplace heat exchanger will usually consist of a mass of metal, such as a grate, intended to absorb heat from the fire and radiate it out to the rest of the room. Unlike a space heater, it is not expected to retain the heat for an extended period of time, although some do – this depends most on the properties of the metal itself, the total mass and equally importantly, but perhaps less obviously, on the shape. Whether heat retention is a desirable characteristic is arguable and is likely to vary depending on what your use case is. The most common complaint in this regard comes from people who need to clean their fireplace after each use and prefer not to be forced to deal with an extremely hot, large and heavy metal piece sitting in their way.

  2. Active fireplace heat exchanger.

    An active device, conversely, will typically have a number of moving parts which are likely to be powered externally. In the case of a fireplace heat exchanger, this is usually an array of blowers or fans of one kind or another. Their purpose is typically threefold:

    Tube fireplace heat exchanger

    Tube-based fireplace heat exchanger (active, unit only)

  • Improve the efficiency of heat transfer from the heat retention component to the air around it.
  • Provide for an increase in warm air circulation.
  • Extend the reach of the “warm zone” further from the device itself.

How to choose between an active and passive fireplace heat exchanger?

A passive fireplace heat exchanger is, in essence, a simple and straightforward device – with no moving parts there’s really nothing in one that can break (shoddy workmanship, bad metal quality and metal fatigue notwithstanding) and with no power needs there are no requirements for a nearby electrical outlet, steady and reliable power supply or indeed any ugly wires potentially shattering the idyllic fantasy world of a log cabin just off the edge of the map. If you’re looking for an easy to install, set-and-forget type device that will do a passable job for a smaller space, a passive unit is a good choice.

An active unit, on the other hand, does require you to give thought to its installation and a moderate amount of attention to ensure that it continues to operate properly, effectively and safely. With that said, an active fireplace heat exchanger will reward you with substantially higher performance and consequently a greatly increased level of comfort. If you’re OK with the extra work and a look that’s not entirely appropriate for a historically correct Victorian townhouse, by all means do go active and enjoy the exquisitely warm and fuzzy feeling of having one purring away in your fireplace.

Fireplace Heat Exchanger: Introduction

What is a fireplace heat exchanger anyway?

Fireplace Heat Exchanger

A fireplace begging for a heat exchanger

“Fireplace heat exchanger” seems like a mouthful – and it is. With that said, the egghead-sounding term belies the simplicity of its function – letting your fireplace heat your entire space, rather than just your feet. It’s like calling a mitten an “upper extremity heat dissipation inhibitor” – whoever comes up with this stuff will be first up against the wall when my revolution comes.

Heat exchange explained.

Q: Isn’t heat exchanged with the room a fireplace is in without any fancy devices?
A: Well yes, but not all that efficiently.

Your fireplace is designed to ensure that CO2 and other gases released by burning your fuel get channeled up the chimney and that’s a good thing – you certainly wouldn’t want them filling up the room (most likely after a while you would no longer care, being deceased and everything). Unfortunately, most of the heat escapes the same way. The thermal energy (heat) from the exothermic reaction (fire) is transferred to the air immediately adjacent to it and as we all know, hot air rises – straight up the chimney. Since it does that at a rather brisk pace, it has precious little time to convey any of that sweet thermal energy to the air outside of the chimney catchment zone (directly underneath the chimney opening). The trick is in getting something to retain the heat and the air in your space warming up as it absorbs the heat from the retention vessel. Wikipedia has a good explanation of the physics involved if you’re curious.

So how does a heat exchanger work?

Heat exchange is simply a transfer of energy – thermal energy to be precise. Any heat exchanger will operate according to the same basic principles:

  1. A heating element, whether external (a fireplace) or internal (an electrical heating element in a space heater) supplies the heat.
  2. A retention element retains the heat (metal, oil etc).
  3. A transference element (e.g. metal casing) aids in transferring heat.

A wood-burning metal stove is a good example – the metal retains the heat and radiates it for a long time after the fire goes out. An oil-based space heater is another example – the oil is heated by electrical heating element, the metal casing is in turn heated by the oil just as the air adjacent to the metal absorbs the heat, thereby cooling down the casing. Incidentally, this is why radiators of all kinds have grooves and ridges – the larger the surface area, the faster the heat is transferred to the air around it. Heat exchangers can be used the other way around too – they are at the core of cooling systems used in datacenters housing servers such as the one our humble blog is hosted on, for example. This may seem counter-intuitive for someone looking to extend the effective cuddling range around a fireplace, but the first law of thermodynamics says that for something to get warmer, something has to get cooler too (more or less).

Fireplace heat exchangers throughout history.

Fireplace heat exchangers are not a modern invention per se. Historically, a metal or stone hearth has been the primary means of staying warm in a typical Eastern European peasant home for many centuries. The hearth would be lit to prepare dinner and put out just before the family went to bed (if the wood they had managed to gather would last that long). The bed would consist of quilts and pieces of cloth on top of the hearth (I’m still looking for a picture to illustrate the construction – for now, take my word for it) and the entire family would sleep up there snug and warm from the retained heat. If you’ve ever wondered why Albanian peasants are so gruff in their demeanor, rest assured it is because of the adverse effect such sleeping arrangements must surely have on their love life.