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.

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