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I have a fireplace insert at home, whose top is behind a very thick slab of stone, so it is not possible to open a proper vent for the hot air to reach the room.

As a result, the room is only heated by the front of the fireplace and is always cold.

Fortunately, there are two pipes that penetrate that thick slab of stone and enter behind the fireplace and I would like to attach extractor fans to them in order to blow that hot air in the room.

The picture looks something like this:

The picture looks something like this

What I was thinking of doing is to create a "Y"-shaped pipe and attach a blower motor to it in order to suck the hot air from behind the fireplace and blow it into the room through the two pipes.

NOTE: The fireplace insert is a solid metal body with a sealed pipe that goes directly to the chimney on the roof, so no carbon monooxide gases are released around it.

What I was imagining is something like this:

Something like this

I have a few questions about this project:

  1. Does that sound like a good idea and does anyone see any pitfalls or dangers in it?
  2. For the purposes of this project I will have to buy an axial ventilator, but I was wondering whether there are some specific parameters for the ventilator that I have to look into? Specifically power, temperature resistance etc?
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    Hot air rises, so if those pipes are useful, you should feel hot/warm air coming from them without a fan. If not feeling warm air from them, then much more work needed than just adding a fan.
    – crip659
    Commented Mar 3, 2022 at 12:47
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    It might be easier to put a fan at the outlet points instead of behind the fireplace. If it's behind the fireplace, A) access is difficult, and, more importantly, B) all electrical work is exposed to much higher temps than at the pipe exits. Also, be aware that you may end up pulling carbon monoxide from the burning wood into your house instead of letting it go up the chimney. That's been shown to be less than ideal for the health of humans.
    – FreeMan
    Commented Mar 3, 2022 at 13:16
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    This is just a comment, since I'm no expert on this, but in two houses I lived in had fans, one was a fireplace insert, the other was a "heat form" (formally called a Heatalator) which is basically a metal fire box built into a brick fireplace. Neither would provide much heat unless the fan(s) were running. So I have to politely disagree with @crip659. While product recommendations are OT here, I will say Fantech would be a good place to start your research.
    – George Anderson
    Commented Mar 3, 2022 at 15:52
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    If you blow air out through both pipes, where will the cool air come in to that space to get heated? Maybe you should consider using one pipe as the inlet and the other as the outlet ... ?
    – brhans
    Commented Mar 3, 2022 at 16:31
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    If you already have an air pathway that enters under the sealed fire chamber and exits through these pipes, you might want to consider buying a "fire place blower kit" rather than cobbling together something yourself. The kits contain everything you need and are cheap.
    – jay613
    Commented Mar 3, 2022 at 20:40

1 Answer 1

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For the purposes of this project I will have to buy an axial ventilator

There's no reason to default to that, and good ones not to.

You'll have relatively high friction/flow-resistance/back pressure with 3" tubes - as you already know because convection isn't doing much for you.

Axial fans are in general terrible at that sort of job. Centrifugal fans are in general really good at that job. There are some weird hybrids made for duct boosting that try to get more of what a centrifugal offers packed into an in-line package, but there's no obvious reason to limit yourself to an axial arrangement when a centrifugal fan will work much better for your load.

Under the "and how do others do this?" sanity check, virtually every fan-assisted "heatilator" fireplace or stove I've ever met uses a "squirrel cage" blower - which is a type of centrifugal fan.

Squirrel cage blower image from (ew) walmart.ca

There's even a common variant with two blowers on either side of a central motor, which would be perfect (probably costs a bit more) for your two tubes.

Temperature in operation should be very close to room temperature, as you'll be pulling room air into the fan intakes. Non-operating temperature you'd have to measure as it is now set up, but probably suggests "metal good, plastic bad" for blower construction.

Speaking of squirrels and their relatives, you might want to start by applying a shop vacuum to the 3" tubes to remove any accumulated debris (or nests) blocking airflow - before blowing that stuff all over the room.

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  • Thanks a lot for the exhaustive response! Just a couple of questions: 1. The centrifugal fans that I am seeing have a large square output shape. I will have to make something that "morphs" and splits that into two circular ones that are much smaller. Do you see any issue with that impacting the effort on the motor unduly? 2. Is there some math I can use to calculate what kind of throughput/power of a fan I need to use so that the air has enough time to warm-up before it is being extracted, etc? I believe the space behind the fireplace is about 10 cubic feet in volume.
    – kaloianm
    Commented Mar 5, 2022 at 9:37
  • Fun counterintuitive fact about centrifugal fans - "wide open unrestricted" is working the motor hardest (because it moves more air mass, so it's doing more work) so restricting the output actually reduces the load on the motor. However, there are certainly small centrifugal fans that are roughly 3" outputs as built, so you might be shopping a bit on the over-sized-size if the ones you are finding are much bigger. While there might be math, it would depend on things we don't know like the flow resistance of the tubes.
    – Ecnerwal
    Commented Mar 5, 2022 at 13:12
  • I think I actually found one. Now I only need to find a Y-shaped pipe and my project can start :) Marking the question as answered. Thanks a lot for the help!
    – kaloianm
    Commented Mar 5, 2022 at 13:51

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