Hot air ducting from a woodburner

mr brightside · Post by **mr brightside** » 07 Oct 2018, 17:47

Anyone ever done this before? It's a freestanding 4.5kW with nothing to facilitate any kind of hotbox arrangement, so i'm looking at simply moving warm air from one room to another over a distance of about 8m in a straight line. I need a duct fan and the ducting needs to be insulated as it'll be running in the loftspace of a bungalow.

Early research is suggesting warm air is problematic to move effeciently.

fuzzy · Post by **fuzzy** » 07 Oct 2018, 18:09

If you are going to blow cold air into an airtight burner and use the exhaust, can't you feed it underfloor, then out the house, so that the heat works best?

Post by **kenneal - lagger** » 07 Oct 2018, 19:00

Get some 100Ã˜ flexible ducting, a 100mm computer fan and a 12V battery with a small trickle charger. Lay the ducting under your loft insulation and Bob's yer uncle!! Works in our house. As long as there is a return path for the air it will work so you might also have to take 5 or 10mm off the bottom of the doors.

Post by **adam2** » 07 Oct 2018, 19:34

Yes, no need for any connection to the stove itself, simply extract hot air from ceiling height near the stove and blow this hot air into another room that would otherwise be cold.
Since heated air rises the outlet into the second room needs to be at floor level.
A refinement would be to control the fan with a thermostat in one room.

Remember that the duct and the fan will spread fire and smoke from one room to another, or into the loft space.
It would be prudent to fit an intumescent grille at each end.

Post by **kenneal - lagger** » 07 Oct 2018, 19:40

The hot air introduced at ground level would rise, Adam, so you would end up building the heat down from the ceiling anyway. Might as well just blow it in from there and let it build down from there. It's one of the problems of hot air heating.

mr brightside · Post by **mr brightside** » 11 Oct 2018, 20:31

kenneal - lagger wrote:Get some 100Ã˜ flexible ducting, a 100mm computer fan and a 12V battery with a small trickle charger. Lay the ducting under your loft insulation and Bob's yer uncle!! Works in our house. As long as there is a return path for the air it will work so you might also have to take 5 or 10mm off the bottom of the doors.

The internet says 150mm is what you need, and yes i'm just extracting from over the stove with a vent in the ceiling. Really i need insulated ducting, but where do you get it from? Even better i need a company who specialise in warm air heating systems, but who the hell bothers!

vtsnowedin · Post by **vtsnowedin** » 12 Oct 2018, 01:27

mr brightside wrote:
kenneal - lagger wrote:Get some 100Ã˜ flexible ducting, a 100mm computer fan and a 12V battery with a small trickle charger. Lay the ducting under your loft insulation and Bob's yer uncle!! Works in our house. As long as there is a return path for the air it will work so you might also have to take 5 or 10mm off the bottom of the doors.
The internet says 150mm is what you need, and yes i'm just extracting from over the stove with a vent in the ceiling. Really i need insulated ducting, but where do you get it from? Even better i need a company who specialise in warm air heating systems, but who the hell bothers!

People in the profession will of course have precise guidance they know you should follow. I the layman with a few decades of heating with wood look at it a bit differently. Any duct that moves hot air from above the wood stove (a space no human could or would want to occupy) and into a room or space where people want to spend time , working or otherwise, to blend with the air that would otherwise be there unheated is a good thing. Gathering it at the ceiling above the stove makes sense as that is where the hottest air is. Discharging it low in the desired room lets it rise through and mix with the air already in the room creating a circulation that blends the two together without creating uncomfortable drafts. Ceiling to ceiling ducts will work but slower and leaving bare feet and ankles on the floor uncomfortable for more time then you want to hear the nagging from the female creatures you are in charge of making comfortable.

You can ignore this advise if you wish but I for one like sleeping indoors.

Mr. Fox · Post by **Mr. Fox** » 12 Oct 2018, 11:18

We did this... I used one of these '12v marine bilge blowers', with one of these PWM speed controllers to set the speed (& to keep noise down) and a mechanical thermostat (in the room with the burner) to automate it.

As Ken suggests, just run the ducting under the loft insulation (or wrap it in any spare that you have). Maybe consider suspending the fan by a bungee to further minimise noise.

It worked OK, 100mm seemed to be enough, but I still had to move the sodding GCH thermostat out of the room with the woodburner in, though.

Mr. Fox · Post by **Mr. Fox** » 12 Oct 2018, 11:22

mr brightside wrote: Really i need insulated ducting, but where do you get it from?

Here - but be careful when you open the package... It leaps out like a jack-in-the-box and smacks you in the face!

Post by **kenneal - lagger** » 12 Oct 2018, 12:01

If you put the ducting under the insulation but ensure there's 100mm of Rockwool under it as well that will sound insulate as well as anything.

vtsnowedin · Post by **vtsnowedin** » 12 Oct 2018, 17:20

Mr. Fox wrote:
mr brightside wrote: Really i need insulated ducting, but where do you get it from?
Here - but be careful when you open the package... It leaps out like a jack-in-the-box and smacks you in the face!

Been there done that!!!

Inside a big walk in closet that suddenly wasn't big enough.

mr brightside · Post by **mr brightside** » 17 Oct 2018, 20:38

That's really great that, cheers. The ceiling vents should be the easy bits to get hold of.

mr brightside · Post by **mr brightside** » 04 Nov 2018, 14:27

Starting to accumulate some parts now. I've gone for this stabilised IP2X power supply unit with 40% more capacity than i need, and a 12v 270cfm bilge fan.

Does anyone know whether your ceiling vents can end up being a restriction point? It's hard to gauge by looking at them online. I'm looking at a standard 4" round one something like this.

Mr. Fox · Post by **Mr. Fox** » 04 Nov 2018, 15:13

Those type of vent are kinda designed to restrict... you control the airflow by screwing that centre disc in and out (useful if you have multiple vents and need to balance the airflow between them). That said, screwed all the way out, they don't restrict much.

I used them for the out-take vents on our PIV/heat exchanger setup (one in each bedroom), but used this type for the (sole) fresh air intake.

I guess it comes down to aesthetics and whatever looks right in you home.

Post by **adam2** » 04 Nov 2018, 15:15

A boat bilge fan will no doubt work to an extent and is well worth trying as you already have it.
Not in my view the best tool for the job.

Bilge fans are often fitted to motor boats and are intended to be run for some minutes BEFORE starting the engine(s) so as to remove any petrol vapour or LPG. Without such a fitment, boats have been known to blow up at the moment of starting when a spark from the starter ignites flammable gas or vapour.

Bilge fans are only intended for short term operation, they are not needed once the engine is running.
Possibly therefore not durable in long hour use.
Bilge fans are also designed to be as compact as possible and to move as much air as possible through relatively narrow ducting.
They can be rather noisy and use a lot of current. Who cares about noise on a boat when a noisy engine is about to run.
And who cares about use of 5 amps for 5 minutes from a large battery.

I would prefer larger ducting of about 150mm diameter, and an equipment cooling fan of a similar size. These are available in both 12 volts DC and in 230 volts AC.
They are intended for long hour use with minimal noise and energy use.