Airtightness with direct air stove

[LinearPancakes]

We're finishing the last details on our home, on which we've attempted to make as air-tight as possible. One of these details is the inset cassette, direct-air, wood burning stove.

 

Originally there was a fireplace. We've had brickwork done to accommodate a stove. We installed a 100mm diameter duct through a hole in the floor through to the vented space underneath. (Yes, we need to make a hole through the lintels 🙈)

 

The plan is to line the flu, and connect the stove to the flu liner and 100mm air inlet. Since stoves have airflow controls, I would imagine that this part of the system is reasonably closed air-wise.

 

The detail I'm stuck on is the gap between the flu liner and the chimney pots. This will open up into the area where the stove is installed. Even if this area itself is airtight, I imagine the stove will not seal tightly to the wall, and there are likely gaps in the stove frame.

 

Originally I was thinking to seal between the flu liner and pots, using a suitable high temperature sealant, near to the spigot on the stove. This way, continuing the airtightness layer around the flu.

 

However, our stove installer is recommending against this, due to risk of condensation build up in the chimney. I can only imagine the increased risk, compared to the chimney being blocked up without a stove at all, is because the flu liner is metal and will get colder causing condensation on that.

 

But.. to avoid the condensation, airflow would be required, which would by definition mean an air-tightness issue. And if the rest of the house is pretty tight, a. this seems like a big shame, but b. would there even be a challenge getting sufficient airflow there, as where will it draw air from? (It will probably find a way, by pulling air in from weak spots causing draughts)

 

I'm starting to think maybe this is the price you pay for having a fuel burning stove in an otherwise airtight home. It would be a shame to abandon it now and install an electric feature heater - it would be so nice, and be a nice backup for electricity given we're on a ASHP now.

 

Has anyone hit this before? Anything I'm getting wrong about this?

 

[LinearPancakes]

After a bit of research I realise we could install a register plate below the lintels to seal it. Then to address the moisture issues, fill around the flu with Leca. Seems like some kind of insulation around the flu is important independently to improve the liner performance and lifetime.

 

Will discuss with the installer soon. The installer wasn't keen on my first version of this solution, using vermiculite, as:

- It can turn into a soggy mess if there's ever a water leak

- Hard to get it to go all the way down reliably when the pots are about 8" and the liner 6"

 

Hoping the extra weight of Leca will help it go all the way down more easily, if using 4-10mm, and the better performance when wet will be suitable.

[LinearPancakes]

Latest update on this, in case it's helpful for anyone in the future.

 

The installer was confident that Leca wouldn't go down either and there wasn't enough space spare to insulate with Chimwrap or a mineral wool wrap either. So we're not going to have insulation there.

 

In terms of airtighting the outside of the flu liner, given space challenges around the stove, we're just going to have to bung up around the bottom of the liner with mineral wool. Not ideal, but that with the limited space between the liner and pots, the cowel on the top and the stove being in the recess, means there will at least be resistance for any air trying to escape up.

 

A separate issue we've encountered, which seems kind of obvious with hindsight, is that inset cassette style stoves are inherently bad for airtightness. That is, they're not a closed system:

- Direct air in, connects to the frame, not the cassette. And there's a big gap, so you're basically connecting it to your room.

  - We're solving that by adding a shutoff valve (Lindab DSU) on the air inlet, so we can close that when not in use

- The flu is directly connected to the cassette. However, the cassette takes air in from (for our unit) 3x places. Two can be closed with the controls on the stove. One remains open, which means again, connected to your room.

  - We're planning to solve that by sealing the responsible holes when the stove is not in use, in a way that's both obvious and easy.

 

If we'd known all this upfront, I think we'd have probably done more research into which stoves are suitable for use in homes attempting to be airtight. A freestanding stove within the fireplace would probably have been a better option.

 

But it's probably going to be good/easy enough in practice and it will look/feel great, and very nice to have a backup heat source when using an ASHP. Not quite installed yet, but should be soon. I'll send a finished picture. Picture of the blower test attached!

[Andehh]

We have similar, but only realised it part way through fitting. So we've got a property we worked hard on getting a reasonable air tightness....then stuck a great big Stovax dual sided stove in it with the air inlet kit creating a 4" direct feed sat underneath it. Live and learn

 

The stoves worth it though, even if we live with a knock to the air tightness, and despite wibbling about it for ages... I have NEVER noticed a draft around it, even on windy days.

 

And with our total energy bills averaging out at about £150/month for a very large and sprawling bungalow, with very heavy usage and a large background drain, (server, CCTV, full networking setup, dehumidify in garage etc) ... I'm delighted all round!

 

 

Edited 15 hours ago by Andehh