Sealing up an external air duct to gas fire
Information, help, tips and advice on cavity walls, ceilings and lofts etc....

huwrugby
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Sealing up an external air duct to gas fire

by huwrugby » Thu Dec 14, 2023 10:31 am

I am relatively new to home improvements but having done the simple things to the loft etc of my 4 storey 1999 townhouse I wanted to improve the airtightness of the building. Ultimately, I would like to install an ASHP but currently the property is borderline to install one, not least as the houses all have integral garages. It has partial fill cavity walls, and I am in the process of removing a gas fire with a flue to the roof on a party wall, exiting via a chimney that had a rain excluding cap on already but appropriate for the fire to continue to be used. The gas fire is on the first floor, so I anticipated that was a sizable stack effect. It was disconnected from the mains supply, and I had hoped to at least block the flue with a balloon or chimney sweep and leave the disconnected fire as a feature for now. Unexpectedly, there is also a plastic ventilation duct behind the fire which is fed from an air brick type vent on the exterior. I hadn’t considered this possibility to explain the duct. I would like to seal the vent but am concerned about leaving it in situ. A thermal camera doesn’t suggest the duct is contributing a lot of heat loss in the assumed route. Vinyl flooring was installed over the chipboard deck in the room some years ago & there was nothing at the time suggesting a ventilation duct. If more images would help, I can upload them but essentially there is a knocked hole in the base of the fireplace where the ducting appears as shown and externally the airbrick/vent opening is illustrated. I assume this was excavated when the house was built & fire installed. The internal run is about 1.5m.

Could I simply fill the interior end of the duct with insulation or better, seal over with cement mix?

On the exterior I would be able to access the vent but only able to seal the external block & not the inner without major excavation within the room. Am I better leaving the duct & vent externally open or somehow insulating the ducting without ending up with the ducting being open to the cavity.

I hope this forum may be able to offer some advice & suggestions.
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stoneyboy
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Re: Sealing up an external air duct to gas fire

by stoneyboy » Fri Dec 15, 2023 6:58 pm

Hi huwrugby,
Ideally remove the plastic duct - if it's been installed sloping downwards from the outside it may be a source of water ingress.
If not possible, suggest you remove the outer grille, stuff insulation into the duct, plug the duct with a weak mortar mix for about 75mm, then render the outside to match the existing surface.
Regards, S

huwrugby
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Re: Sealing up an external air duct to gas fire

by huwrugby » Sat Dec 16, 2023 9:20 pm

Thank you. Since post I had a look with a flexible boroscope. Can't speak to the incline of the duct but looking from inside the duct appears to already be stuffed at the end with what looks like insulation board. A length estimate would suggest the duct bridges the cavity & that this is in line with interior surface of external block. Looking from the outside there was no obvious boroscope light visible suggesting it is already "blocked" to some extent. Externally, the scope would not go easily between the grilles except the lowest and it didn't seem to want to pass into what looked like a brick sized plastic end to the duct. This plastic appeared separate to the most exterior grille. It would appear that if I hacked out the exterior grille then I would have a depth of about 25 to 35mm to fill.

I've looked at the gas regulations & I think that if the fire generated 7kW then a duct was needed but the installed fire was 6.85kW: I wonder if they prepped it then made an effort to block it when fire was actually installed.

The ducting run is roughly adjacent to the skirting board & not really accesible without a lot of cosmetic damage & repair costs.
At this point would there be any benefit in tipping in more insullation such as polystyrene balls from above if it already has an effective wedge of insulation at the bottom. Presumably I could also cement over the internal gap in the corner of fireplace to optimise the airtightness?

stoneyboy
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Re: Sealing up an external air duct to gas fire

by stoneyboy » Sun Dec 17, 2023 10:55 pm

Hi hwrugby,
I still suggest you remove the outer grill and use some coarse sand plus cement to fill the hole, then make good with a render mix so you create a waterproof outer surface.
You could squirt expanding foam into the duct rather than poly beads, then concrete over the surface of the hearth to make good.
Regards S

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