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 sheep 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.

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.

Huw

Plastic ducting visible at back of after removal of gas fire

If it was mine I would foam fill it from outside trying to fill the end of the duct through the cavity and a bit under the floor. Then fill the air brick, it may get foamed up so cut flush, finish same as wall with sand and cement. If you turn it into an open fire or other combination appliance it may need an air vent and I doubt that the existing one would comply.

Thank you both. Since post I had a look with a flexible borescope. 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 borescope 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. 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 tried to block it when fire was actually installed.

At this point would there be any benefit in tipping in more insulation 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?

On the subject of airtightness I am concious that whilst it is a relatively leaky late 90's house with timber sash windows with trickle vents, 3 bathroom extract vents (on-suite running on continuous background with boost), as well as a toilet extract, a kitchen cooker hood and utility room with alternate flow heat recovery we run humidity at around 60% but retrofiting MHVR over 4 floors would only be feasible as part of a very large scale project.