Sealing up an external air duct to gas fire

I am relatively new to home improvements but having done the simple things to insulate the loft, loft hatch etc of a 4 storey 1999 townhouse I wanted to improve the airtightness of the building. We have solar panels, Tesla battery & an electric car. Ultimately, I would like to install an ASHP but currently the property is borderline to install one, not least as the 5 houses all have integral garages. It has partial fill cavity walls but unfilled cavity party walls. 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 might be making the room colder. The fire 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 vent externally, mistaking it for an air brick. I would like to seal the newly discovered 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 across the floor. The internal run is about 1.5m. Vinyl flooring was installed over the chipboard deck in the room some years ago & there was nothing at the time suggesting a ventilation duct. I can upload images but hadn’t checked how to do that before composing this so will submit & try. Essentially there is a knocked hole in the base of the fireplace where the rectangular ducting appears and externally the airbrick/vent opening. 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.

If you can access the external airbrick try probing it with something fairly rigid, such as a straightened out wire coat hanger. You are trying to find out whether the cavity is lined. If it is lined there will be little air movement into the cavity from that area. You can’t assume that all is well elsewhere.

An alternative method of checking for a cavity lining is to use a borescope, if one will fit through the airbrick or it will reach from the other end.
Carbon Coop has one to loan out.

If the cavity is lined you could get away with sealing both ends of the duct. Sealing one end would still allow air movement. If there is no lining you need to seal inner and outer leaves and the other end. Don’t forget to insulate that area of the cavity.

In both scenarios note to remove the duct when working in that area.

Separately, I recommend insulating the garage, all walls and the ceiling.

Thank you. I can access a boroscope so can have a look one way or both & report back.
Is it reasonable to try a Chimney sweep in the vertical gas flue while leaving the chimney uncapped?

Garage insulation is on the To do list - isolating the gas fire means I can remove the gas piping in the garage which tracks along both walls that would benefit from the insulation. Unfortunately the garage buts against a similar garage with unfilled cavity wall and the remaining downstairs corridor abutts another garage on the other party wall. All in all a cold ground floor. The garage is actually not too cold now after replacing an electric up & over metal for a reasonably insulated Hormann 2 door solution & now with the Tesla battery giving a little background heat. Multiple bike users meant lots of cold ingress now significantly reduced with Hormann door

Definitely. You should see a marked reduction in air movement while keeping the flue ventilated. If you have a metal flue lining you could completely seal it if capping the top is near the top of your to do list.

Have you considered filling these cavities? And full filling the partial fill bits?

At the very least fill the bits against and between garages, but you will have a problem finding a contractor to do such a small job. The rest of the party walls may not be heated to your standard but if they heat more than you then you don’t want to insulate as their waste heat is good for your fuel bill.

The flue liner in the hearth itself & the initial neck narrowing is not metal - looks more like some form of heat resistant liner.
I investigated the party walls. Not sure of the state of the party cavity wall junction to the front & back walls re their sealing etc. The main structure appears to be partially filled with board but the party cavity is ~100mm but empty. The initial installer wasn’t keen on just the garage & no chance of disrupting the decorative state of the other 3 floors. Could see if anyone would be prepared to do just garage but the ground floor also contains the kitchen to the rear of the garage so again access limited. I did wonder about blowing in fibre fill but would have to scaffold the building and make good. My reading is that blown fibre has a bad damp reputation but these are party walls not exposed walls. The party walls (& I suspect the exterior walls) are almost certainly not capped: they have fibre insulation stuffed in the tops. End in a cold loft. None of the blown fibre installers came back to me anyway.

Thank you

Since post I had a look with a flexible borescope: from inside the plastic ducting 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.

The ducting run is roughly adjacent to the skirting board & not really accessible without a lot of cosmetic damage & repair costs. 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. I will cement over the internal gap in the corner of fireplace to optimise the airtightness.

All seems good. There isn’t really any benefit in filling the duct with insulation after the outermost 15-20 cm but if you use loose PS balls you might have to fill the duct just to hold the end bit in place.