Tracking a Draught

Letterbox should go. Awful yolks for draughts. Prob not the issue here but it will be.

What's the point of the airtightness test BTW? What's it for in this scenario.

House sounds like an odd spec for a new build.

That stove needs to be swapped out if not the correct type. It would be lethal in a properly airtight house

If you say so.

You are requesting info on random draught.

A stove drawing air from the house internal means a draught is a given. You say there is a vent near the stove but thats not the same as direct air feed.

An efficient stove doesn't need a lot of air, but it definitely needs some. Probably not enough to cause a significant draught.

If your stove does not have a direct air connection, you can still provide a vent to the hearth area so that the air gets to the stove without travelling through the rest of the room.

Failure to provide air can lead to fatal carbon monoxide poisoning.

Have you got carbon monoxide detectors in the house? Did architect say they were mandatory? If not, pay no more attention to this architect.

If the above is in place, I find that incense sticks are a very cheap and cheerful tool for tracing draughts.

Your plumbing suppliers should stock smoke pellets. RGI fitters use them to detect leakage in flues.