i know this sounds a bit mad at first, but please bear with me, i'd be glad of a second opinion. my house has 4 independent chimneys in a poured concrete chimney breast. the 2 upstairs fireplaces i inteneded to seal off, the 2 downstairs are getting room-sealed stoves, with chimney liners. i will be installing HRV in the attic beside the chimney breast. i'm trying to be clever with the duct routes, and i had the idea of using the 2 retired chimneys as air supply vents from the HRV into the 2 upstairs bedrooms. chimneys were originally designed as exhaust vents and now since they are to be retired, why not convert them into fresh air supply vents? you could core drill into the 2 chimneys beside the HRV and run the duct down into the bedrooms. sealing off and insulating the chimney above the entry point. it sounds like a nice idea to have fresh air gently coming in from the fireplace.
the obvious concern is that somebody (say me, if i was sleepwalking or something?!) could forget and try to light a fire in the fireplace. but i would make sure to leave the vent outlet visible if you got down to look at the fireplace. and also since it would be blowing fresh air in to the room (not sucking the smoke/fire up into the HRV), any attempt at lighting a fire would quickly fail and smoke out the person and the room (i have mains-wired fire alarms in every room and in the attic). i could probably live with a discreet "no fires" safety sign in the fireplace or something. has this ever been done before?
i've considered contamination of the air from fumes from the stoves, but since they are in separate chimneys with sealed liners, i'd say no issue here.
also a potential issue of heat in the chimney breast melting the ventilation duct etc., but i could use flexible stainless steel. i know it's not ideal for air flow compared to a fixed duct but i still think it could work. any loss of heat due to the duct going down a cold chimney could be offset by the added warmth of the chimney when the stoves are lit.