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Does anyone have any knowledge or expertise, or advice on fitting passive stack ventilation in chimneys?
I previously found a company, I think they were called Ventive, and they did a thing where they put a heat recovery unit on top of a chimney, and fed two pipes down the chimney, one with incoming fresh air to the top of a room, and one to the bottom of the chimney for outgoing 'stale' air. The warmth from the expelled air heated the incoming fresh air, and the fitting on top of the chimney was such that it always had a flow of air across it, using the stack affect in the chimney for the warm air to rise and be expelled, pulling in cold fresh air which is heated through the heat exchanger.

only now, I can't find that system, and that company does other heating/ventilation type stuff on a larger scale.

I've seen a thread where someone was putting in ducts up/down the chimney, and then filling the voids with vermiculite.

obviously this is on chimneys that don't have a fire or wood burning stove.
we have 6 rooms that could benefit from this over two different chimney stacks.

This a timely post for me. I am considering blocking off one of our two chimneys and fireplace due to not using that log burner.

I have been contemplating an MVHR system sat atop the chimney. Needs to be one with enough pull and a way of removing the condensation.

Once the outgoing air gas been cooled somewhat, its buoyancy, relative to incoming air that's taken on that energy, will be reduced.

Instinctively it feels like either the HX performance will be very low or, if it's not, the air temperature differential will be too marginal for the stack effect to drive the system. That's just gut feeling though, maybe it does work.

A few years ago we had an old single duvet that was ready for the bin. I put it in a bin bag and jammed it up an unused chimney. No more draughts and still snug years later.

Consider small in-the-wall room units - Fresh-r, DUCO, Renson make these in Netherlands. Well-suited to refrbishment projects - require no ducting - come with humidity/Co2 sensors. Try to attend a trade exhibition in Netherlands (they lead with small MVHR) eg. VSK Utrecht. Consider accessibility for annual cleaning (filters) and servicing. I have 3 Fresh-r units (+1 spare) in Scotland fitted into external walls of a passiv haus envelope - but Fresh-r are a bit expensive. I would probably go for DUCO or Renson if replacing (because of price). Most are available through UK stockists.

our house is a 3 floor 1900 ish mid terrace, in a conservation area. single room external vents wouldn't work on the front rooms, but might on the bathroom and kitchen which are in the back of the house, where there is an extension, so external walls on the side of the house.

Are there companies that can do full environmental surveys and suggest the best options for the most gain, least spend, least upheaval, etc?