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It won't be and I had given consideration before to this problem. Some MVHR systems have the option of a positive pressure setting for when they are used with a stove. It was another reason why I'm only considering MVHR for half of the downstairs as that room won't be covered by MVHR. I could go further and give up on MVHR in the dining room too so that downstairs it will only be the utility and kitchen covered by MVHR, these 2 rooms being removed from the living room by quite some distance, and 2 doorways.

Thanks for the replies. I don't have any actual evidence but 'fairly' airtight is what I would say. The windows will be new and there won't be any trickle vents, plus we are changing the roof to be a warm roof as part of the works so the roof should be well sealed above. The weak points will be the front porch and WC which stick out from the front of the house and the living room which will have a multifuel stove in it. I'm not sure I can share drawings openly online for privacy reasons but the flow of the rooms downstairs that we are proposing to ventilate is: Supply in Dining room (23m^2) > Kitchen (22m^2) including recirculating cooker hood > Extract in Utility room (4m^2) The semirigid ducting we are planning to use. The issue is that to get in to some of the downstairs rooms will be difficult. For example for the WC we would need to drop a duct though a wardrobe of an upstairs bedroom down to ceiling level, then the hard part would be to coredrill through a cavity wall into the WC which sticks out of the front of the house.

We are shortly going to extend and remodel our 1970's 4 bed detatched house taking the total floor area from 108 to 170 square meters and have a few questions. As part of these works we intend to have MVHR installed. Ventilating the upstairs (4 bedrooms and 2 bathrooms) will be easy through the roof but we are not sure for the downstairs which rooms we can ventilate. The kitchen and utility room will be in the extension so getting pipes to them seems easy enough. What we are contemplating is only running one input and one 'in' and one 'out' valve downstairs. Fresh air will be supplied into the dining room (open plan into the kitchen) which will then be extracted in the utility room. This would leave the living room, study, hallway, porch and WC not covered by the MVHR system. The WC already has it's own extractor fan and is almost sealed off from the rest of the house as it comes off the front porch. Does it sound like a good idea to only ventilate the rooms listed? Doing others will be more difficult to duct to and add more to cost as well as needing a much bigger system. Or is it worth just considering single room MVHR in individual rooms where there is a problem? Any observations and advice much appreciated.