Our house has gravity ventilation, ie each room is connected to the chimney shafts through a hole in the wall. During cold days heat from the upstairs rooms escapes through one chimney shaft, while very cold air comes into the downstairs and basement though another. I've closed some of the holes, some only partially, but cold air still seeps in through windows, doors, etc. This kind of ventilation can account for up to 40% of a house's heat loss, in our case I estimate it's at least 15%-20%. To would be much more efficient to warm the incoming air with the heat that would be normally lost, so I plan to close the chimney shafts and run the air through a
heat exchanger. I'm also impressed by heat pumps that use ventilation exhaust to warm hot water, like Viessmann Vitocal 160-A WWK or Stiebel Eltron WWK 300. An underground heat exchanger would further increase the efficiency of the system.
This set-up would be ideal, but since recuperators can have efficiencies as high as 90%, then the exhaust air may not have enough heat to be used for water heating. I read somewhere that there would only be 400-500W of usable heat in the post-exchanger exhaust. The heat pump in a water tank system uses over 400W, so it can (should) pump at least 1000-1500W of heat, much more than the exhaust air can offer. Increasing the ventilation speed will increase heating needs in the house.
So maybe I should ditch the hot water tank/heat pump combo and use a small (100-150W) heat pump to transfer heat from exhaust to fresh input air? Hot water would be an electric/solar setup, independent from ventilation.
But I haven't seen heat pumps rated at below 400W, so I figure I'd have to DIY it from a fridge compressor (rated at 50-250W).
What do you guys think?