3 ms·

In US construction, a heat-recovery ventilator (HRV) would be used to mitigate losses from replacing indoor air with outdoor air. These are not heat pumps, but

In US construction, a heat-recovery ventilator (HRV) would be used to mitigate losses from replacing indoor air with outdoor air. These are not heat pumps, but passive heat exchanges to transfer heat between the outgoing (stale) air and incoming (fresh) air. The HRV is commonly added as a dedicated, central loop to do air exchanges for the whole house, while the regular central HVAC system maintains a closed loop as described previously. A pair of fans pushes stale air out and draws fresh air in through the exchanger. Tightly sealed houses will have ventilated "make-up air" pushed in to offset exhaust flows, to avoid creating a pressure gradient. In principle, you could also use an HRV with each exhaust duct for kitchen, bath, etc. However, in most houses these are separate vents to outside local to each room, not a centralized ductwork. To use an HRV you would need to add one to each vent location along with an intake and extra fan for the local make-up counterflow. It also seems like you would have to worry more about condensate and maintenance issues with pushing highly humid cooking or bath air through the HRV...

alkonaut 4y ago
The kitchen fan duct in a Nordic house or apartment is separate from the regular ventilation. I assume thats pretty universal for fire safety reasons. It’s a 125mm or 160mm pipe with 50mm fire insulation using a fan in the hood or on the roof that runs only when cooking. But there is also a vent in every kitchen (ideally at the opposite end) which draws the regular exhaust air continuously (much slower and quieter). Only that vent is connected to the heat exchanger or exhaust heat pump. They must be separate because the kitchen vent should manage a grease fire and you don’t want the grease into the ventilation. Similarly in bathrooms there is a vent drawing a base flow of air to the hrv/heat pump but it’s also common to have an additional direct exhaust e.g on an outer wall which only runs when needed. So the ventilation system with heat recovery is only for the continuous ventilation. Then short term ventilation like after a shower or when frying meatballs is in addition to that.
- saltcured 4y ago
  I thought you were worried about optimizing the loss through the exhaust/extractor systems, i.e. the intermittent fans used to remove steam or bad odors by dumping conditioned air to the exterior. With a central HVAC, there would of course be some kind of return to take from inside the house and send it back through the system again. I think others in the thread are taking it for granted that any system will be an inherently recirculating design. Some modern houses might have return ducts in many or all rooms. But, it is relatively common to just have a few centrally located returns, relying on open floor plans and/or leaky interior doors to allow circulation from rooms back to the return vents. For example, our ~45 year old house has a gas furnace with heat vents near the outside walls of each room, and air flows back through hallways to reach centrally located return vents on each level. The return vents are in a stack quite close to the furnace which is also centrally located.

alkonaut 4y ago

The kitchen fan duct in a Nordic house or apartment is separate from the regular ventilation. I assume thats pretty universal for fire safety reasons. It’s a 125mm or 160mm pipe with 50mm fire insulation using a fan in the hood or on the roof that runs only when cooking. But there is also a vent in every kitchen (ideally at the opposite end) which draws the regular exhaust air continuously (much slower and quieter). Only that vent is connected to the heat exchanger or exhaust heat pump. They must be separate because the kitchen vent should manage a grease fire and you don’t want the grease into the ventilation. Similarly in bathrooms there is a vent drawing a base flow of air to the hrv/heat pump but it’s also common to have an additional direct exhaust e.g on an outer wall which only runs when needed. So the ventilation system with heat recovery is only for the continuous ventilation. Then short term ventilation like after a shower or when frying meatballs is in addition to that.

saltcured 4y ago
I thought you were worried about optimizing the loss through the exhaust/extractor systems, i.e. the intermittent fans used to remove steam or bad odors by dumping conditioned air to the exterior. With a central HVAC, there would of course be some kind of return to take from inside the house and send it back through the system again. I think others in the thread are taking it for granted that any system will be an inherently recirculating design. Some modern houses might have return ducts in many or all rooms. But, it is relatively common to just have a few centrally located returns, relying on open floor plans and/or leaky interior doors to allow circulation from rooms back to the return vents. For example, our ~45 year old house has a gas furnace with heat vents near the outside walls of each room, and air flows back through hallways to reach centrally located return vents on each level. The return vents are in a stack quite close to the furnace which is also centrally located.