2 ms·

Return (exhaust) air pumps just use the heat in the air that was being evacuated anyway. Requires only that you have ducts from those rooms that are ventilated

• LeifCarrotson 4y ago
  It's strange to me that bedrooms/living rooms take in fresh air directly from outside. Wouldn't that incoming air be very cold and have super low humidity? And in the summer you'd have enormous amounts of hot, humid air coming in? I suppose that in a way, American-style forced air is kind of like a heat exchanger between the return air and the intake air...except that instead of using a thermally conductive plate or heat pump to move heat from one mass of air to the other, it just redirects the return air itself with all its entrained heat energy straight to the intake. I am a bit curious how the different solutions deal with issues of air quality. American houses often have small ~50W fans in the kitchen (over the stove, sometimes integrated with the microwave, called an "exhaust hood" and engaged only when the cooking gets smoky) and in the bathroom, engaged only when someone's showering and wants to remove the steam or defecating and wants to remove the smell. You often don't put cold air returns in these places, because you don't want those things to be immediately plumbed to the rest of the house, you put cold air returns in living rooms, hallways, and bedrooms. But that means you're re-breathing the same air over and over again. It usually passes through a filter in the furnace, but still... I think new builds with terrific air sealing often need a little 3" PVC direct vent to admit fresh oxygen.
  • alkonaut 4y ago
    The really old buildings obviously had no ventilation other than simply being not-very-sealed. Only a couple of generations ago there were still houses with single glass windows, there weren’t any moisture barrier film in the building shell, doors weren’t well sealed and so on. The second gen with forced exhaust emulates the old self-draft by forced exhaust ventilation and that exhaust air has to be replaced by air that enters somewhere and since the air is evacuated from kitchen/baths it’s let in through all other rooms. Yes that air can be cold/dry and it’s also uneconomic if it’s -20C so these vents are adjustable so you can close them somewhat when needed. Heat exchanged air forces both from and to the building is of course an improvement over the from-only, but the ductwork required is much more complex so it’s mostly done in new builds. The great thing about forced exhaust only is if it’s combined with a heat pump that reclaims the heat so it’s not wasted. As far as I know the rule of Thumb regardless of system is that all the air in the house should be replaced every two hours using only the base flow i.e excluding the kitchen fan. So air quality is ensured for both systems as they shift the same volume of air, but comfort and economics is better with the more expensive system (no freezing fresh air entering).
• saltcured 4y ago
  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.