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Return air heat pumps? It's telling that I have no idea what you're talking about, when I just this weekend pulled the wiring for the brand new heat pump that's

Return air heat pumps? It's telling that I have no idea what you're talking about, when I just this weekend pulled the wiring for the brand new heat pump that's literally being installed at my parent's house as I type this. We both have (had, in their case) a forced air natural gas heating system, with "cold air return" ducts which transfer air volume from the bedrooms, kitchen, etc. back to the furnace room so that the heating vents aren't fighting back-pressure. Air drawn from kitchens, bedrooms, living spaces, etc. goes back into the bottom of the furnace, where it's heated and returned to the house. There are two sets of ducts, one going away from the furnace and one going back to it. It's a closed system. You can't pull heat out of air in your air return, then pump that same return air through a heat exchanger to make it hotter than before. Sometimes, yes, air from the kitchens and bathrooms when you turn on the stove exhaust fan or bathroom fan is conveyed out of the house through a baffled wall vent by a PVC pipe (note that a sad number of bathroom fans eg. in old hotels and apartments, where getting ducts out of the building is impractical, are not actually moving air: they just make noise and don't go anywhere). But those dedicated fans are only turned on for a few minutes per day. Just about everything else is a closed system...older atmospheric venting water heaters and One thing that I would be interested to see is a clothes dryer that was energy-aware; I've got a newer, reasonably high-efficiency gas dryer but it does dump a lot of energetic hot and humid air into the sub-freezing exterior in winter, which is replaced by cold air getting pulled into the house, and it pulls my expensive cooled air into itself in summer, heats it, and blows it out, to be replaced by pulling in hot and humid summer air.

alkonaut 4y ago
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 (usually kitchen and bathrooms) to the pump, and from the pump out to ambient. It’s not a lot of ductwork. This is for older houses here which have no forced incoming air. Bedrooms/livingroom just take in fresh air directly from outside. A more modern home would have heat exchange between in/out air to move the heat from the incoming to the outgoing air. Return (or exhaust) air pump: https://assetstore.nibe.se/hcms/v2.3/entity/document/121339/storage/MTIxMzM5LzAvbWFzdGVy https://assetstore.nibe.se/hcms/v2.3/entity/document/121339/... Those are cheap as hell. Handles all the hot water, space heating and ventilation in my home with a $5k device which has a COP similar to a wall unit + works when it’s really cold outside too.
- 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).

alkonaut 4y ago

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 (usually kitchen and bathrooms) to the pump, and from the pump out to ambient. It’s not a lot of ductwork. This is for older houses here which have no forced incoming air. Bedrooms/livingroom just take in fresh air directly from outside. A more modern home would have heat exchange between in/out air to move the heat from the incoming to the outgoing air. Return (or exhaust) air pump: https://assetstore.nibe.se/hcms/v2.3/entity/document/121339/storage/MTIxMzM5LzAvbWFzdGVy https://assetstore.nibe.se/hcms/v2.3/entity/document/121339/... Those are cheap as hell. Handles all the hot water, space heating and ventilation in my home with a $5k device which has a COP similar to a wall unit + works when it’s really cold outside too.

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).