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.
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).
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.
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