I was wondering about the physics for mechanical ventilation with heat recovery. My understanding is that it's a passive system that has a head exchanger formed of many thin layers of a metal (presumably aluminium) which then alternate airstreams entering and leaving a house to heat up the incoming air
The diagrams you'll look at will show an example like it being 0º outside and 20º inside, then after the heat exchange, it heats up the incoming air to 18º
Is this not bad science? The pressure of the house has to remain constant, so the incoming volume of air has to equal the outgoing volume of air. At best - if the air had infinite time to exchange heat, the best you'd achieve is 10º for the incoming air.
In the real case, I'd assume your heat exchanger would reach 10º, and the incoming air would interact with it for at most a few seconds. I just can't see any real heat transfer happening here
What's your thoughts? A scam, or something that has actual benefits?
Edit - I've left the original post in tact - but I did find an answer. It's a real phenomena called countercurrent flow/countercurrent heat exchange. It's very important that the flows are in opposite directions - if they're not, you'll just reach the equilibrium temperature. But when they flow in the opposite directions, it is possible exchange nearly all the heat. The phenomena also shows up in nature - ducks have a countercurrent heat exchange system in their legs so the are able to recover heat losses from their feet being in water
I personally like your initial sceptic attitude; it's very healthy, especially in the present times. And equally I like the "I checked, it seems to make sense" spirit of inquiry and open-mindedness. It seems these are becoming more and more rare qualities nowadays!
I know little of this technology, but the process seems to be a thermodynamic, not thermostatic one. That is, we're not at equilibrium. Then many interesting things can happen.
My intuition at the moment is that it may work. The point is that an incoming parcel of cold air is not just being put into contact with an equal-volume parcel of hot air, and left there. Rather, it is for a short time in contact with a parcel of less cold air – the parcel that's going out of the house soon, and receives some heat from it, getting slightly hotter. In the meantime it's moving, and next it meets a parcel of air warmer than the previous one, and receives more heat. And so on, up a gradient. From the perspective of an incoming parcel, it's as if it were put into contact with a "reservoir" that's getting hotter and hotter. The opposite is happening to the outgoing air. So when it gets out it is colder and therefore it's transporting less energy out of the house as it exits.
This intuition could be completely wrong, but it's enough to leave the possibility open for me.
Will check the explanation that you found!
It isn't.