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

  • @outstanding_bond@mander.xyz
    8•1 year ago

    I’m more of a chemist than a physicist but I had a similar difficulty understanding fractional distillation.

    If the concept maps, the key thing is that the length of the path through the heat exchanger means a lot here, and that different parts of the heat exchanger can have different temperatures.

    With short path lengths/little exchange time you’d have a heat exchanger like you describe - It just reaches a temp half way between the two regions it connects. But, imagine you replaced the single heat exchanger in this setup with two heat exchangers hooked up in series with a unheated middle room connecting them - Then the outside temp could be 0C, the first heat exchanger temp would be 5C, the small room’s temp would be 10C, the second heat exchanger would be 15C, and the heated room would be 20C.

    In this situation, the air coming out of the second heat exchanger and into the heated room is 15C instead of 10C.

    There’s no actual need for the middle room, you could directly attach the two heat exchangers end-to-end to achieve a similar result.

    In essence, a heat exchanger that heats the incoming air to 18C is just a bunch of the heat exchangers that you are thinking of stacked so that the output of one is the input of the next, with a temperature gradient that forms between them.

  • stravanasuEnglish
    4•1 year ago

    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!

    • @John_HaslerEnglish
      2•1 year ago

      This intuition could be completely wrong

      It isn’t.

  • SalamanderM
    2•1 year ago

    I am not familiar with the specific technology or claims that someone selling you MVHR technology might make, and so it is possible that someone selling you an MVHR system could be over-promising or outright making misleading claims.

    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

    Yes, that sounds scammy. Unless there is some thermodynamic concept that I am unfamiliar with, in the best case the final temperature without active heating would be about 10 C for a perfectly equilibrated heat exchange process. But such diagram does not appear in the wiki page nor in the articles that I have found while looking into MVHR.

    From looking at the wiki page on MVHR, this is what I understand:

    • The air inside of a home can become “stale”. As an example, a common source of ‘staleness’ is the concentration of CO2, which is given off by breathing humans and accumulates inside of human-inhabited enclosed spaces.

    • A way to reduce the staleness of the air is by exchanging the air inside of a building with fresher air from outside.

    • The most comfortable temperature inside of a home is often not the same as the temperature of the fresh air from outside.

    • Exchanging stale air at a comfortable temperature with fresh air at an uncomfortable temperature will require an additional energy input to either cool or warm the fresh air.

    • An MVHR strategy is one in which a passive heat exchanger is used to equilibrate the temperature of the outgoing stale air with that of the incoming fresh air. This step brings the temperature of the incoming fresh air closer to the desired optimum at no additional energy cost.

    To me, there is nothing obviously scammy or unphysical about this approach. If you do not use a heat exchanger, then you simply throw out the stale air without taking advantage of the energy differential that you have already paid energy to create. The heat exchanger helps you recover some of this differential for free.

    So, I’d say no, MVHR is not a scam. But I wouldn’t be surprised if someone makes outlandish claims about it to scam people.