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 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.
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.