How to minimize heat loss in an air extractor?

Question

In an air-conditioned house equipped with an air extractor that removes for instance wet, smelly, etc. air from the bathrooms and kitchen, what are sensible cost-effective systems to prevent:

• Heat loss during the winter by bringing in cold air and extracting hot air from inside the house
• Heat gain during the summer by bringing in hot air and removing the chilled air inside

That is, I would like to minimize the energy cost of extracting chilled/heated air by somehow recycling that energy before the air is dumped outside.

Answer 1

What you want is called a heat exchanger. Imagine two long air tubes with a thin wall between them. The air exiting the house travels in one tube, and entering air in the other, but in opposite directions. Over the length of the tubes, heat transfers thru the thin wall. Ideally, by the time the house air gets to the far end where the outside air comes in, it's at the outside air temperature, and vice versa for the outside air at the other end.

In practice, there are various ways to construct heat exchangers, and these things are either far from ideal, or very bulky and expensive. The cheapest commercial ones are usually cross flow instead of opposite flow. This is much easier to construct, but the efficiency is lower. Cross flow units in series increases efficiency.

Most of the time, the payback for a heat exchanger for something like a bathroom vent is too long to make sense. Price out some units and do the math before jumping into this.