Something that I would consider a heat-exchanger for is to recover heat from moist air expelled from bathrooms and kitchens.

There is quite a lot of info on this idea if you search for it. Basically you extract warm wet air from your bathroom/shower/cooker hood and get take the heat out and use it to warm up some dry air from outside, usually taking it from the loft space.

I have been to a lecture for landlords some years ago that was to do with creating positive pressure in a house, taking the dry air from the loft and using a slow pressure slow running fan to push it into the house. The idea was to keep the air in the house drier and so reduce condensation and mould.

As the air is colder outside it cannot have so much water vapour held in it and is thus drier than the air inside the house, which is warmer and thus can hold more. The air inside will be picking up water from lots of sources within the house, cooking, baths, showers etc.

Combine this with a heat exchanger and you are re-claiming the heat that you have already paid for.

I have this on my plans for when I've done everything else already on my DIY list!