Joseph Meehan wrote:

There have been some heat exchangers that use the inside air that is
exhausting to warm the air coming in, but the theoretical best it can do
is 50%.

Not so. According to the National Research Council they're typically
70-80% efficient.

http://oee.nrcan.gc.ca/residential/p...rks.cfm?attr=4

The theoretical best it could do is higher than that, of course.


Chris