I am not sure if I can explain the question correctly because I don't know the name of this mechanism in English.
This is my explanation attempt: In a house, a tube is expelling the air from the inside to the outside, and a tube is aspiring the air from the outside to the inside. The 2 tubes are interlaced in order to transmit the heat between their contents. The goal is to have a good thermal isolation between inside the house and outside:
- When the house is more hot than the outside, the outgoing air is warming the incoming air.
- When the house is more cold than the outside, the outgoing air is cooling the incoming air.
Question: In theory, what is the maximum efficiency of such thermal exchange? Could it be possible to reach something close to 1 or the upper bound of the efficiency is well under?