How is Efficiency of Heat Exchangers (HRV) determined

[whatmough]

I've be looking at the specs for Heat Recovery Ventilators (HRV).

They commonly state that they can recover 85% of the heat. How is that possible? To me you could only every recover 50% in a perfect situation because of the equal volume of airflow's. Won't the air temperatures just stop at equilibrium?

 

[AlephZero]

They used counter current flow heat exchangers, which have a theoretical peak efficiency of 100% not 50%. Of course you never achieve 100% heat exchange in real life. http://en.wikipedia.org/wiki/Countercurrent_exchange

 

[enorbet]

Commercially designed heat exchangers must be designed with a fairly strict operating (especially ambient environmental) temperature window in mind to keep to high levels of efficiency. The common household so-called "heat pump" systems efficiency drops precipitously below 45 F (7.2C) for example.

 

[whatmough]

Thanks a lot Aleph you answered my question, that wiki was helpful.