I used to install them years ago. But the higher efficiency units 16 seer and higher would cause problems until water heated up. I have not installed any in probably 10 years or so. The quality of the hru's went down. The newer ones would not produce enough heat to even be worth while. Then good luck finding parts for them. Usually the company that made them where either out of business or moved to China.
I'm installing a new split system that users R410 (5-ton) and was looking at adding a HRU. If the unit is 15 seer, do you think using a HRU with that would be if no use and/or cause problems.