I had emailed Fitech about this situation back in December/January, not sure, and they said the 5/16 return would be fine. So, I went with it and yanked the original 1/4 return. Put a new 3/8 supply line and used the original 5/16 supply as the new return. Haven't experienced any issues that would indicate an issue with the return's size. Edit: Maybe they have changed their "tune", since then?


That being said, I noticed one day, many months ago, that I thought the tank was warmer than maybe it should, but haven't dealt with it, nor asked around as to what others are experiencing.

Also, in putting together my system, I did find some fittings from various manufacturers, seemed to be a bit smaller than what I thought I was getting. I made sure that if I was running a 5/16 return or 3/8 supply, that it was actually 5/16 or 3/8 (or as close as could be within reason) all the way from the pump to the Fitech.