Hello All, First time I have posted but I have scowered the internet the last few days. I have 2 (50G) water heaters installed in parallel for a total of 100G and the "master" failed on me. It has started to leak from 2 spots and needs to be replaced. So I did what anyone should do, call a plumber and ordered a replacement water heater. Unfortunately very water heater I see around only has 1 input for cold, not 2 and Home Depot, Lowes, my plumber and a half dozen other people have never seen. What I mean by this is summed up in the picture below where a secondary pump throws water into the master water heater. The labeling on the water heater at this spot says "space heater return" and it seems to be a separate water tank from the normal one. My guess is that it is an old school hot water recirculating pump. This "space heater return pump" is connected to the cold input in some way, in that if I turn off the cold input it will also turn this off. Any ideas what this is really used for? (I do not have a boiler, water heated home, or heated floors which is the closest I can come to for this. Thanks all -russ PS: I will be donating if I get the answer.