My wife and I just moved into a new house. The builder bankrupted on us and left us with several problems. Most are readily fixable, but one has me a bit puzzled. There are two water heaters, one at each end of the house, connected properly. They connect to the water lines in parallel so that the entire house has their combined capacity available. Weird problem is that cold water is going in one tank, out the hot side, back to the second tank (hot side), and back out the cold side (Backwards!). The tanks are connected correctly, seems to be a path-of-least-resistance problem. I think that a check valve at each tank would solve this strange looping problem. Any one have any other thoughts? My question is this: Should I install the check valves on the cold side or hot side? (I think hot side, so any expansion would go back out the cold side instead of building up pressure). Comments, please.

If the idea was to supply hot water faster to the farthest bath room then the two heaters should have been hooked up in series and certainly not in parallel.
I have installed two water heaters in series, one at one end of the house feeding the kitchen, utility room and wet bar but looping the hot water over to the cold water inlet of the second heater, which supplys the guest bath and master bathroom. This insures that there will always be plenty of hot water at both ends of the house.
Lay it out on paper. About the only advantage I can see with a parallel hookup is the added capacity and the fact that the second heater is closer to the master bath. However as you've found out a parallel hookups will present problems. The solution isn't check valves but to get the hookup done right. Good luck, Tom