The decision to install two water heaters was a good one. The implementation is where the problems are. To fix it can range from fairly simple to "oh my gosh", depending on what he did. However, your problem might have been the same even with one heater, since he apparently mixed the pipes when he created the "manifolds" tying the various pipes together. The repair usually consistes of finding where those manifolds are located in the walls on the first floor and then deciphering where each pipe goes to and whether it should be hot or cold water and then what is actually connected to it. That is the worst part of the job and the most time consuming. If you were in this area, I might be able to help, since I find it fairly frequently given the expertise of the plumbers dong new construction.
: Prior to our new home foundation being poured we had decided to install a hot water circulating system. The home is two story and the plumbing is basically split to two sides of the house. One side, downstairs is the MSTR and GUEST bath. On the other side, downstairs is the kitchen, utility, another bath and service to the garage. Upstairs, same side, is two bathrooms. The plan was to place one hot water tank (gas) in the attic on the two story side and there would be the circulating pump. During the plumbing rough-in, we came by another hot water tank (electric). I question the plumber about the feasibility of vacating the circulating pump idea and instead install two hot water tanks; one for the two story side (gas) and one for the MSTR/GUEST one story side (electric). He said no problem. Little did I know that the decision would give me nothing but problems.
: The rest of the story is nightmarish. I discovered that getting hot water to the MSTR bath was a very long ordeal and the water was never really hot (very warm) and short lived. The Guest bath was only slightly better. On the two story side I found that in the bath after one flush you would be flushing hot water. The sink and shower were both hot water and scalding. The outside faucets (six of them) all, except one have hot water. The service in the garage provides cold water but there is no flow from the hot side.
: The plumber, of course, is long out of business and unreachable. I have contact a maze of plumbers and one by one they fell by the wayside. Either did not want to tackle the problem or provide little support, advice, and no plan to possibly correct the problem.
: I have since helped my situation somewhat. By experimenting, I found that by turning the electric heater (two story side) off, and shutting down its source of cold water the situation improved. Now I am supposedly using only the gas water heater installed in the attic on the two story side.
: Now I do not have hot water in the outside faucets. I do not have hot water in both sides of the bath on the two story side. In fact this shower is the most appealing because you do have a source of hot water and it is controllable; I found that the temperature of the water varied with the pressure (ie, if you turn the pressure down for a light shower the water would cool accordingly).
: The MSTR bath has improved some. The hot water to the tub arrives much quicker than the sinks (they are still very very slow), however the tub hot is short lived.
: I will be the first to admit this is all very confusing. I am not a plumber by trade so think of how confusing it is to me.
: It would seem to me, of course, that our plumber crossed some pipes somewhere. I have a number of photos of the house going up, which includes a number of photos of the rough pipe extending from the slab. There are two main manifolds: One, going up to the attic on the two story side and another going up the attic on the one story side. In the trenching you can see, what appears to be additional piping which I take to be the piping for the circulating system and it appears to be running from one manifold to the other. There are other minor manifold to serve the baths, kitchen, garage, and utility.
: If anyone is willing to discuss this dilemma with me I would be greatly appreciative. I can add more detail to the narrative that might clear it for an experienced plumber. I have purchased some software (home building, all phases) in hopes that I can plan for my original system and then maybe I can add in faults that would provide symptoms similar to ones I am experiencing.
: Thanks for listening,
: Al Stewart