Duel on-demand water heaters

Saw this today. 2 on demand water heaters hooked in parallel, not series. (meaning one or the other, not one to another) the idea here is, if the first one isnt cutting it the second one kick on. Problem is, the hot water will run, and when it switches, there will be up to 2 minutes of cold water. A little annoying when you are taking a shower. Looking at the plumbing, there seems to be no mixing valve. I wondered if they shouldnt be in SERIES, perhaps with a 5 degree temperature difference to prevent the second one from working too hard when it doesnt need to. What do you guys think?

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wondering would not the water be pulled through both as if it was running as one i don’t see how they would determine which would lead and which would lag. series sounds better but don’t see how it could tell what temp to turn on they are operated with water flow correct.

Neither is a good practice. Instead isolate fixtures and use 1 for one group and the other for the second group. In practise they have a tendency to both operate unless set at grossly different temps.

i wonder if a mixing valve would work. Take a little from each.

Besides being a plumbing nightmare, I don’t see the sense in running 2 heaters. If one is inadequate, install a bigger one, or install a small storage-type heater in series with it.

I think there should be a iNachi literacy course. “Duel” heater system, eh?

Parallel heaters are often installed because the flow from this type of heater is relatively low. I have two such heaters in my basement, but they each serve a different part of the house. However, there is crossover piping to allow for one heater being out of service (has happened twice in 18 years).