We recently tore out the floor and wall of the upstairs bathroom looking for a leak in our new home. We found and fixed the leak , reconnected the sink and now have running water upstairs. The next morning we realized we were only getting lukewarm water everywhere in the house. The water heaters check out OK -one is new and the second has hot water when we open the release valve. The set up is a little weird with the hot water supply from the new heater running into the older one and then out to the house. Is there a reason upstairs plumbing work could have this effect or is it a coincidence?

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open the release valve

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I am assuming by that you mean the T&P (Temperature an Pressure) release valve. That little valve with the lever and a pipe coming down the side of the water heater. Leave that thing alone. Your tank can have sediment in it and if that sediment gets caught in the seat of the valve it can begin leaking and you end up replacing it. If you must check the water in the tank use the drain valve at the bottom of the tank.

Assuming no repairs or changes were made to the water supply piping to the bath, the lavatory should not be a factor. You can isolate the lavatory from the question by turning off the stop valves underneath.

My first guess however would be, that you forgot to turn on the breaker to the second water heater if it is electric, or to relight the pilot if it is gas.

Water from first water heater moving to second water heater and cooling before you use it.