We recently remodeled a bathroom with new shower plumbing with a Grohe thermostatic valve and body sprays. Additionally we replaced the tub rough and trim. The problem is that the pressure on the hot water side drops significantly after about 5 seconds when running either the tub or shower in hot only. When switching on the cold only the pressure is great. If the tub or shower is running hot only and we turn on the lav faucet the hot water only trickles out. This bathroom has a dedicated 40 gal electric water heater located in a closet 10 feet away. One thing we noticed is that the supply line going into the heater heats up as hot as the hot water pipe within 10 to 15 seconds. Not sure if this is part of the pressure problem, but I would think that the supply line to the heater would only get colder as water flows into it. You advice is greatly appreciated.

Answer:

The water heater was there before. It is the cold water supply to the heater getting so hot that has me concerned now. It seems that there is some kind of loop feeding hot water back to the heater after the hot water runs for 10 to 15 seconds. The cold water supply pipe gets as hot as the hot water pipe exiting the heater.

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