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My hot water goes off in front shower any time hot water is used anywhere else in house. It is a single handle delta. The same bathroom sink takes several minutes to get hot water to it, but it's a double handle faucet. There is a tub in that bathroom with a single handle faucet and the kitchen is a single handle faucet as well as the back bathroom shower.

Is this the cartridge in the front shower? Or could it be the mixing valve in the wall?

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Could be insufficient pressure in the hot-water supply. – RedGrittyBrick Jan 6 '14 at 23:51
Does this only occur with the shower or all of the faucets? – mikes Jan 7 '14 at 0:48

When someone turns on another faucet it reduces the flow of water go to the hot water tank so less hot water is available for your shower and then you just get the cold. I suspect if you had the shower set to totally hot, you'd just notice reduced flow.

I believe they make kits for shower values that help prevent this by reducing the cold flow if the hot flow is reduced, I know they make anti-scald ones to prevent the opposite problem. There are also strategies for plumbing to help balance things better so the effect isn't as strong.

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If you have a tankless water heater you may be exceeding the heating capacity of the unit. For comparison say your tankless unit generates 5 gallons per minute. Let's say your shower head is a non water saver using 3 gallons per minute. If a washing machine or dishwasher were started on a hot cycle with no flow restriction they would exceed the 5 gallons per minute the heater generates. The water is flowing thru the coils faster than it can absorb heat.

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