Hot Water Issues

[breland79]

Good Morning and I’m glad to be here. I have a little experience RV’ing but as a kid in a Class A in the late 80’s to mid 90’s. A lot of things are coming back to my remembrance.

Last night I figured that as long as I’m hooked up to house water my gas and electric water heater will run hot for just a second and then turn cold. But when I turn off the water to the house going to the RV the water is hot. I have an idea of why but I wanted to get some more opinions on the matter or if anyone else has experienced this before. Thanks.

 

[Bobtop46]

Sounds like a water heater bypass valve in the wrong position. Did you winterize last fall and recently de-winterize. I would check the position of your valves. The water heater is making hot water, leading me to believe it's working correctly.

 

[Gary RV_Wizard]

The water heater doesn't know where the water comes from, but the house "cold water" supply will likely be a different temperature and pressure (psi) than when using the RVs onboard tank and pump. That can indirectly affect how water passes thru the heater.

You didn't say but presumably when you disconnect from the house water supply, you turn on the RV water pump and draw water from the onboard tank. That pump typically is preset to run at about 45 psi and the tank water will be close to the average ambient air temperature.

The water heater "winterizing bypass" can function somewhat differently when the incoming water pressure or temperature differs. It's not supposed to do that, but a faulty control valve or check valve can let that happen. A clogged heater bypass check valve is a fairly common problem wherever hard water is present.

First check to be sure the heater bypass is NOT in the Bypass position. My next check would be the check valve at the heater hot outlet, which will be present on a bypass system that uses either one or two control valves (a 3-valve system usually has no check valve).

 

[Kirk]

Another possible cause would be if the water is shut off at the shower head leaving both hot and cold water open at either the inside or the outside of the shower valves.

 

[Gary RV_Wizard]

Good point from Kirk. And the temperature mixing that occurs through a faucet like that is also sensitive to temperature and pressure differences in the lines and thus may act differently with onboard pump vs city (hose) water sources.