Rinnai False start...

kcopp

I have a customer who has a 2 yr old Rinnai tankless.
They made the comment hat every time a Cold water fixture is turned on the Water heater turns on for a moment.... quite strange. Is there an internal check valve that may have failed? thanks for any input...

Big Ed_4

Maybe a bypass though a fixture , the unit sensed an flow ...

To find it you need to go though the process of elimination .

Larry Weingarten

Hi @kcopp , A possibility is that the supply pipe is not flowing well, (old steel?) and if there were an expansion tank downstream of the heater, water would flow backwards through the heater every time cold water pressure was lowered with a cold water draw. Even with no expansion tank this could happen a little.

A completely different possibility would be a cross connection out in the plumbing someplace. Perhaps a spring check valve on the cold supply to the heater could help in either situation, though I'd check for cross connection and see if there is a problem that can be found and fixed before doing any other modifications. Do you know what brands of fixtures they have?

Yours, Larry

kcopp

This house is less than 4 years old. All the fixtures are of the same age. I am not sure what faucets and valves they are but probably kohler as the rest of the development.
Since it is a tankless there is no expansion tank as there is very little or no thermal expansion.
There is a watts #7 at the meter.
Never seen this on a Rinnai before.

Alan (California Radiant) Forbes

I've seen it once before. It was in the mechanical room, so no one ever heard it or did anything about it, including us.

Like others have said, check for a cross-connection. You probably know how to do that, but just in case, I would turn the cold water off at the Rinnai, open a hot water tap and see if you get any steady flow. If you do, there's a cross-connection.