I have a Rinnai Infinity XR20. It's about 3 years old. I've just recently noticed a peculiar issue with it that I hadn't noticed before. If I turn on a cold tap like the one to the garden hose and then turn it off, the unit momentarily fires.

I've verified this occurs now on all cold taps in the house even those that aren't connected to the hot via a mixer.

My guess is it's the shock wave caused by water hammer which is causing the unit to think that water has started flowing when it hasn't.

I asked my local plumber and he agrees but isn't able to offer a solution. He says he hasn't seen this problem before and will ask around for me. In the mean time I rang up Rinnai and the technician suggested I could fit a non-return valve on the outflow of the Infinity. However, that doesn't make sense to me.

What suggestions do you have to fix it? Should I fit a water hammer arrestor to the inflow of the infinity?

  • a water hammer arrestor may fix the problem but the unit firing over such a small wave seems strange. Ill look up the flow switch assembly in a minute and get back to you
    – UNECS
    Commented May 23, 2013 at 8:06
  • 1
    Just a guess, it probably has internal pressure balance valving, similar to the anti-scald on shower valves. The anti hammer cartridge (for both supplies) would be a reasonable response. Adding a pair at the washer hoses is a DIY friendly exercise, available as an inline 'T' that has dual hose threads.. no soldering. (though I LIKE soldering)
    – HerrBag
    Commented May 23, 2013 at 12:49
  • I have the same problem with a Rheem hot water system. The non return valve either before the heater or after the heater does the same thing. It prevents the pipes on the hot side from depressurising when a cold tap is turned on. Thus when turning off the cold water they aren't repressurised, Thus no flow through the heater. This may be more of a problem if you have long hot pipe runs or are using plastic pipes. Commented Mar 14, 2020 at 0:49

1 Answer 1


I had the same problem with the heater firing up when the toilet was refilling from the cold waterline. I put a non-return valve on the pipe just below the heater on the cold water inlet. It has fixed the problem: no odd firing of the heater any more.


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