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?