Water flowing through tankless heater at beginning and end of cold water draws

[davida]

I installed a rheem gas condensing tankless water heater yesterday, and it seems to be providing hot water just fine. My problem is that for about a second when you first open a cold water tap in the tub (or when the clothes washer begins to cold water, or a toilet begins to refill, but not with low-flow fixtures like kitchen/bath faucets), a bit of water seems to be flowing through the heater, causing the fan to come on. It does the same thing for a second or so when you turn the cold water tap off. It doesn't happen throughout the cold water use, just at the very beginning and end. Someone suggested installing a check valve at the WH inlet, so I did that and it seems to have solved half the problem--now the fan does not come on at the beginning of a large cold water draw, but still does at the end. The WH and attached pipes also jerk a bit when the cold water is turned off. I don't think I have a hot/cold crossover problem, because when I turn off the shut-off valve on the WH supply line and turn on a hot water faucet somewhere in the house, the flow comes to a stop fairly quickly. The house has mostly pex pluming, if that's significant. Thanks for any advice you can offer. Feel free to ask questions if you need more info.

 

[DonL]

What model is it ?

Some of them measure water Pressure, not just flow.

My guess is the pressure change/drop is what is causing it.

Do you have a expansion tank ? What is your water pressure ?


Good Luck.

 

[Sponsor]

 

[davida]

What model is it ?

Some of them measure water Pressure, not just flow.

My guess is the pressure change/drop is what is causing it.

Do you have a expansion tank ? What is your water pressure ?


Good Luck.

Thanks for your response. It's a Rheem RTGH-84DVLN. I don't have an expansion tank. I'm not sure what the water pressure is, but we're on the city water supply.