Rinnai Tankless Heater: turbine turns on when cold water is drawn

poriordan

We had an Eternal tankless for a number of years. We just replaced it with a Rinnai RU 199IN. The unit works fine but we have one problem I'd love some help with.
Whenever we run a cold tap anywhere in the house, we hear something turning on in the Rinnai. The "In Use" light does not turn on but something is running - I assume it's the flow turbine.
Following a suggestion from Rinnai's tech support, we installed a check valve on the cold inlet and also added a 2 gallon expansion tank. I got conflicting info on what pressure to set the expansion tank to - our static water pressure is about 67 PSI. I was told to set it to 12-15 lbs below that so I pumped it to 55 PSI. Neither of these changes have helped.

The problem seems to stem from the fact that we have 2 "on-demand" recirc pumps in the house. These are the under-sink pumps where you push a button when you want to get hot water. A pump drives the hot water back down the cold water line until the water is hot, which saves wasting all that water. If I shut off the valves to *both* of these units, the problem goes away. If I leave the valves to either of them open, the problem happens. One is a more powerful "Taco Genie" unit; the other is a residential "Chili Pepper" unit. Just to clarify - we do NOT have the RUR continuous recirc model.

- Would changing the pressure on the expansion tank to match our static water pressure help?
- Any other suggestions?

Thanks so much for any help.

rick in Alaska

The expansion tank is not going to do any good for you as near as I can tell.
It sounds like you might possibly have a cross connection or a bad check valve. I would try running the water heater until it has hot water flowing out of it, and then stop the hot water flow. Then I would have someone turn on a cold water tap and see if the cold water line coming out of the water heater feels like it is getting warm. There is a possibility the water is going backward through the water heater and making the turbine spin backwards. The hard part would be finding out how it is piped to determine how it is doing this.
Rick

poriordan

Thanks so much for the suggestions, Rick. We never noticed a problem when we had the Eternal heater and the plumbing hasn't changed since then so I don't think there's a cross connection. The fact that we have to shut off both of the recirc pumps to make the problem go away makes me think it's not likely to be a bad check valve as both of them would have to be bad. I I tried your experiment but did not get any sense that the water on the cold inlet was getting warm. I suppose I could put a check valve on the hot outlet of the water heater to confirm there's no backflow.

I should also say that whatever turns on does turn off after ~10-15 seconds even if the cold water keeps running. So it seems like some transient issue caused by the cold water turning on is the trigger. Also if I turn the cold on very slowly it avoids the problem.

rick in Alaska

Not sure what else to tell you without seeing how things are piped. I am wondering if there is just a little flow going through the water heater, but not enough to make it fire off. I suppose you could check that by getting the water heater hot again. Then run some cold water and see if the hot outlet pipe feels like it is getting cold.
Rick

poriordan

Thanks Rick. I did try putting a check-valve on the hot water outlet of the Rinnai but that didn't help. I think you are right - there must be a temporary draw on the hot line via the recirc pumps when the cold is turned on. Not sure what (if anything) can be done to address that - I will try calling Taco Comfort to see what they say about their recirc pump.

poriordan

Just to let everyone know - I have found a solution to half of the problem. The Chili Pepper unit has a spring-loaded check valve whose tension can be adjusted. I was able to tighten the tension on the spring so it no longer temporarily opens when the cold water is turned on. I have not had time to examine the Taco unit to see if it too is adjustable.