Tankless water heater runs when toilet stops flushing

[ChuckDIYHack]

Hi,

I recently had a Rinnai tankless water heater (TWH) installed. It runs unexpectantly for a few moments whenever:
1) any toilet is flushed or
2) a cold water faucet is shut off.

Other observations:
3) It doesn't run while the toilet is flushing or while a cold water faucet is running, only when they stop.
4) If the cold water is shut off slowly, it doesn't run.
5) When the flushing stops, the unit's flow sensor indicates a 0.8 gal/min flow. During the flush this reads 0.0 gal/min. This is the same when a cold water faucet is on and when the hot water is off.

Install context:
1) The TWH does not have a recirc pump.
2) There are no recirc pumps in the house.
3) We do have single handle faucets so there is mixing in those faucet valves.
4) The default turn on mode is low flow just incase there is a recirc pump.
5) The only setting changed was for the hot temp to go to 125F.
6) I think city water pressure is 80 psi.
7) Water pipes are 3/4 inch.

Replies from Rinnai:
1) Rinnai engineering has replied that there is something wrong with the plumbing system. They also said it is causing excessive wear and I should get it looked at.
2) One chat said I needed a check valve and surge pot (thermal expansion tank?)
3) Another chat said I need a pressure valve. When I explicitly asked, he said not a pressure regulator. ( I was not aware of a pressure valve). I don't like the idea of this as it would open up every time the cold water was shut off.

Installer reply:
When I asked the installer what was going on, he called Rinnai and said it was purging. I think he didn't understand the context.

What's going on?
What are some solutions?

I think it is water hammer and the pressure wave coming back through the pipes is triggering the TWH to come on?

Thanks,
Chuck

 

[GReynolds929]

Sounds like water hammer is creating small back flow activating the water heater for a second. Rinnai is correct that this will cause excess wear. Need a pressure reducing valve, hammer arrestors, or both. Water pressure should be less than 80 psi.

It has nothing to do with purging, a check valve would be useless in this case unless it is upstream of the water heater, and even then I wouldn't recommend it.

 

[Sponsor]

 

[Reach4]

If you put in a water hammer arrestor, you prefer to put that close to the fast valve. https://www.supplyhouse.com/Sioux-C...F-Swivel-Ballcock-Nut-x-M-Ballcock-Thread-Tee would go right under the toilet tank between the flex connector and the input to the fill valve.

There are also fill valves that close more slowly. Korky calls their valve "quiet fill" but I am not sure how much they slow the close.

https://www.supplyhouse.com/Sioux-C...on-x-3-8-O-D-Female-Compression-Tee-Lead-Free would go between the stop valve and the flex connector, if your valve has the common 3/8 compression output.

It may be sufficient to just partially close the supply stop valve.

 

[Fitter30]

Valve the gas off, open the bath tub on full hot. Gpm still should still read. Measure gpm with a know bucket size stopwatch. Have some one shut the tub off. See what happens to the gpm . Could be a bad flow sensor.

 

[ChuckDIYHack]

Sounds like water hammer is creating small back flow activating the water heater for a second. Rinnai is correct that this will cause excess wear. Need a pressure reducing valve, hammer arrestors, or both. Water pressure should be less than 80 psi.

It has nothing to do with purging, a check valve would be useless in this case unless it is upstream of the water heater, and even then I wouldn't recommend it.

I put in a PRV and set it to 60 pounds. It did not fix the problem. I'm sure it reduces the water hammer effect.

 

[ChuckDIYHack]

If you put in a water hammer arrestor, you prefer to put that close to the fast valve. https://www.supplyhouse.com/Sioux-C...F-Swivel-Ballcock-Nut-x-M-Ballcock-Thread-Tee would go right under the toilet tank between the flex connector and the input to the fill valve.

There are also fill valves that close more slowly. Korky calls their valve "quiet fill" but I am not sure how much they slow the close.

https://www.supplyhouse.com/Sioux-C...on-x-3-8-O-D-Female-Compression-Tee-Lead-Free would go between the stop valve and the flex connector, if your valve has the common 3/8 compression output.

It may be sufficient to just partially close the supply stop valve.

I wondered if these were available. Thanks for the link.

 

[ChuckDIYHack]

I have called the installer to come out because the manufacturer has stated there is a problem.

There was no check valve installed nor a surge pot (thermal expansion chamber?) nor a hammer arrestor.

 

[ChuckDIYHack]

The installer put in a check valve and an expansion tank on the cold water line going to the heater. That fixed the problem.
Rinnai had told them there is probably a leaky mixing valve and to install the check valve.

Probably every house has a leaky mixing valve.