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Old 02-18-2018, 08:24 PM   #1
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Knocking sound from the thermal bypass valve


I recently installed a tankless water heater with a recirculation pump. As part of the recirculation feature, I installed the thermal bypass valve (aka sensor valve by Watts) under the sink at the farthest bathroom away (also most frequently used) from the water heater. The valve connects the hot and the cold water inlets. When the recirculation pump is off, I get some light hammering sounds from the bypass valve when I turn the cold or hot water taps off. How can I get rid of this problem? This problem goes away when the pump is turned on. And also I didn't have this problem before I installed this bypass valve.

I have tried changing the bypass valve, using hammer arresters (putting them near the shutoff valves or near the tap), and lowering the water flow with a new aerator. They helped only slightly.

There's a spring inside the bypass valve and I suspect it is being opened and closed inadvertently when the water pressure increased momentarily due to the hammering.

Any help would be greatly appreciated!
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Old 02-19-2018, 08:40 AM   #2
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Re: Knocking sound from the thermal bypass valve


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And also I didn't have this problem before I installed this bypass valve.
Welcome to the forums hannes.

In the instruction manual for water heater, does it recommend it or is it optional?

If it's optional, leave it off. As you stated, the hammering started happening when you installed it.

The plumbers will be along with more advice shortly.
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Old 02-19-2018, 10:22 AM   #3
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Originally Posted by HannesAlven View Post
And also I didn't have this problem before I installed this bypass valve.
Welcome to the forums hannes.

In the instruction manual for water heater, does it recommend it or is it optional?

If it's optional, leave it off. As you stated, the hammering started happening when you installed it.

The plumbers will be along with more advice shortly.
Thanks! It's required according to the instruction manual.
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Old 02-19-2018, 12:14 PM   #4
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Re: Knocking sound from the thermal bypass valve


I've never used this type of setup. Usually places with recirc pumps have a dedicated recirc loop. From reading the instruction manual, I don't think there's much you can do.

Cheers!
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Old 02-19-2018, 12:24 PM   #5
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Re: Knocking sound from the thermal bypass valve


What is the recirc model # and type of heater? Describe your system better, please
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Old 02-19-2018, 02:56 PM   #6
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Re: Knocking sound from the thermal bypass valve


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What is the recirc model # and type of heater? Describe your system better, please
I have a Rinnai RUR98iN tankless water heater. The recirculation pump is inside the unit (I didn't buy it separately). It's installed in the garage. I don't have a dedicated return line for the hot water so the cold water line is used for recirculation. The thermal bypass valve connects the hot and the cold line together to complete the recirculation loop. The bypass valve is installed under the sink upstairs in the bathroom farthest away from the water heater. The bypass valve is normally open and it closes when the water reaches above 98F.
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Old 02-19-2018, 03:00 PM   #7
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Re: Knocking sound from the thermal bypass valve


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Originally Posted by HannesAlven View Post
I have a Rinnai RUR98iN tankless water heater. The recirculation pump is inside the unit (I didn't buy it separately). It's installed in the garage. I don't have a dedicated return line for the hot water so the cold water line is used for recirculation. The thermal bypass valve connects the hot and the cold line together to complete the recirculation loop. The bypass valve is installed under the sink upstairs in the bathroom farthest away from the water heater. The bypass valve is normally open and it closes when the water reaches above 98F.
I have talked to a plumber and I was told to install a pressure regulator to reduce the pressure to 55 psi (I don't have one). I am reluctant to spend more to do that because I am skeptical that it would fix the problem. My water pressure right now is at 70 psi.
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Old 02-21-2018, 04:14 PM   #8
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Re: Knocking sound from the thermal bypass valve


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Originally Posted by HannesAlven View Post
I have talked to a plumber and I was told to install a pressure regulator to reduce the pressure to 55 psi (I don't have one). I am reluctant to spend more to do that because I am skeptical that it would fix the problem. My water pressure right now is at 70 psi.
when the pump is not running...


1. Close the cold water stop valve below the sink with crossover.
2. Open the cold water side of faucet.
3. Water should still slowly flow from the cold outlet side.
This flow should gradually decrease to a trickle of water until finally
no water is coming from the faucet once hot water reaches the crossover valve.


If flow does not stop when water is hot, the crossover is defective.
once the hot water reaches crossover see if it continues to make sounds...
be sure to open back the cold stop valve below sink... and check cold side
Is the crossover valve secured with the ears and screws provided?

Lastly, close the hot stop valve & open the hot side of the faucet...
there should be no flow if internal check valve is operating correctly...
if flow or noises occur, the crossover is likely defective.

if all the above checks out then I agree that your
noise problem may be from too high a static pressure.

Edit: noise could also be coming from an internal check at pump

Peace
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Last edited by DR P; 02-21-2018 at 04:17 PM.
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Old 02-22-2018, 12:38 PM   #9
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Re: Knocking sound from the thermal bypass valve


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Originally Posted by DR P View Post
when the pump is not running...


1. Close the cold water stop valve below the sink with crossover.
2. Open the cold water side of faucet.
3. Water should still slowly flow from the cold outlet side.
This flow should gradually decrease to a trickle of water until finally
no water is coming from the faucet once hot water reaches the crossover valve.
Yes, I did this and the cold water from the tap stopped almost immediately.

Quote:
Originally Posted by DR P View Post
If flow does not stop when water is hot, the crossover is defective.
once the hot water reaches crossover see if it continues to make sounds...
be sure to open back the cold stop valve below sink... and check cold side
Is the crossover valve secured with the ears and screws provided?
There are no hammering sounds with the above experiment. Yes, I checked and the crossover valve is secured.

Quote:
Originally Posted by DR P View Post
Lastly, close the hot stop valve & open the hot side of the faucet...
there should be no flow if internal check valve is operating correctly...
if flow or noises occur, the crossover is likely defective.
Yes, I did this and the hot water from the tap stopped almost immediately. There were no hammering sounds.

Quote:
Originally Posted by DR P View Post
if all the above checks out then I agree that your
noise problem may be from too high a static pressure.

Edit: noise could also be coming from an internal check at pump

Peace
When the recirculation has stopped running for a while (>15min, not quite sure), the hammering sound comes back. It seems like the hammering sound comes back when the hot water in the pipe has cooled down past 98F and the crossover valve opened up. When recirculation is running, there's no hammering sound.

I may just live with this since it's not a big problem. Thanks for the help!
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