Gas heater switches on with cold tap running

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gzt

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#323989 12-Feb-2026 16:45

I have a Rheem 20 instant. Since installation, when a cold tap is turned on it always runs for a few seconds and then turns off.

This is irritating and probably emits a puff of gas each time. With cold input appliances that do not use the hot water line at all now like the dishwasher and washing machine it will trigger frequently. I have minimised our gas consumption elsewhere so it's time to deal with this minor annoyance.

As I said it has always done this since installation. Is this a minor adjustment required to an existing input valve or do I need to install backflow prevention or non-return or something additional on the cold input?

richms

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#3461039 12-Feb-2026 17:10

Do you have a problem with water pressure dropping when things are turned on? The sense for starting continuous heaters is just when it sees pressure drop across it, so if there is a lowering of water pressure because you have bad supply or a poorly set up pump that would cause it.

Richard rich.ms

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#3461055 12-Feb-2026 18:10

Rainwater tank supply and a domestic water pump.

The led display on the water pump auto pressure switch shows "P55" when idle.

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#3461058 12-Feb-2026 18:30

My theory is adding a backflow preventer to the instant heater cold input will prevent any pressure drop occuring on the cold side when a cold tap is turned on. Technically, that's not what backflow preventers are intended for and dunno if it will work that way, but maybe it will help eliminate one possible cause.

After that if the issue continues to occur with less frequency then a remaining cause might be momentarily higher pressure on the cold side when a cold tap is turned off and the pump continues until whatever the cutoff is. No ideas how to mitigate if it's the latter.

WWHB

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#3461086 12-Feb-2026 21:03

You need a non return valve on the water inlet to your gas unit, when a tap is shut the shock wave travelling through the pipework is causing the unit to start up

Saor Alba

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#3462299 17-Feb-2026 18:32

Google tells me NZ mains pressure is between 2 and 5 bar. Pump controller has a bar pressure readout.

Pump controller default start pressure is 2.2. bar and that is active. Pump stop is based on flow rate. After detecting low flow the pump run stops with the pressure readout somewhere between 5 and 5.4 bar. That's all good.

Experimenting I can see the Rheem 20 does not activate when the pressure drops on the input side. That is good. I let off a little cold tap pressure and can get down to a steady 2.8 bar pressure with no Rheem activation. The Rheem seems to activate momentarily only when the pump turns on. On the other hand if I fully open a cold tap the Rheem does not activate - and then activates momentarily when I close the cold tap.

gzt

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#3462302 17-Feb-2026 19:10

WWHB: You need a non return valve on the water inlet to your gas unit, when a tap is shut the shock wave travelling through the pipework is causing the unit to start up

With the pump and Rheem turned off I made hot water from the elevated shower come out of the cold tap outdoors.

Bonus points for reverse flushing some crap out of the line. Anyway..

That proved it does not currently have non-return or backflow prevention imo. The Rheem 20 manual states in three places "a non-return valve must not be installed" on the cold water line. So, that kind of approach is firmly ruled out by the manual.

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RunningMan

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#3462305 17-Feb-2026 19:17

It's probably detecting the small amount of flow through it as the pressure of the pump increases on the cold side.

Is there a way of reducing the pressure variance?

tweake

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#3462306 17-Feb-2026 19:18

if its the shock wave triggering it, try installing a water hammer arrestor(s) on the pipes.

WWHB

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#3462307 17-Feb-2026 19:25

gzt:
WWHB: You need a non return valve on the water inlet to your gas unit, when a tap is shut the shock wave travelling through the pipework is causing the unit to start up

With the pump and Rheem turned off I made hot water from the elevated shower come out of the cold tap outdoors.

Bonus points for reverse flushing some crap out of the line. Anyway..

That proved it does not currently have non-return or backflow prevention imo. The Rheem 20 manual states in three places "a non-return valve must not be installed" on the cold water line. So, that kind of approach is firmly ruled out by the manual.

I am a plumber/Gasfitter and this is a know problem on both Rheem and Rinnai continuous flow units. I have done training with Rheem and they brought this up and talked about adding a non return vale to the unit, this valve works on either the hot or cold just make sure it’s installed in the correct direction.

unfortunately I can’t seem to upload the image, Rheem prefers it on the hot outlet but this fix will work on either

Saor Alba

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#3462308 17-Feb-2026 19:25

WWHB:, when a tap is shut the shock wave travelling through the pipework is causing the unit to start up

It does seem related to water shock wave in some way aka water hammer.

There is no massive hammer problem. But, for a long time I have heard an annoying pipe movement in an internal wall when a tap is turned on or off sharply. It is annoying because I'm guessing the Buteline pipe in there it won't last forever that way.

I'm thinking adding a pressure vessel to the pump near the output will smooth that out and maybe might address the Rheem activation annoyance at the same time.

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#3462309 17-Feb-2026 19:28

WWHB: I am a plumber/Gasfitter and this is a know problem on both Rheem and Rinnai continuous flow units. I have done training with Rheem and they brought this up and talked about adding a non return vale to the unit, this valve works on either the hot or cold just make sure it’s installed in the correct direction. unfortunately I can’t seem to lad the image, Rheem prefers it on the hot outlet but this fix will work on either

Wow ok thanks!!

Geekzone requires an image upload of some kind before the link will appear. I think pasting an image directly using Compose Reply box can also work, assuming it is converted automatically to base64 there or something like that..

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#3462310 17-Feb-2026 19:30

Nuisance Start-up

Continuous Flow water heaters may start operation when cold water is used within a building. Working pressure changes in the system pipework may cause water movement through the CF unit. If the inlet water turbine detects a flow equivalent to 2 litres per minute the combustion fan will start. This issue is a nuisance and not ideal but is not a safety concern.

A single check valve installed on the hot water out (Preferred position) in the direction of water flow will prevent water movement during cold water use to overcome the issue.

NOTE: preferred position of check

ALTERNATIVE

valve is on Hot water outlet but may also be installed on the Cold water inlet

this is copied and pasted from my Rheem training booklet

Saor Alba

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#3462311 17-Feb-2026 19:50

Awesome thanks I wish that note was easily available from the Rheem site. I may need to show something from Rheem to my plumber if they have any doubts.

I'm inclined to get a check valve with a bypass in case someone feels the need during future maintenance.

If time allows sometime can you recommend one with bypass, and one without?

Shindig

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#3462468 18-Feb-2026 07:27

This happens when the washing machine is running, I'll be outside near the vicinity of the Rheem 27 and would hear the unit spark up and then stop.

Thanks for the replies, going to sort this bug bear out as it's a waste IMO.

Also, I was tempted to remove the hot water pipe to the washing machine. I made sure when purchasing the unit I bought a model that heated the water up itself as I had read it was more efficient to do so.

Thank you

The little things make the biggest difference.

gzt

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#3462477 18-Feb-2026 08:32

Shindig: Also, I was tempted to remove the hot water pipe to the washing machine. I made sure when purchasing the unit I bought a model that heated the water up itself as I had read it was more efficient to do so.

Accurate comparison does depend on how your usual hot water is heated, and if you're paying a different price for hw.