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Wed Nov 28, 2012, 02:17 PM

Issue with a Rinnai Tankless water heater

We had one installed recently, and noticed that it turns on for a few seconds any time water is used - even cold water. When a toilet is flushed even. Sometimes it just revs a few seconds when no one is using any water.

I've called the installer about this, and he's stumped.

Any ideas?

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Reply Issue with a Rinnai Tankless water heater (Original post)
ehrnst Nov 2012 OP
OffWithTheirHeads Nov 2012 #1
ehrnst Nov 2012 #2
Warpy Nov 2012 #3

Response to ehrnst (Original post)

Wed Nov 28, 2012, 02:48 PM

1. Well, lets start with the obvious.


I assume your heater has a flow sensor of some sort that tells it to start up when it senses water flow. In theory, your hot water piping is completely isolated from the cold water side. Since your heater tries to fire when you use cold water, I'm guessing there is some kind of crossover between the hot and cold systems that shouldn't be there.

Obviously, I'm not there to actually look at your system, but, thinking logically, I suspect that at one time your old water heater had some kind of recirculating system to provide hot water whenever you turn on the hw tap. My house has this. In Arizona where water is precious, some think this is better than having to wait for the cold water to flush through the pipes before the hot water gets from the tank to the tap. One way or another, your heater is sensing water flow when it shouldn't which tells me that, somehow, the separation between your hot and cold water piping is compromised.

That's my two cents anyway.

Edited to add, "even when no one is using water" do you have an icemaker? A drip irrigation syster on a timer? Anything else that might use water without a concious act by you?

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Response to OffWithTheirHeads (Reply #1)

Wed Nov 28, 2012, 04:20 PM

2. thanks for the info!

We just moved back into the house and the only things that are hooked up to water right now are the bathrooms - with new toilets that we made sure weren't leaking. The tank revs at 4:00 am when all are asleep.

The remodel included all new plumbing since the old bathroom and kitchen were demoed and moved.

I'll contact the GC.

Thanks again!!!

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Response to OffWithTheirHeads (Reply #1)

Thu Nov 29, 2012, 12:15 AM

3. or the split could be very close to the heater

so a change in water pressure can cause the unit to cycle.

Other than that, I got nothin.

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