Rinnai v65 no hot water when second cold faucet open

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Santa Barbara, CA
I have had a Rinnai v65 for years. I just replaced it with a brand new one because I kept having problems with cold water during showers. It worked fine as long as the only water running was the shower. If the toilet was flushed or the kitchen sink with even just cold water was on, it would start just putting out cold water and since the heater is in the closet behind my shower, I could hear the fan turn off. Because I assumed the water heater was old and dying, I replaced it.

With the new one the same thing is happening. I have the temp set hot at 135 F. If I turn on a cold faucet elsewhere in the house, it stops putting out hot water. I checked the water PSI into the heater and it is 70 psi, which I believe is what it is supposed to be. Since it is brand new and the same thing is happening, I guess this means my old one was probably working fine.

I decided to see what would happen if I turned down the temp to 130. It now works fine with one hot water faucet running and 1 cold water faucet running. I then checked what happens with two hot water faucets working at 135 F, and low and behold it works.

Does this make any sense? It looks to me like it does not like that the flow rate drops to the water heater when a cold faucet is open at the same time when heating to 135 F. Should I adjust the PSI limit device coming into the house to allow a higher PSI?


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Peace valley missouri
If the heater turns on jt seeing a flow. A cross connection, bad check valve. Must be getting hot water out of the cold side somewhere.
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