Rinnai V75i Water Heater - Hot water in other rooms, but not the master

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Hello all

We just purchased a home a year ago and they installed the Rinnai V75i tankless water heater. We like our water a little hotter, so we had already increased the temp to the 140 max per the instruction manual a while back. Everything worked great.

In the last few days our master shower has been lukewarm, while our other sinks/showers are hot. For the life of us we cannot figure out why.

FYI - We did have a pool put in and a gas meter upgrade. The issues started after this.

Any suggestions are greatly appreciated!


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Rochester NY
Do you have a single-handle shower valve? If so you probably just need to change out the shower valve cartridge. There is a rubber diaphragm inside there and it can wear out and develop a hole or a tear. This allows too much cold water to mix in with the hot & it then comes out lukewarm. Some other symptoms you might notice are that the cold water will also come out lukewarm instead of cold; or that you sometimes find unexpectedly warm water coming out the cold line in the nearest sink faucet when you first turn the sink on. Replacing the shower valve cartridge can be a "intermediate level" DIY if you're handy (although some shower valves can be pretty tricky, it completely depends on the model#).


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Ontario, Canada
Agree with Phog.

The issue should not be the gas supply as the water heater is operating as it should to deliver hot water to every faucet other than your shower. Since only the shower is affected, there must then be an issue with either the control valve for that shower, or the feed which supples that shower.

Inspect the supply pipes feeding the shower to look for a partially closed or defective isolation valve which is restricting the flow of hot water to the shower control. Alternately, if there is a mixing valve to reduce the temperature supplying a nearby faucet, that could be sticking open which may cause cold water to mix with the hot supplying the shower.

Although you didn't mention if your home is equipped with a hot water recirculation system to speed up delivery of hot water to faucets, if so, the check-valve maybe stuck open which is allowing the cooler water in the return line to backflow your shower control.
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