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We recently brought a house that was built in 1927, and much of it has been updated. The previous owners put in a tankless cold water heater two years ago. Our hot water is very inconsistent.

When someone is in the shower for instance, its a big no-no to flush the toilet or turn on the sink. Ok. We can live with that. The problem is, however, even without someone using water elsewhere, the shower temp is likely to change any moment.

I have an even bigger problem in the kitchen, which was updated eight years ago. The water temperature used to change constantly when I was washing dishes, however in the last few weeks, I just never seem to get hot water to my kitchen sink. When hot water was coming to the kitchen, I'd have to let the water run for a good minute before I got hot water, but if pretty much let it run indefinitely now, it stays cold.

Does anyone have any suggestions of what this might be, and how I could fix it?

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The first thing I'd check is the temperature from the faucet closest to the hot water heater - run it for a few minutes and monitor the temperature. If it isn't stable then its the hot water heater. If it is then it could be an issue with the plumbing –  Steven Aug 17 '13 at 2:45
    
It would be useful to determine if the flow is variable or the hot supply temperature is variable. An old shower mixer valve is unlikely to be pressure balanced, and that would explain variable temperature behavior with the water heater working perfectly. Or maybe the water heater itself is misbehaving, but that's unusual unless the flow rate is close to the threshold at which it turns on. Some simple science experiments should indicate where the source of the problem might be. –  wallyk Aug 17 '13 at 7:07
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