OK, this is a weird one. Follow the bouncing ball:

  • I have a tankless water heater
  • I've had problems the last few weeks with the water running lukewarm instead of hot. Someone suggested power-cycling the water heater, and I did, and the problem went away for a couple weeks, then came back. Power-cycling didn't work a second time.
  • Following suggestions found in the trouble-shooting guide, and various YouTube videos, I decide to clean the water filter.
  • I turn off the power to the heater, the gas, and the cold water supply.
  • I turn on the kitchen hot water to relieve pressure, water flow stops, and I close it again.
  • I open the bathroom hot-water tap just for good measure. I close it again.
  • I remove the water filter from the heater. There's a brief spurt and then nothing.

So far, everything is as expected. Then …

  • I go inside, turn on the cold water in the kitchen, and clean the filter.
  • I go back to the water heater, and water is gushing out where I removed the filter.
  • I go back inside, double-check that all the taps are turned off. No help. I double-check the cold water valve to the heater; still off.
  • I turn off the main water feed to the house so I can put the filter back in.
  • I turn the main water feed back on.
  • Hot water taps in both kitchen and bathroom are now providing cold water when I turn them on.
  • Not knowing what else to do, I put everything back and come here.

This looks to me like somehow there's a cross-over between the hot water and the cold water in the house. That would certainly explain the lukewarm water problem as well.

For the record, the guy who originally installed the heater was a general contractor but not a plumber, and he did originally install a cross-over which we discovered and made him remove. But that was two years ago and there haven't been any problems since.

So how could a cross-over spontaneously appear? Could there be some sort of pressure-relief mechanism that failed? Is there such a thing? Why would it be installed?

Edit: If Chris H is right, and I suspect he is, the kitchen faucet is letting cold water into the hot water system.

When the water filter on the heater was somewhat clogged, there was a pressure drop coming out of it, and so the cold water from the kitchen faucet was able to dominate the hot water from the heater. Cleaning the filter shifted the balance enough that I can get water hot enough to shower, but the problem isn't really solved.

And the problem in the kitchen faucet is intermittent enough for the problem to come and go.

plumbing diagram

  • Annnnnd, I just took a shower and the hot water is just fine. I'm at a loss. (I also don't expect it to last) Jul 25, 2016 at 1:22
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    A non-thermostatic mixer valve can make things tricky. If this is combined with a missing or malfunctioning one-way valve, it won't help.
    – Chris H
    Jul 25, 2016 at 7:23
  • The kitchen faucet is the kind with a single lever that goes side-to-side to adjust the temperature and forward-and-back to change the volume, and it was the last thing I used before the problem happened this morning. I did suspect it was the problem, and fiddled with it, with no improvement. Maybe I need to go take a second look. Jul 25, 2016 at 8:17
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    Edward did you ever figure out the problem? I have installed quite a few more on demand units since answering this none had any “electronics” but all had flow / differential pressure switches and I have found mixing valves in the shower to be a problem 2x ( back then I had not seen mixing valves in the kitchen sink but have since, so any mixing valve could cause problems. This is the only other problem than build up in the valve I have seen other than a failed diaphragm on the flow valve that allowed water in the gas side and was not serviceable. The flow valve was part of gas safety circuit
    – Ed Beal
    Jan 19, 2020 at 5:33
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    Edward, what was the eventual outcome of this? I have a similar problem. May 30, 2020 at 13:30

1 Answer 1


Power cycling a tank less gas water heater? Yours must have electronics that I haven't seen on the models I have installed. There are demand valves in the water heater that sense the flow. These valves can become sticky with lime & rust scale buildup. The reason to turn the water off then open a hot water faucet is to make a drastic change and hopefully the the scale will break loose.Most faucets don't have mixing valves these are usually found in shower / bathtubs that could cause part of this problem. I believe it is more likely the water heater is not turning on correctly or sensing the flow. Some units can be cleaned by disconnecting and pouring CLR (Calcium,Lime & Rust remover) into the inlet and flushing with fresh water. If the sensing valves directly regulate the Gas flow like on a Bosh aqua star you need to get it cleaned out before the seals fail because they are not repairable and the system will have to be replaced.

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    But any single-lever faucet will allow backflow from cold to an unpressurized hot, as I noted in a comment to the OP. Jul 25, 2016 at 15:15
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    All the modern tankless gas water heaters I have seen (about 20 models) have a microprocessor and electronics to sense the output water temperature and adjust the flame to achieve target temperature. Other functions include running the fan to cool it once the water stops flowing, positive ignition sensing, digital temperature setting, etc.
    – wallyk
    Jul 25, 2016 at 16:48
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    Yes, power-cycling the heater seemed like a shot in the dark, but it also seemed to work. That was probably a coincidence though. The fact that I was able to shut off the cold water inlet to the hot water tank and still get full flow out of all the hot water taps in the house definitely pointed to something letting cold water cross into the hot-water pipes. The fact that it didn't happen at first, but did happen after I used the kitchen faucet points to that faucet as the culprit. Jul 25, 2016 at 16:48
  • Yes to wallyk's comment. I figured there was some sort of sensing electronics and likely a microprocessor at work. That's what made power cycling seem plausible. Only the fact that the problem came back so soon made me look elsewhere. Jul 25, 2016 at 16:50
  • My kitchen faucet is a Kohler, similar to a K-99259 so it does have a mixing valve. Jul 25, 2016 at 16:53

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