We replaced the hot water heater with a tankless version. We now only get warm water from the faucets and showers even though the temperature is set at 140 0F on the heater and the outlet pipe from the heater is very hot. Also when the cold water supply to the heater is turned off, we still get cold water flowing when we turn the hot water faucet on. So two questions 1. Why cant we get hot water from the faucets and showers 2. Something is causing cold water to be introduced on the hot water line

  • 1
    What is the manufacturer and model of the tankless heater? It is a natural gas fired heater, right? May 24, 2020 at 12:48
  • 2
    Do you have one of those gadgets that makes the hot water come on right away by circulating water through the system? May 24, 2020 at 12:54
  • Natural gas fired
    – chris
    May 24, 2020 at 16:32
  • Model number ECOH200DVLN-2
    – chris
    May 24, 2020 at 16:33
  • No there is no recirculation system to constantly keep the water hot
    – chris
    May 24, 2020 at 16:33

2 Answers 2


It could be a thermostatic shower faucet, where the backflow preventer is defect. Or a backflow preventer is defect/removed at any other place. That way the cold water is getting into the warm pipes/hoses, sometimes even if the shower faucet is closed. Normally the warm side has lower (dynamic) pressure, sometimes tankless heaters have a throughput reduction. Backflow preventers can be easily missed since they are available as small coin sized inlets for water valves.

  • This is what I am afraid of. For the sinks I am planning to shut off all the cold water valves and run only hot one at a time to eliminate the faucets as a source of problem. Then remove the cover plates and manually do the same with the shower faucets and see if I can locate the problem
    – chris
    May 24, 2020 at 16:38
  • Thank you for your hints
    – chris
    May 24, 2020 at 16:39
  • A standard non-thermostatic faucet normally does not mix both warm and cold water before the closing system, i.e. no water can mix resp. enter the other side if the faucet is closed. The mixing of water most likely is only due to the failing back flow preventer(s) of the thermostatic faucet(s), if any other reason is excluded. Behind the cover plate there are most likely 2 normal valves (warm/cold water side) which can be closed thus avoiding any mixing by failing BFP.
    – xeeka
    May 24, 2020 at 17:27
  • I turned all the cold supply valves for each sink off and started with the farthest sink. I turned the hot water on which it did get hot for about one minute and then it became warm. Followed the same process with other sinks and the same thing happened. I am almost ready to go back to a tank type hot water heater
    – chris
    May 25, 2020 at 1:35
  • The tank type boiler does only hide the problem since it can deliver much higher temperatures that will have to be mixed with cold water anyway. Removing covers of thermostatic faucets should not be a big issue if the service manual is followed. Normally they are made for quite easy exchange of critical components like BFP or sealings. And with the help of the internet, there might be even additional information or tips by other users.
    – xeeka
    May 25, 2020 at 11:18

What type of faucets do you have ? I like tankless but with thermostatic faucets it may be better to turn The temp down. Flow will increase on the hot side and will reduce the cold mix I think you are seeing. This as a problem , I found 120 a good temp and since tankless the only issue was to make sure the dish washer did heat the water to sterilize the dishes.

  • I will reduce the temperature and see if this will resolve the issue. Thank you!!!
    – chris
    May 25, 2020 at 1:30
  • All the faucets are Grohe including the shower faucets and the kitchen sink
    – chris
    May 25, 2020 at 1:31
  • Sorry all the faucets are Kohler. The shower faucets are model K-11748-KS-NA
    – chris
    May 26, 2020 at 3:14

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