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Hot water issue in shower

ab123 Member Posts: 4
We get hot water from oil burner. (Not hot water heater and not tankless). In the winter, when the heat is on and running through the baseboards, we noticed the hot water in the shower seems to stay consistently hot. But when the heat is not running through the baseboards on warmer days, the hot water in the shower doesn’t seem to last. It starts out hot but doesn’t seem to be enough for a full shower. We have a single handle in the shower for cold/hot. We haven’t noticed a problem with the hot water from the kitchen or bath faucets. What could the problem be?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,378
    So you have a tankless coil in the boiler?
  • ab123
    ab123 Member Posts: 4
    Yes, I think we have a tankless coil.
  • mattmia2
    mattmia2 Member Posts: 8,874
    or do you have a tank heated by the boiler?
  • ab123
    ab123 Member Posts: 4
    We have an expansion tank, is that the same thing?
  • mattmia2
    mattmia2 Member Posts: 8,874
    No. that is just to allow the water in the closed boiler system to expand. (or a domestic hot water system with a check valve).

    Pictures of the boiler would tell us, but does your domestic hot water come from some pipes that connect to a plate on the side of the boiler?
  • ab123
    ab123 Member Posts: 4
    Pictures attached.

  • psb75
    psb75 Member Posts: 756
    Your tankless coil may need flushing/demineralizing with a cleaner.
  • pugdad
    pugdad Member Posts: 23
    Possibly bad mixing valve . Low limit set to low or possibly the shower valve needs rebuilt 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,753
    edited March 2022
    pugdad said:

    Possibly bad mixing valve . Low limit set to low or possibly the shower valve needs rebuilt 

    I agree with @psb75 But you are not set up for the needed maintenance. There is no mixing valve. I will follow with an explanation why you need one below You also need to add a boiler drain on the cold inlet to flush the chemicals needed for maintenance. And you might want to add isolation valves to keep the chemicals from entering the potable water system during the cleaning procedure. See the illustration

    If you are a DIY plumber, you can easily do this yourself. If you are not, you need a Pro that is a littler smarter that the average plumber (like smarter than the original installer) to get it right. If needed I can also explain that.

    End of necessary info. If you want to understand why the mixing valve is necessary, read on:

    The control logic or sequence of operation of a tankless coil water heater is as follows
    1. Maintain a minimum boiler temperature in order to provide sufficient temperature transfer from the boiler water to the potable water (or Domestic Hot Water/DHW) in the coil at a given flow rate. There is a specified transfer rate based on the coil capacity rating (in your case, about 3 gallons per minute at a 50° temperature rise)
    2. Allow the burner to operate at temperatures higher than said minimum when needed for space heating.
    3. Shut off the space heating circulator if the space heating return water causes the boiler temperature to drop near or below the minimum DHW needs


    4. In a situation where the boiler is operating for heat and the high limit is set at 180° (so the boiler water around the DHW coil is 180°) AND there is no DHW tap open, the water in the DHW coil can get to 180°.
    5. if someone is in a shower and turns on the hot water, the first few seconds of water from the shower head will be the cool temperature in the piping, then the 180° water will flow out of the shower head for up to 10 seconds. This can cause 3rd degree burns on the skin in that short time.
    6. Once the water flow is established, the transfer rate of the boiler heat to the DHW in the coil takes over, and the shower head water may deliver 140° water to the shower occupant. this can cause 3 degree burns if exposed for 30 seconds.
    7. The reason these temperatures are this hot are because the desired 110° to 115° are delivered at the 3 GPM flow rate when the boiler water temperature is at the 150° that is set by adjusting the Low Limit setting of 160° (that is 160° less the 10° minimum differential of the control)
    8. So any temperature that the boiler may be above 150°, as a result of normal operation, will show up as a higher DHW temperature.

    Since at any given moment in the heating system cycle from no call for heat to operating at the high limit temperature, the water in the boiler can change, Your system REQUIRES a mixing valve. By adding a mixing valve (as shown in the manufacturer's installation instructions) the hot water leaving the DHW Coil will be "MIXED" with cold water to maintain a constant 110° or 115° temperature. The mixing valve will adjust the amount of cold water required to maintain the desired water temperature.

    Here is the mixing valve recommended piping from the manufacturer of your heater.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Larry Weingarten