CH flow too great through HW cylinder circuit

Discussion in 'Plumbers' Talk' started by Davidws, Feb 6, 2022.

  1. Davidws

    Davidws New Member

    I have a Potterton Profile 80e driving CH and HW through a 3-port valve. On the HW side, there is an indirect cylinder with only a short circuit between boiler, pump, valve and cylinder. I have noticed that, when there is little heating demand and HW is not up to temperature, the boiler cycles on for only a max of 3-5 minutes at a time, without completing heating of the water.

    I can see that the HW is not reaching temperature because the grey lead on the motorised valve doesn't show full voltage for many cycles (ie the cylinder thermostat is not closing). The thermostat seems to be working OK.

    From observation, when there is no heating demand and the HW temperature is within say 10C of the thermostat setting, the return water from the cylinder is nearly as hot as the flow. I am guessing that this is what is then turning the boiler off prematurely (from a HW perspective).

    Is this a common problem? I haven't found much on the internet. Is this because the pump is too strong for the HW circuit alone? Should there be a flow limiter in the HW circuit, which would mean that more heat was extracted to the DHW, the return water would be cooler and the boiler may stay on.

    This can't be an efficient way for the boiler to be working or special for its longevity (although now 20+ years old). Yes, I know I could replace the boiler, fit an adaptive pump, etc but is there a simple (and cheaper) short-term solution? Will appreciate any suggestions or comments.
     
  2. Jimbo

    Jimbo Screwfix Select

    The cylinder stat should be about 5 or maybe 10 degrees below the boiler stat otherwise it will basically never be satisfied.
     
  3. Davidws

    Davidws New Member

    Thanks, Jimbo. The boiler stat is well up and the cylinder does get there eventually. I think the diifference is a good 10C.
     
  4. Mike83

    Mike83 Screwfix Select

    It’s not an efficient way for the boiler to work but things have moved on these days.
    Boilers now modulate unlike older boilers that simply turned on and off.
    Your boiler could start to cycle more than it did years ago due to being a bit older.
    Your boiler will fire and maintain full rate regardless of what load is required. This will cause the cycling.
    New boilers will modulate and reduce the load as the return temperature increases.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice