CH with HW Demand on Combi

Discussion in 'Plumbers' Talk' started by fred812, May 6, 2021.

  1. fred812

    fred812 Active Member

    Thought I had asked this before but can't find it.

    So the CH is on and running, somebody has a long shower, does the combi do both or does it divert entirely to the HW. Does the CH pump still run?
  2. pppmacca43

    pppmacca43 Super Member

    Hot water will take priority
  3. fred812

    fred812 Active Member

    Appreciate that but does the central heating pump still circulate the water through the rads even though its not being heated? Just wondering.
  4. exbg

    exbg Active Member

    No. Any combi can only do one or the other, and water takes priority. The pump (in most cases) will be used to circulate water through the hot water heat exchanger

    Ths is important to realise if quoting for a combi in a commercial environment. If the business uses a lot of hot water, say a caterer or it has a large waiting room where a bathroom may be in constant use.
  5. Bob Rathbone

    Bob Rathbone Screwfix Select

    The only time the combi will circulate hot water through the radiators during DHW heating, is if the output of the primary heat exchanger exceeds the sink capacity of the DHW, for instance, if the secondary heat exchanger is blocked. This will not happen with all models.
    pppmacca43 likes this.
  6. fred812

    fred812 Active Member

    Ok thanks, hadn't appreciated that the same pump was used to move water through the DHW heat exchanger. Thought that was done by mains pressure.
  7. Jimbo

    Jimbo Screwfix Select

    The mains pressure pushes cold was through the boilers plate heat exchanger yes. But the heating circuit waters travels in a loop between the main heat exchanger with the burner within it, and the plate heat exchanger, via the pump.

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