I'm looking at a P&ID of a heating circuit. The burner/boiler is non-modulating. Red is flow/hot, blue is return/cold. AFAIK the pump is fixed speed (no FI).

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As far as I understand the control logic, the temperature sensor on the return line (or rather the control circuit it's a part of) will open the three way valve and mix hot water into the return, and close when the return flow temp is warm enough. The boiler delivers a constant $\Delta T$, controlling for return temperature ensures that the flow temperature is correct.

  • Is this control scheme likely?
  • Why not control for flow temp. directly?
  • $\begingroup$ Please do spell out technical abbreviations, to help others to find this question: people won't find this question by searching for "FI" or "TC" as they'll get too many irrelevant hits - they'll use the full terms. $\endgroup$
    – 410 gone
    Commented Feb 3, 2016 at 7:14

1 Answer 1


Typically a three way valve is used in these systems to maintain minimum flow through the boiler. There are many ways of configuring such a system but the objective is to maintain flow through the boiler rather than allowing it to cook itself.

You can use the return temperature to modulate a three way valve to either direct more flow to the load (on high return temperature) or more flow through the boiler (on low return temperature).

Note that the boiler will also need to be switched off on high inlet temperature to prevent damage.

  • $\begingroup$ The flow rate through the boiler is determined by the pump, do you mean something else? i think the second paragraph is exactly backward. $\endgroup$
    – mart
    Commented Feb 3, 2016 at 11:46
  • $\begingroup$ Sorry, yes I had it the wrong way round in the second paragraph, I've edited that now. Yes the pump will dictate the flowrate through the boiler in the schematic you have given, the three way valve is just at the opposite side to what I'm used to seeing. As long as the valve purchased is designed for that flow direction then that should be fine. $\endgroup$
    – dhewett
    Commented Feb 3, 2016 at 11:57
  • 1
    $\begingroup$ Also I noticed I hadn't answered your second question. I believe the control is based on the return temperature as this will be the first to dip as the demand increases. $\endgroup$
    – dhewett
    Commented Feb 3, 2016 at 11:59

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