So let's say you set your thermostat to 20 degrees centigrade, and the current room temperature is 10. A flow of water will gush into the radiators. What I'm wondering, is this flow constant the whole time, or does it slowly descend as the room temperature approaches 20 degrees?

Another way to put it, in the same scenario as above, will the room temperature get to 20 degrees faster if I put the thermostat on 30 degrees? Or will it take the same time?


1 Answer 1


A hot water based central heating system is fed by a pump and the only signal from the thermostat is heat/don't heat.

The temperature regulation is done by duty cycling the pump so that when the temperature is low the pump will be started and when it's high the pump will be stopped. There will be some hysteresis of a few degrees to reduce the number of times it will cycle.

So no, changing the temperature only changes how long the pump will be active until it turns off again.

  • $\begingroup$ I'll just add that this might not be true everywhere. It is definitely possible to set up a system that uses flow control and variable speed drives to modulate the flow through the radiator. In most buildings you wouldn't see that. I'd expect it to be a bit more likely in systems that are newer and have a focus on efficient design. AFAIK it's more common in radiant in-floor heating systems, which are generally more modern anyways. $\endgroup$
    – JMac
    Commented Mar 8, 2017 at 17:23

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