Vaillant Ecotec Plus slow to fire up

28 Nov 2004
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United Kingdom
A strange problem - my central heating, although working, does not seem to be doing so correctly.

The call for heat comes from a Drayton Digistat receiver. It shows all the correct lights (eg green for heat required) and I can clearly hear the relay clicking when I turn the RF thermostat control up or down. So I assume that is passing instructions to the boiler.

The boiler fires up initially and heats the rads, but it is when it reaches its set temperature that it seems to be behaving erratically.

I have set the central heating temp to 55 degrees. The display will go beyond this by a few degrees before shutting off. It then goes down in temp, sometimes into the 30s before firing up again - seemingly ignoring the green light on the digistat. The display is flashing the radiator symbol to show it is in central heating mode.

I have only recently observed this behaviour, so it could have been doing this for a long time. Maybe it is behaving as it should! But my understanding was that if the Digistat calls for heat and the water temp is below that set, then the boiler should fire up.

All suggestions gratefully received! (NB the boiler is serviced annually and was last done a couple of months ago)
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If the heating demand is lower than the boiler's minimum output then the flow temperature will start going up and the burner will trip at SP+5C, in your case 60C, it shouldn't trip at 62 or 63C so check this, when it does trip the circulating pump continues to run until the anticycle time has timed out, the boiler temp will go down to 30C or so as the rads are still emitting heat.
Thanks, John, it seems that my limited knowledge is seeing a problem that doesn't exist. Your clear answer makes perfect sense - and thus the boiler is working as it should.

Do appreciate the time you have taken to share your knowledge - many thanks!

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