WTF is going on?

13 Mar 2016
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United Kingdom
Hello all -
Some of you may have read my previous posts about my woes relating to my knackered central heating system! Those of you who have previously replied to me will probably be glad to hear that I have a heating engineer coming tomorrow to take a look...

Something odd has happened however in the last couple of days. We are having some demolition work done and in the process of this a central heating pipe was whacked and there was a slow leak from the system for a couple of hours before I could cap it off. Following this the system clearly had some air ingress as I had to vent the pump several times and manually release air around the 3 way valve ( open vented Y-plan system). The pump seems to be circulating ok again now.

Heating seemed to work ok for 24 hours after - ie not great but as good as its been since November. The following day boiler cycling again and minimal heat getting to rads.

Then today - even when the thermostat is set to max (heating and DHW cylinder) the pump will come on for 5 minutes or so, no heat comes round and then the pump promptly switches off again even though the controller appears to be calling for heat still.

I am confused by this as I thought all the time the system is calling for heat the pump at least should stay on, even if the boiler isnt actually firing for some reason. The pump has no 'feedback' from the boiler does it ? As in, if the boiler isnt heating, the pump doesnt 'know' and turn itself off? I know some boilers can take an input from a pump (what for exactly Im ignorant of) but our pump only has a live, neutral and earth connected to it and I cant see how it could get a signal back from the boiler to tell it its faulty and therefore no point running the pump....

Whats even more odd, is that if I change to CH od DHW to off, and then move it back to continuous on the programmer (basic 2 channel Honeywell) then it doesnt restart. But if I turn the power supply to the whole system (ie programme, boiler etc) off and then on again, then switching either to continous on starts the pump up before promptly turning off..

I really am at a loss as to understand this and am happy to be gently led by the hand of a hopefully competent heating engineer tomorrow towards the sunny uplands of functional central heating - however Id rather have some vague understanding of what this latest issue might be related to!!

Any bright ideas? In case anyones interested about my travails please see my previous recent posts. I wont bore anyone by retreading old ground..
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Air lock stopping the flow. Can you turn the pump speed up or down a bit?
Thanks - i did that - no air noise now all day from the pump - was rattling away yesterday.
Bizarrely after turning boiler off for a while and then resetting it it seems to be working again as before.

I dont really understand how the electrics all inter-relate it would seem...
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Boiler is pretty simple. There is a call for heat from the controller and the circulation pump starts diverting the water to either the hot water, central heating or both. It all works fine unless the boiler temp exceeds the limit. In which case it shuts down. I suspect you were not getting the water to circulate properly and the boiler temp exceeded the limit. Air in the pump will do this

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