CH not starting after switch-off

Before starting to fit a new kitchen, the fitter turned off the CH+HW, then disconnected and bypassed the kitchen sink. We were out last evening, but the heating didn't switch on this morning, which has me concerned. Another point, I left the thermostat on ~15degC overnight and the heating on; around midnight, there were strange gurgling noises compeletely unlike anything I've heard before from the system, so I just turned everything off and went back to bed. I'm waiting for the fitter to arrive this morning, so I can find out what sequence he followed, but what I've tried is: - ensuring the stop-cock that feeds the pump is fully open - starting up the HW and CH independently (we've had issues before where HW would come on and not the CH, but it's been fine for a while and wouldn't want it to become a red herring) - turning up the wall thermostat to max
I can hear the pump chrning away, but no change has resulted, the system doesn't appear to be heating up either water or rads. Boiler is in the garage, soI'll go out and check it's lit when I've written this, but can't see that being the likely problem.
Could an air-lock have been introduced when the fitter turned off the system initially ?
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Combi boiler (no header tank)? Traditional (hot water cylinder, header tank)? What kind of system is it?
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Sorry, was in a hurry to post !
Yes, it is a traditional gravity pump system. Installed 2001 by the wonderful BG. Boiler is BG-branded F2 (Potterton?).
In answer to the other post, the gas supply definitely wasn't switched off.
Update: -------
Gets more bizarre. I thought it would be unlikely to be the boiler, but when I eventually went to take a look, the boiler had switched off (boiler is in garage). The boiler has an Overheat Limit Thermostat, which appeared to have tripped. I had to reset it by pressing a recessed button on the underside of the boiler casing. By resetting this thermostat, and powering the boiler off and on again, I managed to get it to light. This worked fine for the rest of the day, and we had heating and HW throughout. Overnight, however, the system wouldn't switch on at the programmed time. Had a look at the boiler again this morning, same story - it had switched itself off. Same procedure sa yesterday lit the boiler temporarily, but the CH+HW wouldn't get going. The boiler also cuts out after a few minutes, with quite loud spluttering and banging. The pump+valve are in the airing cupboard above thestairwell, along with the HW cylinder. When I switch either function on, there is a low grinding noise for about 45 secs, then a constant very low rumble for the next few minutes, then silence. Looks like the pump is trying to turn itself over, but can't.
Any help please, or is it best to have someone in to look at it ? Can't really do w/o CH with the kids at home.
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Almost certainly a dodgy pump - which runs once it gets started, but sometimes fails to run at all.
If the pump fails, the boiler WILL trip - because the heated water isn't being removed from it. It turns its burner off with the normal thermostat, but the residual heat in the metal parts continues to heat the water to an unacceptable temperature - whereupon the overheat stat trips.
Some pumps have an end cap which you can remove termporarily (with very little loss of water) behind which is the end of the shaft. You can see whether this is rotating, and give it a nudge with a screwdriver if it is stuck. This sometimes gets it going again - but if this happens often, you'll need to replace the pump.
Roger
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Are you sure that the fitter hasn't turned off the GAS supply? The pump and stats would still work - but the boiler would churn out COLD water!
Roger
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