Worcester Heatslave 12/18 Oil boiler

3 Jul 2021
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United Kingdom
Hi All,

I have a Worcester Greenstar Heatslave 12/18 oil boiler which stopped heating domestic hot water or central heating hot water today. The pump is running when the controller is set to either HW or CH but the boiler isn't firing up. When it first happened this morning the pump was red hot, couldn't touch it and have had this problem before, about 6 months ago. Then it cost us about £600! The engineer changed the pump, etc and then changed the DHW thermostat and that seemed to resolve it. I don't really want to call them out again (another £600 later!) when I am comfortable is changing the stats and thermocouples (if that is the problem) but having looking inside the control unit there appears to be at least thermostatic units (DHW and CH, attached to control knobs and one, with a thermocouple but no control knob - see photo).

So my question/s:

Is it likely to be one of thermostats?
If so which one?
If not, any ideas?




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It would be better in plumbing and heating sub-forum, as opposed to forum information. If your pump was red hot, then I’d be suspecting the pump, but could be motorised valve (if you have one) not opening.
Thanks CBW, will do that. I wondered the same about the motorised valve but that's working fine when I go from DHW to CH.
Are you referring to the central heating circulating pump?

If that is jammed again it would often be because of dirty system water.

With no circulation I would expect the boiler might fire up for a couple of minutes until it gets hot and then shut down for a long while and repeat.

£600 seems a bit expensive to replace a circulating pump.
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You got ripped off if I did that repair it would cost £250-00 I think the pump has air locked check the pressure first when cold should be 1.2 bar undo the screw in middle of pump let water out for a couple of secs then do up see if that makes it run better if not new pump. Bob
when you remove the screw on the pump see if you can spin the shaft with a screwdriver, might just be jammed
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Thanks all for your advice. Will check the pump over but I think it's working fine. Should've also said that the £600 wasn't just for the pump, but 3 call outs the pump and new stat

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