Grundfos Alpha+ pump running continuously

4 Nov 2021
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United Kingdom
I’m currently experiencing an issue with a Grundfos Alpha+ pump running continuously in my unvented heating system and would appreciate any advice. It continues to run when both the heating and hot water are turned off. When there is demand, the radiators do get hot as expected. Both zone valves appear to work, at least there is resistance when there is no demand and they activate when there is. Resetting or turning the boiler off doesn’t stop the pump either, only turning it off at the wall socket does. Any suggestions?

The boiler is an Ideal Icos HE12 and it displays ‘C’ with the burner light flashing while the pump runs (with both heating and hot water off).

The earliest I can get someone to look at it is early next week unfortunately, but if there are things I could check in the meantime that’d be great.


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which boiler do you have ? sorry just seen you had said , the C and burner light flashing means the boiler has reached the set temperature, or there is a blockage, sounds like you have a sticking micro-switch on one of the zone valves
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Thanks, is it possible to check the micro-switch easily or best left to someone qualified?
easily checked , do you have and know how to use a multi meter ?
Sorry for the slow reply, the heating system isn’t the only thing that’s packed in today. Yes I have access to a multimeter and can use to troubleshoot.
Edited - as inaccurate information
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check between the grey and orange wires with no demand, should be 0v, anything more, and it’s likely a stuck microswitch.

Isn't that the wrong way around? If the microswitch is stuck 'on' (i.e. the contact is closed) then both wires will be at the same potential (230V) therefore the voltage difference between them measured by the multimeter will be 0V.
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Edited - due to lack of accuracy
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Great, I’ll give that a try next thanks!

I’m not sure if this helps at all, but I’ve had everything turned off most of the day and before turning everything back on to get some heat in the rads I turned down the thermostat on the hot water tank to its minimum at 25C (it clicked below 45-50). As soon as I turned the pump back on the valve on the pipe leading to the hot water tank opened. That’s with the thermostat on the tank turned down and no heating or hot water turned on at the controller. Would I be right in thinking that this might indicate the micro switch in the hot water valve as the likely culprit, or is that normal behaviour?

I’ll see what I can do with a multimeter after there’s a little heat in the house and I turn things off again. Thanks again.

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