Grundfos Pump Overrunning

4 Jan 2009
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United Kingdom
Hi there

I have a CH system in my house, not sure how long its been there, but it looks old!!

last night before going to bed, I turned the themostat down to 14-16, as normal but the pump carried on running. This the first time it appears to have happened.

The water was switched off and so was the heating. In order for me to stop the pump I have to swith off the power at the CH mains supply. On turning the system on again this morning, the same thing appears to be happening, water and heating is coming on as needed, but the thermostat is not kikcing the pump out when I turn the temp down...the pump just carries on humming and getting hot!

Is this a problem with the Grundfos pump? Do I need to get a new pump, as it seems to be overrunnning itself!!

Your comments would be very much appreciated
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It won't be a problem with the pump. Probably a fault with the boiler if it's got a pump overrun facility.
Thanks Mike

How will I know if I have an override boiler? Is this the matchbox size stat on the side of the boiler? This has a wire leading out to another box?

Post the make and model of the boiler and one of us on here is bound to be familiar with it.
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The matchbox size box on the side of the tank is a Honeywell

The make and model of the boiler is Potterton Netaheat Profile

The pump is a Grundfos Selectric

The boiler actually seems to be working fine, its firing up and going out as we need it too. Its on turning the system off, when the Grundfos just keeps running, whilst everything has stopped!!?
Hi Mike

A little more info for you.....

Its a Neat Heat Profile 50 HAJ015757....if this makes sense to you?
This boiler has a pump overrun thermostat incorporated which will keep the pump running until the boiler temp drops below 80C. It's unlikely that the boiler is remaining over 80 all night so I suspect this stat is faulty.
OK thanks Mike....

Is it likely I can fix this myself, or do I need to get an engineer out to asses the work? Ive worked with pumps etc in the past, but im unsure on intergrated stats on boilers.
It needs a registered installer to carry out the work as it requires removal of the combustion chamber cover and this a positive pressure boiler which means that products of combustion can be leaked into the room if the case or seal is faulty.

When you contact someone it's worth mentioning that you think the pump overrun stat is faulty but bear in mind it's not possible to be 100% sure when diagnosing over the net.
Just a thought - is there a frost thermostat anywhere that could be causing the pump to run?
Hi mogget

Ive checked the boiler and under the actual main on/off power supply switch there is a Honeywell froststat located directly under the switch...both wires seem to run into the boiler.

Could this be another possible cause? - If so, do you have another solution?

your comments are very much appreciated
The frost stat should run the pump and boiler but it could be wired wrong. It's worth checking out before spending money on calling someone in. Are you proficient with a multi meter?
Hi Mike

Im aware of what its used for, but Ive not had personal experience of using one myself. Looks like the bucks will have to come out!!
The frost stat should run the pump and boiler but it could be wired wrong. It's worth checking out before spending money on calling someone in. Are you proficient with a multi meter?

With the heating and hot water turned off, aim a hairdryer or fan heater at the frost stat for a few minutes and see if the pump stops spinning.

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