HW / CH pump won't switch off!

[mawsonics]

Hello all. I have an irritating problem with our central heating / hot water system. Basically, when the HW is up to temp, everything switches off (inc motorised valve) but the pump stays on, and will stay on unless we do something about it - this has included disabling it by jamming the speed selector in between settings (prob not a good idea) or simply turning the whole system off. When the system's turned back on, say in a few hours time, the pump is back to off again.
Now here's the even weirder thing - this doesn't always happen; in fact it might occur once every ten or twenty times. Our engineer's genuinely stumped - anyone got any ideas? It's a pressurised system based around a Warmflow Kabin outdoor oil-fired boiler.

 

[D_Hailsham]

Is this a new problem, i.e the system previously worked correctly?

Full details of your system please - makes and models of all components.

 

[mawsonics]

Motorised valves are Danfoss HPA2's, the controller is a Honeywell, pump is a Grundfoss UPS25-55 (?), HW tank stat is Honeywell - does that help? Thanks, Chris

 

[D_Hailsham]

Motorised valves are Danfoss HPA2's, the controller is a Honeywell, pump is a Grundfoss UPS25-55 (?), HW tank stat is Honeywell - does that help?

Yes. I now know you have an S Plan system.

However, I still don't know if it is a new problem, i.e the system worked correctly before.

 

[mawsonics]

Hello again,

This is a new, but intermittent problem - that's the odd thing. In fact over the last few days the system's been behaving itself. One of the MV's had to be replaced earlier this year but that was the CH one (it was stuck on), not the HW, and it seems to be the HW circuitry that's causing the pump to stay on - every now and then!

 

[D_Hailsham]

Sorry, I should have asked which make and model boiler you have.

 

[mawsonics]

Hello there - sorry for the delay in getting back to you. It's a Warmflow KP120HE. Funnily enough, after a week of behaving itself, the pump got stuck on this evening, after the HW had been on.

 

[D_Hailsham]

after a week of behaving itself, the pump got stuck on this evening, after the HW had been on.

Thanks for the boiler info.

When you say the pump got "stuck", I assume you mean that it carried on running.

The only thing that could cause this situation to occur is if the boiler thermostat has turned the boiler off, but the cylinder stat (pump connected to this stat) is still calling for heat because the cylinder water is not up to temperature.

1a. When the problem occurs, is the HW turned ON or OFF at the programmer?
1b. If ON, what happens if you turn the cylinder stat down?

2. What temperature is set on the boiler stat?

3. What temperature is set on the cylinder stat?

 

[footprints]

Might well be a pump over run thermostat playing up get the service engineer to take a look at it.

 

[D_Hailsham]

Might well be a pump over run thermostat playing up get the service engineer to take a look at it.

Only the combi version of this boiler have an overrun stat; the OP's boiler is not a combi.

 

[Norcon]

The micro switchs in Danfoss HPA2's go faulty for a past time.
They are rubbish.
Best option is fit honeywell or do as I done and fit a timer/relay to each zv.

 

[mawsonics]

Hello again,

An update really. The system was behaving itself for ten days or so and then this morning . . the pump stayed running after the programmer had switched the HW off! Replying to D_Hailsham, the cylinder stat is set to 50 degrees. I thought this could be the source of the problem, though turning it down (whilst the pump's still in this 'overrun' mode) makes no difference. I suppose another potential source of the problem is the MV, which is a bit sluggish when turning on/off but does the job all the same.

Not sure what you mean by 'boiler stat' - would that be in the boiler itself?

 

[Tipper]

Still sounds like failing motorised valvehead to me.