Overheating Boiler/Pump broken?

[Matt21]

Hi all - hope someone can help me here!

I have a CH/HW system fired by a Keston 80 gas-fired boiler. There are two pumps (Grundfos Selectric), the first of which is situated immediately outside the boiler (the second is on a different part of the system and relates to the underfloor heating only (I think). It looks as though water has to go through the first pump regardless of whether it is CH or HW that is being asked for.

Anyway - I cannot tell if the first pump is running. When I fire the boiler, it (the boiler) runs for about a minute but then the pressure valve needle starts 'jumping' and it makes knocking noises until it eventually shuts the gas off and the boiler stops (not the electrics). At this point, if I open the tundish valve, boiling hot water comes out. Logically, it seems that the water in the boiler is being heated but has nowhere to go ie the pump has failed.

How do I know if it has (I have opened the inspection port and turned the impellor which turns fine)? Also, could it be the capacitor or anything else, and therefore might I not need a whole new pump (there did seem to be some water in the plastic box that houses the pump speed switch)?

Any thoughts from those more competent than me much appreciated!

Thanks

 

[simond]

Change the pump.

And start saving up for a new boiler. Kestons are not known for longevity.

 

[Matt21]

Yes, I know about the Kestons - big job to change it though - different sized flue pipes etc mean ripping our half the ceiling. I think i am stuck with it for the time being!

So you reckon it is the pump and I should just replace it? No point faffing about with capacitors? I don't know why it doesn't work if the impellor spins freely.......

 

[simond]

If you have water in the terminal box the bearings have failed. Put your hand in your pocket.

 

[Matt21]

OK cheers. Will do so.