Central heating pump runs constantly

[jimrat]

Last week my Potterton Prima 30F central heating boiler blew the main fuse. I didnt have a spare 3 Amp fuse, so fitted a 5 Amp, which also blew. I then called out a a local heating engineer who traced the problem to a faulty pump, which he replaced (and chargerd me £200!)

Since the repair, the central heating pump runs constantly, even if the boiler is not running. I contacted the engineer who fitted it, who suggested that the pump over-run thermostat may be damaged. I have since checked this (by dipping the probe in hot water) and it is switching OK.

The boiler also seems to be only partially controlled by the timer switch- it follows the preset CH + HW settings, but also has also started firing up in the middle of the night or if the temperature dial on the front of the boiler is turned up.

Does anyone have any suggestions? Would a higher rated fuse have wrecked the PCB?

Many thanks

Jim

 

[ollski]

I would first make a quick check to make sure the pump isn't wired into a permanent live connection......do you have a frost stat?

 

[oilman]

This has the characteristics of a faulty motorised valve which needs a new head, but I would be careful with pump changers, (and I don't go to fast fit tyre and exhaust centres for the same reason).

 

[jimrat]

Thanks for your help

I checked the pump wiring straight away- it doesnt seem to be wired up as per the wiring diagram in the inside of the cover. That said, all the engineer did was remove the old pump and replace it, so he shouldn't have altered any connections in the junction box. I dont think there is a frost stat (is this for protection against frozen pipes?)

How do I check for a faulty motorised valve? Can anyone suggest a reputable central heating engineer in the North Bristol area??

Cheers
Jim

 

[jimrat]

Oilman

The motorised valve seems to be working OK: it is a mid position type, and there is a position indicator on the side marked W / M / H (presumably water /mid / heating?).

With the controller set to hot water only, the indicator moves to W. If I manually move the valve to the middle using the plastic lever at the back, the stepper motor moves it back to the W position.

With the controller set to Heating only, the indicator moves to M. If I touch the storage tank temperature sensor onto a hot section of pipe, the valve moves across to H.

Cheers
Jim