Potterton Prima b central heating fault

[stfe1203]

Hi

I am hoping somebody could shed some light on a problem that I am suffering at the moment with my central heating. I have a standard y plan system with Potterton Prima b 40 boiler with cylinder thermostat and room thermostat. The boiler keeps short cycling constantly for very short periods when the programmer and all thermostats are switched to off. The only way I can stop the problem is to turn the boiler thermostat to off but this causes the pump to run constantly? The heating and hot water is working as normal but when we go to bed on a night and switch everything off after a while the boiler fires up and the pump starts to run for about 30 secs and then switches off. This occurs around about a dozen times before a 10 to 15 min break before it starts again. The only way to stop it is to turn the boiler thermostat to off which kills the boiler and pump(in airing cupboard) but then the pump starts up again and runs continuously all night.

Any advise would be great

Cheers

Steve

 

[Newboy]

It sounds like a PCB fault - get a GasSafe registered engineer to double check and replace if necessary.

PCB faults are not uncommon on Primas (I've changed my own twice!)

 

[Gasop]

The Prima B does not have a PCB.

Your problem is very likely a fault with the motorised valve.

Best to get it checked out by someone that knows what they are
doing.

Switching the boiler off at the fused spur or plug will turn off boiler
and pump.

 

[stfe1203]

Thanks for the replies. The actuator to the 3 port valve was changed not so long ago because the motor had burnt out. The spindle on the valve feels free enough tho and the actuator is operating correctly. Also noticed today that with the heating and dht turned on on the programmer/timer the boiler is still short cycling with the pump simultainiously starting up and cutting out constantly. I will contact a engineer but just wanted some advice to see if it was something I could have a look at myself i.e cylinder thermostat ect Also I recently drained down the system to change a rad, would an air lock or sludge perhaps interfer with the boiler regarding my issue.

 

[picasso]

Get whoever you get to repair your boiler to check the pump overrun thermostat, it sounds like its stuck.

 

[stfe1203]

Thanks alot. I will do.

 

[picasso]

Just a thought do you have a frost stat fitted ?

 

[stfe1203]

Just had a look at the manual for the boiler on the potterton website. There is no mention of a frost stat. Just to add also that there is interference with the tv now as well when the boiler turns off and a short rasping noise.

 

[stardanny]

Do you have a motorised valve like this?

Untitled

• stardanny
• 31 Jan 2015
• 1

Daniel.

 

[stfe1203]

Yes I have a myson plus MPE322.

 

[stardanny]

If the pump still running all the time, try this.

Switch off at fuse spur ( the main switch for central heating system )

After a few minutes, with the programmer both hot water and heating off, switch fuse spur back on.

Does the pump run?

Daniel.

 

[stfe1203]

I switched it off at the mains this morning with the programmer both in the off position. The pump started up straight away but then cut out after a few minutes with the 3 port valve in hot water only position. After 15mins or so the pump & boiler started back up again and has been turning itself on and off all morning. Whilst in a period of being off I switched the CH on on the programmer and the pump and boiler fired up and the actuator turned the 3port valve to CH only position which seems to be working fine. Why would the boiler/pump start up randomly when everything switched off and thermostats turned right down?