Replaced Central Heating Pump - still have issue

[RogerNE]

I have a conventional system, Potterton Prima Boiler, into a Pump into a Motorised Valve that feeds the Radiators and/or the pipes in the Hot Water Cylinder. Analogue Controller with slide switches (replaced 2 years ago) and analogue Room Stat and Hot water cylinder Stat.

I'm just an amateur plumber, but am an Electronics Engineer, and do all my own car repairs.

The pump packed up on Saturday (jammed solid) so fitted a new one today. However the Boiler only comes on for 1 to 2 minutes, then the main burners go off. (This is regardless of where the Boiler Stat is set.) Only after turning off for 20 minutes and then turning on again will it re-start for a minute or so.

While the main burners are on, the pipes in and out of the pump and out of the Motorised Valve both sides are all hot.

I am at a loss as to what could be causing this ! Even if there was an air lock, would the boiler turn itself off like that? (Tried bleeding the pipe going into the Water Cylinder and the Radiator where the air normally collects)

I have tried various things suggested by friends who are retired plumbers, but had no joy.

Could the Boiler stat be faulty? But I would have thought it wouldn't come on at all if the contacts were burnt out. (and you can hear it click if you turn it down and up again)

Hope someone can help !

 

[RogerNE]

Well water is flowing round the system, as even in the 2-3 mins that the Boiler runs, the one radiator I left turned on is getting fully hot . . . plus I have hot water out the cylinder.

I tried shorting out the Boiler Thermostat to rule that out, and it made no difference.

One of my friends said that - as there is no PCB in the way - all it can be is the Gas Control Valve . . . tried hitting it when it went off, but made no difference.

So I have ordered a new one . . .

Fingers crossed !

 

[ShaunCorbs]

It’s not a gas valve

It will be air locked upstairs just run one rad with the rest turned off and keep bleeding the air ports by the cylinder if there’s any

 

[RogerNE]

That's exactly what I did . . .

As I think I mentioned, the Boiler turms off after 2-3 minutes (even with the Boiler Stat shorted out). It won't turn itself on again unless I turn the heating off, and wait about 20 mins . . . then when I turn it on it will fire up again for 2-3 mins.

I have done this now about 20 times . . . and periodically checked whether any more air is coming out of the Cylinder bleed valve (there isn't). And no more air coming out of the one radiator upstairs that I left turned on. (that's the one radiator which I would have to bleed occassionally).

An ex BGas engineer said that given all I have done, it could only now be the Gas Control Valve?

 

[RogerNE]

Several people told me it would be an Airlock . . . but there is nothing in a conventional boiler like mine to cause the gas to turn off, even if there was an airlock ! (apart from the emergency High Temp cutout . . . which wasn't tripping).

So I figured it HAD to be the Gas Control Valve.

It was ! I ordered a new one online (eventually found one for less than half the price some companies quoted). The Boiler is now working perfectly.

I had thought I would then need to get someone in to set the Pressure with a meter . . . until a former BGas Engineer pointed out that a more accurate way is to adjust it by looking at the gas usage and timing it. (which I could obviously do myself quite easily).

So it's all set correctly now !