Potterton Lynx 2 sudden water pressure rise on CH start

[garfieldian]

Hi folks, new to the forum but if you can help.I have had my Lynx 2 for 15 years and it’s always had a slight leek, never found it. I top it up occasionally but the gauge always sits on the border between green and red at about .5 bar. Recently I have had a problem, only with CH not HW, sometime immediately when the boiler start the pressure gauge goes through the roof. The boiler sound like its having a fit, some banging, and then I get a whooshing sound and the gauge drops. Then amazingly the boiler starts. I think the PRV is kicking in but not sure and I don’t know what causes the rise. Also the fault is intermittent. However I have found a way of replicating the fault. Turn boiler on from cold CH, then turn off after 30 seconds, turn on again and the gauge rises to above 3 (within ten seconds) then then drops and then the boiler kicks on. Naturally this plays havoc with the gauge pressure. Recently I topped it up to 1 and ran the boiler, the same thing occurs, even bigger whoosh and no the gauge is not registering any pressure at all, nothing! But boiler functions normally. This only happens on cold start and not when the boiler is running. Any help appreciated.

 

[gas112]

Sounds like the pump is not starting straight away

 

[garfieldian]

Why would that happen, faulty pump or something else?

 

[petertheplumber12]

I think your expansion vessel is not functioning properly, try introducing air into a radiator, a decent sized one, half full of air. see if it behaves better, that will prove the expansion vessel problem as well.

 

[gas112]

Expansion vessel would not come in toplay untill the boiler had been on a while .
Noise is the pump not working and the boiler getting to hot and not getting rid of the heat.

OP been a long time since i have worked on a lynx 2 , dont think they have a flow switch on them so yes it does sound like the capacitor is not starting the pump and when the boiler starts to bang with water surging through it this is enough to give the pump that little kick to start up

 

[garfieldian]

I have attached a picture of the water flow. I would assume that the 'Water flow Switch (13)' would shut the system down. Mind you I can't see where the pressure gauge is on this diagram.

 

[garfieldian]

Here is image (hope this works)

Lynx 2 flow diagram

• garfieldian
• 6 Dec 2012
• 1

 

[Agile]

As I recall they DO have a pump proving switch towards the bottom left.

If so and its not operating then all that happens is the boiler does not proceed to the ignition stage. They can also jam in the on position.

But that does not seem to be your problem.

I quite like working on the Lynx but dont see many still around.

Tony

 

[garfieldian]

Well I'm a bit stuck here I think it’s logical that it’s the pump not starting occasionally. But I would have thought a flow control switch would not start the boiler. I will have to check tomorrow from cold. But worried about running the boiler with the pressure gauge not registering. All working but getting trickling sound from downstairs radiator. I assume that’s a symptom of low pressure?

Thanks
G

 

[Agile]

Sounds like air in that rad!

 

[gas112]

looking at diagram i would be looking at possibly 2 faults flow switch stuck which is allowing boiler to fire up without the pump actually running and faulty pump.
If you can see if when boiler first fires you are getting 240v at pump without it running.


It could possibly be a weird board fault allowing things to happen out of sequence