Potterton CH cuts out after 20 seconds

[mabtech]

I'm servicing a Potterton Lynx 2 boiler which is about 8 years old.

The problem is that it seems to cut out about 20 seconds after you switch the unit on when its asking for CH water. If I short across the flow sensor terminal, it will run without a problem and the rads get hot.

The problem seems to be 'caused' by the diverter value on the CH system. Its energised as soon as the boiler's switched on and after about 20 seconds the value is turned enough for the flow to stop in the pipe where the flow sensor is set (just in line with the pump). Therefore the system switches off.

I've put a new (recon) PCB in anyway as it was wearing out (dry joints) and I also checked and replaced the flow switch (mistaken diagnosis but its done now)!

Any thoughts on what could be wrong?

Many thanks in advance.....

 

[brumylad]

dripping Tap ?

 

[mabtech]

i.e. the dripping tap is causing the boiler to switch to DHW mode? So it could equally be a sensor in this part causing the boiler "confusion"?

DHW works fine, by the way. Turn on any hot tap and it fires up without a problem but I guess a faulty flow sensor on DHW wouldn't actually cause hot water problems....