potterton lynx 2

[alexbetts]

Hi

i have a potterton lynx 2 and has the following problem.

1. when the hot water is ran for a couple of minutes it shuts the boiler down.

2. after 3-4min approx the CH also shuts the boiler down.

everything i have rad seems to point to a PCB does this sound right

 

[hammerswinger]

Is the pump working.

 

[alexbetts]

yes pump fine although very hot

 

[hammerswinger]

Could it be the overheat thermostat?

Maybe someone with more experiance of this boiler can advise.

 

[twgas]

Is there water in the system, should be 1 barish. what pipes get hot when you turn it on

 

[dave18]

Alex,

What makes you say that the pump is working OK?

You say the pump is very Hot,

This, along with your boiler problems, point to one thing,

The pump is no longer serviceable.

unscrew the large chrome screw from the centre of the pump and check that the shaft is turning. two things, check when cold.... 1, any water that leaks out will be very hot, 2, if you drop the large chrome screw it will be too hot to pick up.

If shaft not turning, try freeing it with a small screwdriver on the end of the shaft


Dave.

 

[harryfox121]

if the pump wasn't working surely the flow switch wouldn't proe it back to the pcb and the boiler wouldn't fire up at all? Unless it's starting to fail...

 

[dave18]

Harryfox121,

On looking at the manual to refresh my memory,

The flow switch for the DHW is located on the cold water inlet to the boiler
so the pump would not have to be working for the boiler to fire.

But for the heating I agree with you the pump should be running to trip the CH flow switch.

So due to the above I will revise my advise and say there are two problems 1, the pump is not working 2, the CH flow switch is stuck in the on position.

From the problems the OP said is wrong with the boiler I would say it is a problem with lack of circulation of the water from the heat exchanger.

So, 1, check pump as in my last post,

2, with the power to boiler switched OFF check the CH flow switch.

Dave.

 

[harryfox121]

dave18 that is true about the flow switch.

Could be a sludged up heat ex or air trapped in the system somewhere, or possibly a faulty overheat stat. Poster: drain some water from your system at a low level drain valve and see what colour/quality the water is....?

dave18 - the CH flow switch stuck in the on position, that wouldn't make the boiler overheat though would it? It would also prevent HW from coming on at all surely?

 

[harryfox121]

sorry might have sounded confusing : if the CH FLOW SWITCH was stuck in the ON position then the boiler would continue to fire even if no demand was being made from CH or DHW. So if the pump was failing/not working then this lakc of circulation would make the overheat stat trip and send the boiler to lockout.

 

[dave18]

Harryfox121

To answer your point I will try to explain how the boiler works.

1, There are two heat exchangers above the flames at all times.

2, on call for hot water the DHW flow switch is closed the diverter valve moves to circulate the CH water round the boiler tripping the CH flowswitch

3, flames come on heat hot water to taps.

4, flames also heat water in CH heat EX which is pumped round the boiler to cool it down (as the CH is not required). before the water goes through the heat EX again.

5, The above takes place with the CH turned on or off as you do not get heating when hot water is being called for.

6, Due to the above if the pump is not working the water in the CH loop will over heat and the O/H stat will trip out the boiler.

7, If the boiler is set for CH. and the flow switch is stuck in the ON position the boiler thinks water is flowing and the system lights up, and the heating will appear to work as it should (it working under the control of boiler stat room stat ect.

8, if the pump is not working the boiler is alight but the heated water is not getting away from the heat EX so the over heat trips out the boiler.

9, are you still with me

10 I rest my case

 

[harryfox121]

dave18 - yes I understand that. If the CH flow switch was stuck in the ON position though would the boiler not permanently fire/cycle on-off the boiler stat without demand from CH

 

[dave18]

Harryfox121

Sorry I did not mean to imply that you did not know how a boiler worked,
but explained it that way for other readers in case they did not realise this boiler does not have a secondry heat exchanger like most modern combies.

Yes if the heating is turned on, on the timer, it will appear to be working normally, providing the pump is working.

Dave

 

[harryfox121]

dave18 - it's OK I was just trying to work this one out! I know a bit but am no expert obviously.

One point though : if the CH was turned OFF at the timer/stat etc - and the CH flow switch was STUCK in the ON position - would the boiler still fire then continuously, or does it have to have power from the external stat etc to allow it to fire??

 

[dave18]

Harryfox121

The boiler would need power from the external controls to work.

Dave