Potterton Puma 80 perm pilot light switches off

[pennib]

When I turn the CH on the boiler fires up and the rads heat-up nicely, after 15mins approx there is a little 'clunk' and the pilot light is switched off.

This week because of this problem I have replaced the complete diverter valve and wax capsule assembly, water pressure switch, the CH expansion vessel, CH overheating thermostat, CH, DHW temperature sensors. I have also changed the flow switch and micro switch for good measure.
The fan and pump are working because I can hear the fan and pump turning.

Can anyone help please

 

[twgas]

I think I would be looking for a blockage and checking pump is working properly, I'd also check burner pressure/gas rate - but thats a corgi thing.

 

[Agile]

After replacing so many parts I would have expected it might have been cheaper to call a professional to deal with the real problem !

Tony

 

[pennib]

Thanks Mick,

I have tested the pump. I have undone the central screw and pulled it out so that it spins when the CH is switched on. I have drained the system separated the pump by undoing the 2 allen screws. The blade wheel spins freely and the motor works.

Just before doing this I tested the CH again and this time the pilot light went out immediately, before it would take approx 15min before going out.

Can you give me anymore advice on where to search further for this blockage?

 

[pennib]

Hi Tony,
Good point. The £300 pounds I have spent has virtually given me a new boiler, and some knowledge of how those parts work and how to fit them. The Diverter valve and Expansion Vessel were 10 yrs old. The diverter valve was ready to be replaced and the expansion vessel was busted and needed to be replaced. Those 2 parts cost £200. A new boiler + fit = £1300 plus

One of he purposes of this forum surely is to help people who are prepared to roll-up ones sleeves and have ago at repairing stuff, and information on what is dangerous and requires Corgi qualification, hence the sites title, DIYnot.

Neil

 

[Agile]

If there is a blockage then it can be anywhere in the boiler but usually at points where there are restrictions in the flow.

Two possible places you can check by removing the pump head and poking 100 mm of thick wire in each direction.

Another possible place is at the 90°C bend where the flow and return enter the main heat exchanger. Also at the changes in direction within the heat exchanger.

Removing the heat exchanger is a CORGI job!

Tony

 

[stumpydick]

checked min burner pressure? He said rads heat up nicely.

 

[Agile]

There is a simple little thing you can do which will not even cost anything!

On the control PCB there is a black plastic potentiometer shaft towards the bottom and slightly to the right!

Turn that fully anticlockwise and I expect your problem will go away or be reduced.

In the medium term you still need a competent CORGI to set up the maximum and minimum burner pressures.

Tony

 

[pennib]

Thanks Tony,

I have turned the Potentiometer fully anticlockwise but it doesn't seem to have made any tangible difference to the length of time the boiler takes to lock-out, which is roughly 10 mins, when demanding CH (although this has varied, because it locked-out immediately yesterday). I have been testing the pump this morning, again, just to make sure its not failing and the boiler took roughly 10mins to lock-out. This is before I adjusted the Potentiometer. Can I ask what a Potentiometer is please?

How would the burner pressure affect only the CH and not the DHW, which works perfectly?

Yesterday I flushed the boiler and Rads, again, again, and again.

I isolated 1 radiator on the top floor, took the pump off then flushed. I then did the same for the bottom rad, then the whole system.
If there is a blockage , its still there if thats the problem.

But it does take the CH roughly 10mins to lock out. What sort of blockage would that be, because it is not a sediment block. It must be a big chunk of mettle stuck in the boiler, and if that was the case the boiler would shut down more quickly than 10mins?

going quietly insane
Neil

 

[ChrisR]

It's possible your pcb isn't "modulating" the gas , ie turning it down, properly. A cracked solder joint on the board will do that.
Check the legs on the big transistor which is on the heat sink, roughly in the middle. Look at the underside of the board. If you see anything moving when you poke at it topside, you might be able to fix it with a soldering iron and a dot of solder. Having said that, those pcb's don't last more than a few years before multitudinous bad connections kill them, so a new board might be due.

Have you got a voltmeter?

It's also very possible that the boiler is a bit marginal because it needs a service of the gas bits. Corgi territory.

 

[pennib]

Thanks ChrisR,

The solder looks solid on the PCB. The Issue number is 05, WK/YR:43.06
I have looked at the back and it looks clean.

If there is a hairline crack I would need a voltmeter, as per your suggestion. I do have a dusty analogue multimeter (rapitest 7040)

What do I test?

 

[Agile]

I have this strong urge to ask him to reduce the min gas rate!

But if I did that I could be criticised by a mod!

Could I get him to dis the mod con and rate it?

Tony

 

[ChrisR]

With the CH on, measure the voltage on the contacts on the mod coil on the gas valve. Blue I think the coil is. Iirc it goes 11 to 24 volts dc.
Careful, other contacts are live, so test with suitable range for that first!

Just to be sure, to be sure, the red light on the middle of the pcb - it does go off smartly when you turn the HW tap off, I hope???

 

[pennib]

Thanks ChrisR,

The red light on the Modulation Board comes on when DHW is demanded and goes off, promptly, when DHW is not.

If i've understood your instructions correctly, please tell me if I have not.
On the Permanent pilot gas valve there is a red and a black terminal for the Modulator Valve Connections which are situated on a blue plastic cowling on the Modulator Valve. Do you want me to measure the Direct Current Voltage between the negative pole and the positive pole?
When on DHW the DCV starts at 28 DCV then falls to 18 DCV
When on CH The voltage is constant at 8 DCV.

Please let me know if this is OK

Neil

 

[Agile]

Thats fine!

The 8v corresponds with the CH setting potentiometer being on minimum.

That pretty much proves to me that the gas valve is not set up correctly.

Needs a competent CORGI to check/set max/min on gas valve. Not all CORGIs know how to do that!

Tony