Potterton Netaheat 10/16 fails to start

[hartnellg]

I have a Potterton Netaheat 10/16 Mk II (gas) boiler which has recently failed to fire up
The boiler gets part way through its start up sequence, but the main burners never fire up
The boiler has been operating normally and there have been no recent changes
The room and / or cylinder thermostats call for heat, the motorised valves open and the pump starts
The relevant control should then then instruct the boiler to start up

Some time ago, I found a description of the start up sequence for this boiler
I thought it was on this forum, but I can no longer see it - fortunately, I made a hard copy
I think I have been able to identify where in the start up sequence the boiler fails
However, it would be useful for some advice to confirm what I think is at fault

The start up sequence I found for a normally operating Netaheat boiler is summarised as follows:
1) thermostat calls for heat, supplying power to the electronics
2) the fan starts
3) a pressure switch detects that air is flowing through the boiler
4) ignition is delayed for a "purge period"
5) the pilot valve is opened and the spark generator is activated
6) the pilot ignites and is detected by the ignition electrodes
7) spark generator is de-activated, the main valve is opened and the main burner is ignited by the pilot flame

For my boiler, the sequence gets as far as the pilot being lit, but the main burners never light
The Potterton documentation for the boiler shows two solenoids in the gas valve
From the symptoms, it seems to me that the main gas valve solenoid has failed
I am reasonably competent with the electrics and have measured the voltages supplied to the gas valve from the PCB
After the pilot is lit, both BR and R terminals on the PCB measure about 230V AC wrt the B (neutral) terminal
With all power isolated, I then measured across the gas valve solenoid coils:
a) the BR coil measured about 2.7kohms - the documentation shows a resistor in series with one of the gas valve solenoids
b) the R solenoid appeared to be totally open circuit and I correlate this to the failure of the main gas valve

Does this sound like the most likely cause ?
Is there anything else I can check or do to get my boiler working
I've read various threads on here in relation to old Potterton boilers
Opinion seems to be divided about whether to repair the old one or get a more efficient new boiler
As I am recovering from Xmas overspend, the plan is to repair the existing boiler / keep it going a few more years


The boiler was fitted in 1984 and has been regularly serviced - most recent service was July 2007
About 4 years ago the PCB and relay were changed
At the same time I purchased a spare relay - I tried changing the relay, but the boiler still does not start
I did not expect it to as I think it is the main solenoid on the gas valve that has failed
I have checked the fuse on the PCB and this is OK

Thanks for any advice and help

 

[bengasman]

Sounds like solenoid is dead, providing you measured correctly. not sure if you can replace the solenoid or that you need to fork out for new valve. CORGI job
Could cost a few bop, hang on for second opinion. Or third even.

 

[gas4you]

I thought the gas valves were no longer available for these

 

[Gasguru]

I'm assuming this is a MK II F.

If so the main burner should fire up a minute or so after the pilot has lit.

The pilot flame heats a mercury vapour phial. As the mercury expands it operates the switch unit (plugs into the end of the gas valve) to power the main burner solenoid.

The pilot may be partially blocked (flame too small ie insufficient heat) but often the mercury unit fails.

Unplug the switch from the gas valve, when the phial is cold (ie pilot off for a few minutes) you should get zero ohms between pins 4 and 1 (nothing on the other pins). Now plug it back in and run the boiler for several minutes to get the phial hot. Unplug the switch again and across pins 4 and 1 you should now measure 5000 ohms (the high wattage resistor) and get less than an ohm across pins 4 and 2. With the switch unit unplugged you will also be able to measure the resistance of the main burner solenoid.

If either the vapour switch (about £110.00) or the gas valve (maybe £200) has failed its a corgi job since both involve the gas parts. In addition this boiler needs careful checking for safety.

Be careful unplugging the vapour switch...mercury is not good for you. Any signs of leakage then gloves and sand to absorb the mercury.

 

[simond]

I thought the gas valves were no longer available too, Dave!

 

[Gasguru]

The White Rodgers on the MK II F is available, its the Netaheat Electronics that are obsolete.

 

[hartnellg]

Gasguru

Thanks for the input, and yes, the boiler is a Potterton Netaheat 10-16 MkIIF

I have made the following measurements

1) With the phial cold and removed from the end of the gas valve, the measurements across the pins are exactly as you predict

2) After running the boiler with just the pilot light burning and phial present for about 5 minutes, the measurments across the pins have not changed

3) The gas valve solenoid measurements with the phial removed are as follows:
a) pin 2 to neutral: about 65 ohms - I would guess that is the coil resistance
b) all other pins to neutral were effectively open circuit

So I think that looks like:

1) one of the gas valve solenoids has failed
2) the phial has failed or the pilot may be partially blocked

Either way, I think it is now time for a local Corgi engineer

Any suggestions as to where he might obtain the parts

The gas valve is just marked: 'GAS VALVE' with a WR logo

The phial is marked:
Type 3094-101
White Rodgers
St Louis, USA

Thanks for all your help

Geoff

 

[giblets]

The White Rodgers on the MK II F is available, its the Netaheat Electronics that are obsolete.

You are right GG - I have a brand new one sitting on my desk awaiting a suitable job (or amazing cash offer ).

 

[Gasguru]

I'm not sure you are measuring correctly. After you have heated up the phial measure the contacts quickly before it cools...I don't think you have much time.

I think when you measured the gas valve solenoids you didn't isolate the main burner live feed. You need to pull out the relay, in it's un-energised state the fan is in parallel with the main burner solenoid...I suspect this is why you are reading 65 ohms (probably correct for the fan) but far too low for a solenoid.

Parts will be available through the boiler eng.s usual merchants. If it is the switch take up giblets offer....there aren't too many of these boilers around now.

 

[giblets]

If it is the switch take up giblets offer....there aren't too many of these boilers around now.

It's actually the gas valve I have. WR Type 25K49H-47. Potterton Part No 8905371.

 

[Agile]

The fan will not be exactly in parallel with the main burner.

You can measure for voltage across the main solenoid!

It does sound as if its failed if its o/c but you have to be able to measure properly.

To ny

 

[kevplumb]

****ed again glazier

 

[tonybhoy]

These boilers are death traps ,get rid of it!

And,don't take off the combustion chamber cover!

They are positive pressure boilers so should only be looked at by someone who knows what they are doing.

 

[hartnellg]

Thanks for all your advice and replies

I engaged a local Corgi boiler repair engineer and he has fixed the boiler by diagnosing the gas valve to be at fault and replacing it

In due course I would like to change the boiler, but I want to do it at my time of choosing, i.e. in the summer, after having identified the most appropriate boiler for my needs.
Any recommendations gratefully received

Thanks,

Geoff