Potterton Suprima 50L Firing Problems

[gjohn9]

Hi folks.

My Potterton Suprima 50L will not fire and then locks out. When I reset, the fan comes on and I can see the ignitor sparking, but boiler does not fire.

When I check voltage across the wires at the gas valve, between the red and green/yellow wires I see 0v until the ignitor is sparking when I then see 240v, and across the red and black wires I see 160v until the ignitor is sparking when I then see 700v.

I know of the appalling reputation of the PCBs on these boiler, and I'm already on my second, but before I consider buying another one (reconditioned from for £60), does this actually sound like correct operation of the PCB, therefore suggesting that the gas valve has broken? Checking the gas meter, it shows no usage of gas through the 20 or 30 re-starts it's attempted whilst I've been investigating it. Is there any other checks I can do to home in on the problem?

If it's the gas valve I won't be trying to change that myself!

Thanks,

Glenn

 

[Agile]

Do you immagine you are measuring AC or DC or raw DC? Do you know how your meter responds to a non sinewave or to impure DC ?

Rather than thinking about the voltage across the gas valve, have you considered measuring its winding resistance? If its too low it can damage the PCB, if its open circuit it will ( obviously ) not open.

We dont fit repaired PCBs because they are not approved by the manufacturer. In my experience DIYers who change PCBs often try to change their gas valves as well.

Tony

 

[gjohn9]

Hi Tony.

The multimeter was set to measure AC, as I assumed this was what the gas valve would receive as I couldn't see any transformers in the boiler (probably a poor assumption). The fact that I see 240v between red and green/yellow with the meter on AC made me think it was AC driving the valve. The other odd reading was between black and green/yellow, that was always 0v whether it was sparking or not...as opposed to the odd reading between red and black - 160v and then up to 700v!

I'll measure the resistance of the windings - any idea what it should be or between what pins I should be measuring (red-green/yellow, or red-black)?

I presume the electrical side of the gas valve cannot be changed on it's own, without having to worry about opening up the gas side (which I won't do).

Thanks,

Glenn

 

[ChrisR]

There are two coils, resistance approx 1400 and 2800 ohms. Joined at centre contact so if you measure end to end contacts you get 4200. Coils aren't separately changeable.
Sometimes tapping the gv will make it unstick if that's what's happening.

 

[geoff@cetltd]

Hi Tony.

The multimeter was set to measure AC,
I'll measure the resistance of the windings - any idea what it should be or between what pins I should be measuring (red-green/yellow, or red-black)?

I presume the electrical side of the gas valve cannot be changed on it's own, without having to worry about opening up the gas side (which I won't do).

Glenn

If you ring CET, I'l lead you through it

The volts to the gas valve are DC not AC. and should be about 230V. If they are >300 volts as in the manual, you have a gas valve solenoid open circuit

Chris has already mentioned the DC resistance of the gas valve solenoids

 

[gjohn9]

Hi folks.

Thanks all for your help so far. From measuring the resistances it looks like the larger 2800 ohm coil has gone. I measured the voltages as DC across the connector with it unplugged, and the numbers still don't add up, but I guess that may be a consequence of it not being plugged in. From the diagram and figures below does it at least look as though the PCB is OK (the two voltage figures for each measurement are before ignitor sparks/during sparking):

Thanks again,

Glenn

 

[Agile]

As I tried to guide you, its the resistances which indicates if the gas valve is faulty NOT the voltages applied.

If the coil is o/c then pretty obviously its not going to work and should be replaced ( by a CORGI ) if you think the boiler is worth repairing bearing in mind its reliability and efficiency.

Measuring "DC" voltages on gas valves is only an indication. This is because the applied voltage is normally raw DC and digital multimeters give widely varying indications of the voltage.

The better quality meters like the Fluke and its variants give a very good approximation to the RMS equivalent but the cheap ones can vary from rather low to very high so different meters could read from say 180v to 360v when the Fluke would read 230 V. Some will not even give a stable reading.

Tony

 

[gjohn9]

Thanks Tony.

Does this site have gas engineers associated to it that I can get in contact with to do the valve replacement, or is it advice only?

Glenn

 

[geoff@cetltd]

As I tried to guide you, its the resistances which indicates if the gas valve is faulty NOT the voltages applied.

Tony

Both should give an indication

As I said above anout 220-230 v DC indicates that the solenoids are OK, >300 vDC and the pcb is driving into an open circuit