Only reading 100V output from boiler to CH pump

[Dingers]

Just wondered if anyone can help.

The CH pump is not coming on when the boiler fires up. The pump works ok if I wire it independantly. I'm only reading 100V from the boiler terminal to the pump (240V pump). Is this a problem with the boiler pcb or is it something else?

The boiler is a potterson suprima 60L

Any help would be appreciated and might save me time and money

 

[kevplumb]

from the mi's

View media item 16657

 

[Dingers]

Thanks for the reply. There is power at the pump but only the same 100V from the boiler. If I have the pump running independantly but continously the CH & HW seem to work Ok. It seems to me that it is the PCB but not 100% sure

 

[kevplumb]

apologies i didn't realise what a crap pic that was

its from the fault finding in the mi's if you look

 

[ollski]

What voltage do you get on the orange wire at the valve?

 

[Dingers]

Yes, there is 240V on the orange wire coming out of the 3 way valve

 

[Agile]

At the time the boiler is supplying 100 vac to the pump what is the voltage at mains input to boiler?

All voltages should be measured to earth in this situation but also measure from earth ( chassis ) to the neutral in this condition.

Tony

 

[Dingers]

The voltage to mains at the boiler is 240V. The voltage between neutral and earth is 0 volts.

With the CH programmer set to CH/HW on, or CH on only, or HW on only, the voltage to the pump from the boiler is 108V

With CH programmer set to CH and HW off the voltage at the boiler to the pump is 36V, mains voltage to boiler still 240V.

I see on the 3 way valve instructions there is a condition with a 100V output to hold the valve in the furthest condition, but this would be independant from the output voltage from the boiler wouldn't it?

 

[Agile]

The boiler is supplied from the permanent live feed directly to it.

The pump, if correctly wired is supplied from the boiler through the PCB to give pump over runs.

The call for heat connection is a high impedance input which resonds to the voltage without drawing any significant current.

If the pump neutral is as expected at 0V the the only place for a loss of power is within the PCB. Its difficult for me to imagine where that voltage loss is occuring as it will be accompanied by a disapation of 20-30 watts and that is a bit of heat!

I can only guess that its as a result of dirty contacts on the fan relay. To check this it would be necessary to measure the voltage across these contacts when its energised!

The normal solution is to replace the PCB. Some might replace the relay!

Tony

 

[Dingers]

Ok thanks for the advice.

How easy is it to replace the PCB on the Potterton 60L. According to the instructions the control boeads rotates down at 45 degrees. Do you need to take it competely off to replace the pcb?

 

[Agile]

Its not that easy!

The existing PCB was so unreliable that it has been replaced by a modified design which is about £160 !

As it comes with a new wiring harness it needs an RGI to fit it.

This is one of the few situations where a DIYer might consider using a repaired PCB from a firm like CET.

A professional should not use these and is not allowed to by law.

If its the relay as I expect and you can identify it then a local TV repair shop would probably be able to replace it for you. But be aware I have not seen or tested your boiler so I cannot be sure its the PCB at fault.

For the weekend you can power the pump independently if you want heating. Just remove the pump lead from its position on the choc block and put it onto the live feed. ( After dissing the power at the isolator )

Tony

 

[Dingers]

Hi Tony

Thanks for all your advise, it sounds too tricky for me to tackle I'll contact a RCE tomorrow, but at least I know understand the problem

 

[simond]

Stupid question perhaps but are you measuring the voltage to the pump on the terminals within the Suprima boiler itself or in a wiring centre somewhere?

The most common reason for a spurious reading like this is an open circuit neutral to the pump.

 

[Dingers]

Hi Simond

Thats confused me a bit. I was measuring the voltage at the terminal inside the boiler, but with the live wire to the pump connected.

There is no neutral or earth connected to the boiler pump terminal, the neutral and earth to the pump come from the terminal block in the airing cupboard.

With the live wire to the pump conected to the boiler I get a voltage of 108V, with the live wire removed, (no wires in the boiler terminal block) I measured 0 volts between the live and neutral.

All reading taken when the biler was fired up.

Where's the 108V coming from?

 

[Agile]

You have confused me now too!

You dont "measure 108 vac at the pump live terminal".

You measure the potential difference BETWEEN two points.

I asked you to make all measurements between chassis ( earth ) and the pump then neutral and then to the pump live.

Now I wonder if thats what you have done?

Tony