Poterton Suprima lock out - Only when thermostat turned up

[JLow75]

Hi all,

I have a Potterton Suprima 100 with the dreaded lock-out problem.

Basically, in ONLY locks out when you turn the thermostat on the boiler up past say half way It NEVER locks out when you keep the thermostat at say '3:00' or lower.

I have already replaced the temperature sensing Thermistor but it has not made any difference.

It has also been fitted with the updated Siemens 5107117 Control unit about 2 years ago - I have inspected the PCB inside & it is immaculate i.e. no sign of any dry joints.

I have not yet replaced the Overheat thermostat could it be this?.

Also my system is wired 'S' Plan & the pump is wired into the junction box & not directly to the boiler - but surely this wouldn't affect it locking out at a certain setting?

Any help or ideas appreciated!.

Thanks in advance.

 

[Dan Robinson]

If the boiler isn't controlling the pump you are lucky it works at any setting.

 

[JLow75]

I will rectify this at the weekend, I've only just noticed it.

I have been in the house 4 years & it's been working fine wired like this.

Like you said if this was the main problem - it would be locking out at any setting, that's why there is something else up too

 

[Dan Robinson]

If you say so

 

[JLow75]

Well I'm not saying so - that's why I'm asking for help & advice on here

I can only tell the symptoms how they are now & that it's been wired like that since I moved in 4 years ago & it's always worked ok (apart from when I needed a new expansion vessel which cured an old problem)

it literally just takes a small increase past half way on the boiler thermostat & it will lock out

 

[JLow75]

Anyone?

Is it worth investing in a new controller PCB? - as the 'margin' of error on the thermostat increase to when it locks out and not is so tight

 

[MrTherm]

No, even a momentary temperature spike can cause permanent lockout until manual reset.

For the pump to operate correctly it **must** be wired back to the pump section of the boiler terminal block. It is these terminals which will control the timed pump overrun (approx 2 mins) and free run check (2 mins in 24hrs).

Having no timed overrun even with a bypass could be the cause your problem. Your method of traditional 'S' plan wiring (fine as it may be) does not in itself control pumped overrun.

To prove the integrity once and for all, i would suggest as an experiment, that you try the pump wired to the boiler terminals and fine tune your bypass throttle to suit the new situation, then come back and inform us of the results.

Your pump is on the return to the boiler which is allowed but your flow bypass manual throttle is slightly open permanently which might be insufficient for to pass the minimum 4.5 ltrs/min, too much open may give insufficient system flow. Therefore i would further suggest if the above does not yield some good result, to fit a 22mm auto bypass valve set to open when pump overruns and both zones are closed - as per the boiler manufacturers instructions.

A sufficient flow around system loop will let you turn the boiler stat near max as the boiler stat setting is always lower than the overheat or limit setting, even by just a two or three degrees and provided that the pump is running until heat dissipation lowers the water temp.

Good luck.

 

[JLow75]

Thankyou.

Yes the pump is now wired directly to the pump outlet of the boiler & It hasn't made any difference, in fact it seems to have made it worse!.

Unfortunately I'm not a heating engineer - I'm an electrical/electronics engineer, so the wiring part is childs play to me, but I really only know the plumbing basics of an S plan system i.e. the layout, which pipe is what, which zone valve etc etc I do not know much about fitting this Auto-bypass valve, or about the manufacturers guidelines of the boiler etc.

Could you advise where & how to fit this valve?, as I will deffo give it a shot, as you certainly know your stuff

Thanks