Potterton Suprima 50 pump overrun failure

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The manufacturers of my new heat exchanger (fully pumped, indirect open-vented system) recommend setting my Potterton Suprima 50 to MAX, for optimum performance.

This seems to be fine but I am convinced the correctly-wired pump does not overrun. On rare occasions the boiler will kettle (bang) for a minute & also cut out.

I have read here about some basic points to check, such as cleaning electrical connections to the PCB &, on a more advance level checking the overheat thermostat/temperature sensor.

Is Servicesol good for the cleaning & how would you check the overheat thermostat/temperature sensor? I assume I would only need to look at those items dealing with the pump operation.

 

[croydoncorgi]

Are you SURE the pump is wired correctly, from the terminal strip ON THE BOILER?

The 'banging' symptom is NOT good news - the boiler is probably boiling! This is claasic evidence that overrun is NOT happening.

Next check: is the pump turning just after the boiler shuts down (should run on for at least a couple on minutes). Get a big screwdriver, put the end on the pump casing and put your ear to the handle. You should hear a whirring noise.

Finally, have you got zone valves for CH and HW but no bypass arrangement. You MUST have this with a Suprima - see manufacturer's instructions. They do get very hot and bothered without one!

 

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The pump was wired to the boiler a few weeks ago (after several years of being wired to the junction box in such a way that it would always be on regardless of a call from the HW or CH)

Yes, there is a functioning bypass (gets hot when the pump/boiler is on)
CH & HW valves seem fine (Honeywell S Plan)

I have found it difficult to determine the activity of the pump, I usually felt for vibrations, so I will try the screwdriver next time the tank has heated.

Any advice on cleaning PCB & parts?

 

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The pump did overrun for one minute. It shut down & the HW valve could be heard closing at the same time.

Does the overheat stat./pcb control the duration of the overrun or is this fixed? A minute seems a short time to me & leaves me clueless to the kettling (which hasn't happened for a few days).

 

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I think I have figured out the problem but not the solution.

The problem seems to lie within the 10-way Honeywell S Plan.
The boiler Live is connected to the CH/HW valves as a failsafe. When the timer turns off, the valves are closed & the power to the boiler disconnected. Leaving no power source for the pump now linked to the boiler & explains why kettling only occurs when the boiler may have been in the process of heating when the timer shut.

This also explains why the pump overrun seems fine when it is the stat that ceases to call for heat & the timer is still on.

The solution must be a rewiring one but can anyone suggest a safe method?

 

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OK. Have just discovered a relevant wiring diagram in the Potterton manual. This is my interpretation but PLEASE correct me if I am wrong.

The boiler actually needs 4 connections, which is annoying as the installers not only didn't wire in the pump but only ran 3-core to the boiler.
The valves are connected to the Switched Live (not the Live as it is). The Live is the connected to the Permanent Live. Earth & Neutral as before.