Prima 50F - is this likely a PCB issue?

[dingo45]

I have an old Potterton Prima 50F in an S Plan system and an intermittent issue.

Occassionally, becoming more frequent, the following happens:

When there is demand for CH or HW, the relevant zone valves open and the boiler fires.
However the pump (a Grundfos Alpha 2 25-60 130) does not power up (display remain off). Conesquently the boiler starts kettling.
Sometimes, the pump eventually comes on, things get moving in time and the boiler settles down.
Alternatively it will shut off, start again, & sometimes the pump begins running.
Least commonl outcome, it will cut out altogether via the overheat thermostat.

Most of the time it operates normally and I have not managed to determine how often there's a delay vs failure in the pump powering up.

It seems to me the problem lies with the boiler failing/delaying to switch power for the pump.

What are the likely causes of this intermittent issue?
The PCB or some other component in the boiler?

I plan to get a new PCB but wonder if it is actually the most likely culprit.

 

[CBW]

It could actually be the pump starting to fail

 

[dingo45]

Thanks CBW. I guess I need to monitor for power arriving at the pump during an event it fails to start up.
Is an intermittent fault with that make of pump likely to manifest itself as a totally blank pump display? It don't even flash/blink. Normally this grundfos Alpha goes through quite a palava befre finally pumping!
EDIT: Also, is it conceivable that a PCB fault could result in intermittent failure in delivering power to the pump? Is there a dedicated relay on the board for switching power to the pump?

 

[ianmcd]

Thanks CBW. I guess I need to monitor for power arriving at the pump during an event it fails to start up.
Is an intermittent fault with that make of pump likely to manifest itself as a totally blank pump display? It don't even flash/blink. Normally this grundfos Alpha goes through quite a palava befre finally pumping!
EDIT: Also, is it conceivable that a PCB fault could result in intermittent failure in delivering power to the pump? Is there a dedicated relay on the board for switching power to the pump?

The pcb does not control the power to the pump, this is done by the seperate pump over run relay

 

[dingo45]

The pcb does not control the power to the pump, this is done by the seperate pump over run relay

Thanks Ian.
Next step I guess is to check for power at the pump during one of these events.

 

[ianmcd]

Thanks Ian.
Next step I guess is to check for power at the pump during one of these events.

Yes you will know if the pump is getting power, if it isnt it will be the over run relay , if it is getting 230v AC then it is the pump (most likely culprit)

 

[RomanAbramovich]

I keep an eye on my elderly mothers ch system, it is identical to yours - we experienced the same symptoms, replacing the pump resolved the problem.

Back in the spring I purchased a very low cost 'trident' pump from a fleabay store, installed it and all was well again.

 

[dingo45]

I'll update this once I find out the problem. I do have homecare so will probably get an engineer out. But intermittent problem means sods law and it will operate fine when they come.

Something that occurred to me whilst looking at this. With HW demand, the return near the boiler is about 10C cooler than the flow. Is that something to be expected when only HW is being called?

Also, regarding the boiler overrun themostat:
Say, the heating controller stops calling for any heat, the zone valves close immediately. AIUI there's no provision in S Plan for the boiler to keep zone valves open during pump overrun. So presumably water must go through the ABV (which we have) when overrun kicks in.
Pump overrun is something I can say I have not seen occur while looking in to this. Pump always goes off with the zone valves.
How long, typically would pump overrun go for? Is it something you expect to see on a regular basis?

The pump I think is maybe 5 year old and works well, when it's running. My instinct is going toward boiler overrun stat. I have yet to capture voltage reading during an 'event'. I want to have some V measurement arrangement that doesn't leave exposed wires in the airing cupboard! EDIT: I will wire a bulb to the same terminals in the connector box to the pump.

 

[ianmcd]

Pump overrun is something I can say I have not seen occur while looking in to this. Pump always goes off with the zone valves.
How long, typically would pump overrun go for? Is it something you expect to see on a regular basis?

The pump overrun has a Phial same as any other thermostat, and it will depend on the residual temp in the heat exchanger if it actually comes on or how long it stays on, and yes the water is circulated but the ABV then the overrun will control the pump, here isa pic of it

 

[old duffer]

PCB component failure is rare and very limited to the failure point (always high voltage switching). The simple rule is that if it works at all then it works. Suggest you look at the simple low cost things first and I'd start with the thermistors as these will create intermittent problems you outline due to the tiny variation in resistance they monitor.

 

[dingo45]

The pump overrun has a Phial same as any other thermostat, and it will depend on the residual temp in the heat exchanger if it actually comes on or how long it stays on, and yes the water is circulated but the ABV then the overrun will control the pump, here isa pic of it

Thanks Ian. I just managed to find the Potterton installer's manual {original paper copy from when the boiler was fitted).
According to the fault finding chart, the pump overrun not operating -> replace overrun thermostat.
And since I also have the intermittent failure to power pump (still to confirm this vs a pump issue), the overrun thermostat is looking like the culprit.
I checked the availability of Potterton part No. 404507. Whilst it's still obtainable new and used, lots of suppliers show it as obsolete and out of stock.
We have been with BG Homecare since the boiler was installed 30 years ago, I wouldn't be surprised if they've no stock of that part and decline to repair the boiler. Whilst I would consider tackling a PCB swap out, I am not qualified to do the overrun stat since it involves removing the sealed case cover. In a way, I hope it is the pump as BG will have no supply issues replacing that.

 

[dingo45]

PCB component failure is rare and very limited to the failure point (always high voltage switching). The simple rule is that if it works at all then it works. Suggest you look at the simple low cost things first and I'd start with the thermistors as these will create intermittent problems you outline due to the tiny variation in resistance they monitor.

This boiler has had one control board replaced to my knowledge. Probably within the last 5 years. I wished I'd asked to keep it and got it repaired, as a spare! But I agree, this issue does not appear to be a control pcb problem.

 

[ianmcd]

I'd start with the thermistors

Boiler is firing its just the pump is not always running, nothing to do with the thermistors, which this boiler doesnt have anyway, it has a liquid control thermostat and it isnt that either

 

[dingo45]

I have wired up a small bulb to PL in the airing cupboard junction box. Next time the pump doesn't run, I plan to get there quick and see whether the bulb is lit.

 

[dingo45]

Just seen the pump lose power and a variation on the issue I have not witnessed until now:
No demand for CH or HW. Boiler off. Pump off.
Demand for HW arrives.
Zone valve opens, pump comes on, boiler fires.
Pump ran for about 10 seconds then went off. The temporary light also went out.
Boiler continued to run. I turned it off power at the isolator before it started kettling badly.
Waited 10 seconds and turned on power.
Pump and then boiler ran as normal and continued to do so.

So I think this confirms issue is with the boiler. What's more I have now also witnessed the boiler start the pump successfully only to then cut power a short while after.

Does all this still point to overrun thermostat?

Many thanks for the input.