Boiler keeps firing up with no demand

ninjaef

Appreciate this has been asked on the forums however the answers are specific to the system.

*Potterton Promax SL
*S Plan Room Thermostat, CH interlocked by room thermostat. HW and CH also interlocked by 2 channel 7 day *Programmer device.
*HW Storage Tank
*Sealed System


My boiler is firing up even when there is no demand from the control unit (programmer).
This used to happen once every hour maybe a year or two ago. Now it's down to circa every 10 minutes. Every 10 minutes the boiler fires up for about 2 minutes.
I have always disregarded this because I've been advised it is "frost protection" and "normal operation". No, it cannot be surely.
It was serviced November 2018 but the bloody thing didnt do this naughty behavour so the engineer couldnt remedy it - he claimed

Initial thoughts:-
* Frost protection? But why fire up every 10 mins; and it used to be every hour about 2yr ago.
* Sticking HW/CH Zone valve; need to check these but how?
* Faulty programmer; if I take it off the back panel can I still test the system and how?
* Something mechanical in the boiler deteriorating over time?

There's been no work undertaken for 2 years on the boiler; last work was a service.


I'm and electronics engineer but not HVAC or Gas / Domestic so could someone please help guide me through diagnosis.



Thanks all

Ant555

my WBosch combi has a setting where the hot water is pre-heated to a certain temperature so that it gets delivered slightly quicker. I see your setup includes a hot water tank so it might not apply here but I mention it just in case.

When boiler is firing up - does the programmer show that its 'active' - mine has a flame icon that lights up.

Do you happen to hear the thermostat 'click' when its activating every few minutes?
Does it still do this with the thermostat turned right down?

ninjaef

Hi

Don't know if mine gets pre-heated, but why every 10 minutes for 2 minutes 2 time. It never used to and ohhhh my gas bill !!

Programmer is not demanding CH or HW so no - it does not show active

Which thermostat? The room thermo is solid state electronic. There's a HW stat on the tank. No clicking.

There is no user adjustable thermostat on the boiler, and anyway if there is one on the internals which I do not believe there is, then that hasnt been changed or adjusted either. But I turned both the tank and room thermostats down - no difference.


These are obvious candidates so I guess Im looking for a bit more in terms of diagnosing this particular system.
I think what I shall do this afternoon is attempt to find a "live" demand voltage, and trace this back., assuming the demand is coming from somewhere - but I think it's something to do with the boiler internals that is degrading over time.

Alex1983

I would think it's a sticking zone valve, depends on the zone valve manufacturer to how you would test, you can also check with a multi meter on the electrics.

ninjaef

Honeywell V4043H. . I've replaced two of them in two years , £60 a throw each. Might as well buy "eco" cheapies from fleaBay or £22 ones from bes.co.uk ??

I have a decent multimeter so will check the motor inputs and switched live output. I'm sure I saw the arms move though for On/Off cycles, but I'll double check.

Alex1983 wrote: »

I would think it's a sticking zone valve, depends on the zone valve manufacturer to how you would test, you can also check with a multi meter on the electrics.

a&akay

Simple check for a standard sticking zone valve. Set all thermostats to a low reading below the ambient. If the lever arm on the valve is slack then it's seized 'on' and the microswitch on that control line will demand heat no matter what. Happened to me twice over 30 years, but I've got 4 zone valves.

ninjaef

Guys, to clarify...

boiler is TURNING ON when it shouldn't be

ninjaef

apologies. I misread posts a&akay

Alex1983

Boiler will fire with failed zone valves, turn programmer off and check for 230v on the orange, if there is the zone valve is faulty, you will need to turn the power off and split the wires as there will be a orange for each zone, to identify the faulty valve.

southcoastrgi

You haven't said where the boiler is located & if it's an unheated space if there is a separate frost & low limit pipe stat

ninjaef

Boiler is in the kitchen, large double rad nearby.

I have no idea if there is a forst/low limit "stat" in the boiler apparatus - I do not have the install or service manual nor am I a Gase Safe engineer. I am able to replace Zone Valves and Combi Valves and so on as these are simple but I would not undertake work to the boiler. Hence why I am here, to obtain help, if I were a Gas Safe engineer I doubt I would be asking this question on MSE would I ?

southcoastrgi wrote: »

You haven't said where the boiler is located & if it's an unheated space if there is a separate frost & low limit pipe stat