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Question - mend, repair, fix

How can I fix my Potterton Prima 'F' boiler?

When the hot water or heating is switched on the boiler attempts to start up, i.e. the fan starts and stays on for about 15 seconds and then stops for about 10 seconds. This cycle goes on for about 10 attempts before the boiler finally lights up. Then it behaves normally, well almost! Sometimes the fan will run for an hour before it fires up.
I've had a plummer in to service it but he failed to sort out the problem. He said it could be the 'air pressure switch' or the 'black box', but could not say for definite. He suggested that I wait until it gets worse.
Can anyone please help!!

February 2006

There is more help available. Mend - potterton, prima, f, boiler
Need to mend your Potterton home heating system?

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Answers - mend, repair, fix

May I add my grateful thanks to the contributor with the details of the capacitors to change. With three repaired control boards and a spare fan I hope to get another 25 years of service from my Potterton F60.
I consulted a local (older) plumber with the idea of changing it for a newer one but he advised against it as these boilers were designed to last for ever.

William Wombat
March 2019

P.S. For others who may wish to change the electrolytic capacitors on the control board:
Commercial Electrolytic capacitors have a wide tolerance of 20% at least, their value can change over time, and their value in the circuit is not critical.
C4 and C7 (22uF) perform a smoothing function, so any value above 22uF will do - but for circuits working at 50v, a high operating voltage at least 64v, preferably 100v would be better.
C6 (4.7uF) may be part of a timing function, but any value around preferred values 4.7 or 10uF will be fine. I had some 10uF 100v capacitors to hand and used one for C6 and 2 in parallel for each of C4 and C7.
Tantalum capacitors with the right values and working voltages would be better - a little bit more expensive, but everlasting!

Roger Forshaw Romsey Hants
March 2018

My boiler is a Potterton Profile 80e - but you'll find this relevant:-

10 years on .... (you will see my previous entry from Feb 2010) ... control board playing up again!
First suspected the gas valves sticking - but on testing they are fine - then suspected the air-pressure switch as the red & white plastic pipes easily slip off the pipes at the back of the switch. But that was OK. Checked fan which I had replaced in 2002 when a hibernating bumble bee had unbalanced the fan and ruined the bearings!

Still unreliable - the boiler sparked into life with tiniest tap on the circuit board cover - so suspected a dry solder joint or stcking relay. After removing the control board, prising off the covers of the relays was not so difficult with a stanley blade - used some very fine grain emery on the contacts. But still no reliable sparking. So finally, decided to apply my soldering iron to every solder joint on the Printed Circuit Board. Came across several joints which were dodgy - where the solder broke away from the space between the component wire and the copper board. A higher quality (MoD standard) of solder joint would have the component wires bent down against the tinned copper track, so that a mechanically robust joint is formed with the solder just providing electrical connection rather than bridging the annuar gap between component wire & PCB.
Anyway - this seems to have done the trick ..... at last!
Other design issues with this boiler include the frailty of the plastic under-tray - which tends to disintegrate after years in a warm environment. A metal bar across the underside using a longer retaining screw sorted that one. The fancy temperature control knob also suffers the same fate - replaced with a more robust volume control knob on a small shaft extension.
Hope others find this useful.

Roger Forshaw
March 2018

Result !! Many thanks to Tommy Taunton. Got the 3 capacitors from Maplin (67p !) and got a guy who does PCB repairs to swap them (£6.00). Up and running again. All seems good so far. The power of the internet and the generosity of a knowledgeable poster has restored my faith in human nature. I hope I can repay the favour by providing a solution to someone else's problems.

February 2018

Does anyone have the schematic diagram for the 407677 PCB, before I spend hours tracing it out from the PCB ?

H Brunt
October 2017

just did the £2 caps fix from maplins and went around the pcb for dry joints, my 21 year old potty is now working like new, thanks everyone.

March 2016

Thanks forum members the hair dryer trick worked have to find my soldering iron now!

March 2016

Potter ton Prima 60f intermittent starting, changed APS £56 2days ok then same trouble ,changed PCB for a reconditioned one £31plus £7.50 envelope and postage ,1day ok. all the time Wd40 and oiling fan same problem but only worked when I Wd40 fan been running now for 24hrs watch this space (heating engineer)

Georg webber
February 2016

The fan on my 20 year old Prima 60F boiler gave up a couple of weeks ago with seized bearings causing the boiler not to fire up. I took the unit out and managed to free it up, but thinking that would just be a temporary fix, I ordered a new one along with the two fibre gaskets, the originals having disintegrated during removal of the fan unit.

Having fitted the new fan unit, I found that ignition was intermittent and mostly when the boiler was starting from cold, but could be started by thumping the casing. I had had previous trouble with dry joints on the connector at the front of the electronics board so I decided to remove it and check for more dry joints. I couldn't see any so I did a Google search and found this thread and learned about the three capacitors, which I then bought (100C, 100V versions) from Maplin, fitted and re-fitted the electronics board.

This time I had trouble with the fan powering up and down every couple of seconds, so I ordered a new pressure switch and fitted that. The fan powered up properly but the ignition was still intermittent and I discovered that by tapping the metal tray on which the electronics board is mounted, a spark would be generated. Out with the board again and a detailed inspection of the solder joints with a magnifying glass and there it was! The output pin from the spark transformer had a dry joint; it looked OK at first glance but by pulling and pushing on the transformer casing, the pin could be seen moving up and down in the solder. A quick dab with the soldering iron on that joint and a few others just to be sure; back with the board and bingo,the boiler worked perfectly.

So some things to look for based on my experience:-

1) The spark between the electrode and the shield should be a long yellowish arc connecting both. If the spark is bluish-white and about the size of a pinhead located in the gap, then there is something wrong with the spark generation electronics.

2) If the pilot light does not ignite immediately, the spark should repeat every one to two seconds. If it does not do this, there is something wrong with the spark generation electronics.

3) If thumping the case, or tapping the metal tray with a heavy object, such as a screwdriver handle, causes the boiler to ignite, there is something wrong with the electronics board, most probably a dry joint.

4) If intermittent ignition occurs, primarily when the boiler is attempting to fire from cold, then there is something wrong with the electronics board, most probably a dry joint.

5) If the fan only spins up for a couple of seconds and then stops and the cycle repeats, suspect the air pressure switch.

After all the faffing about, I am expecting another 20 years out of this boiler and the electronics board is probably working better and is more reliable than when the boiler was new. The soldering quality was abysmal!

Total cost: £120 for the fan unit, £8 for the fan unit gaskets, £50 for the pressure switch and a few pence for the capacitors.

Many thanks for all the info from the others that have contributed to this thread, especially about the capacitors.

September 2014

Changed capacitors and worked fine, all for an hours work and less than £2!

Scott W
March 2014

Be warned: do not attempt soldering unless you're confident of your soldering (and de-soldering) skills and equipment. I hadn't done any for many years and made a botch of the connections. As a result I was forced to buy a new PCB on eBay, which works beautifully.

PCBs are delicate and I think I may have damaged some of the on-board metal.

All-in-all, spending around £120 is better than the full cost of a call out and fitting, so I'm happy.

I also found the disassembly / assembly instructions here very useful:

Jim N
January 2014

Another success for the hair dryer quick fix. My Profile 50e was trying to fire for over an hour after a few days' holiday with the heating turned down. Three minutes of the hair dryer with the bottom cover off did the trick. My girlfriend now thinks I'm some kind of genius.

I'll examine the PCB tomorrow and report back.

Jim N
January 2014


Less than £3 to repair my boiler rather than £300 odd.

What a result

December 2013

Hi Robert,

Yes, the links were for 105C radial parts.

In fact, I did buy a 105C axial part for C7:

but it was considerably more expensive than the radial ones, and slightly too long for the space available, so I had to mount it proud of the board and fold the wires in a bit. Not a perfect solution.

In hindsight, I would have just fitted the cheaper 105C radial part, but I'd already bought the axial one. I can't be 100% sure, but I think my PCB had holes for a radial alternative.

Boiler still working absolutely fine several months on and now a lot colder.

November 2013

You provide two links (thanks), but neither link to an axial capacitor as required for C7. Do you have a link for it, please?

Robert Reid
October 2013

Thanks everyone. My symptom initially everything dead, fixed obvious dry joints on 6-way connector, then fan cycling on/off every 5-10 secs as air pressure ramped up & down. Initially suspected RL2, but C7 22uF 63V was my problem: -ve rail was wandering 42-48V with 5VAC on top. Now a rock solid 50V consistent with 51V zener. Also replaced C4 & C6 while I was there.

Used 105C parts from CPC, instead of standard 85C:

Remember to check the flame supervision/rectification function after repair - turn the gas off at meter (or unplug connector to gas solenoids) and check that RL2 is energised and the ignition restarts. When gas turned back on (or connector plugged back in) the first spark should ignite pilot and then RL2 de-energises and the burner solenoid (GV2) will open.

FYI, a direct equivalent for the original Omron relays is Schrack/TE T7NS5D4-48 (low power 360mW coil), available from CPC (SW03941), Farnell (965-9650) or Ebay.

September 2013

I have pictures of all of the components if anyone needs to see the values of any of the components

April 2013

I needed two x 22uF and one x 4.7uf capacitors. I used the 100V high temperature versions from Maplins. 22uF = DT58N, 4.7uF = DT54J. Boiler now starting every time!

February 2013

Similar to what Paul H as mentioned below, can somebody please confirm the correct Maplins part numbers for these three capacitors please?

On the Maplins site:

I can find the part number VH28F
I can find part number VH19V

But I cannot locate part number AT40E

Many thanks in advance for any help given.

February 2013

In answer to a query made in an earlier post, the fan can't run backwards - it's a shaded pole ac affair. Potterton manual says that the connectors can be put on either way round.

My problem appeared to be a PCB with dried up capacitors and a ridiculous number of dry joints. After I fixed the PCB, it didn't do the frantic clicking any more, but still took an age to start (minutes to hours).

Followed Potterton's flow chart and found that the air pressure switch contacts weren't changing over. The fan appeared to be whizzing around merrily but I was only getting a differential pressure of 2 mm H2O on my home-made manometer. You need 3.5 mm to actuate the APS. I checked the seal thoroughly for leaks (none), checked the APS tubes for blockages (none) and concluded that it must be a weak fan.

I took the top bearing off the fan and cleaned everything up and sprayed a bit of WD40 onto the shaft near the far bearing that I couldn't get to (due to rusty non-metric grub screw holding impeller onto shaft). Now the fan didn't want to spin at all. I dismantled the fan again (breaking what looked to be a graphite washer in the process), flushed out everything with WD40 and then put a drop of engine oil on the shaft and reassembled. Whoosh! the fan ran at a much higher speed, generating lots of air flow, and more importantly a differential pressure of about 10 mm H2O for the APS.

So boiler is now working, and replacement fan is on order. Wife is warm. Everybody happy!

Paul H
February 2013

In answer to a query made in an earlier post, the fan can't run backwards - it's a shaded pole ac affair. Potterton manual says that the connectors can be put on either way round.

My problem appeared to be a PCB with dried up capacitors and a ridiculous number of dry joints. After I fixed the PCB, it didn't do the frantic clicking any more, but still took an age to start (minutes to hours).

Followed Potterton's flow chart and found that the air pressure switch contacts weren't changing over. The fan appeared to be whizzing around merrily but I was only getting a differential pressure of 2 mm H2O on my home-made manometer. You need 3.5 mm to actuate the APS. I checked the seal thoroughly for leaks (none), checked the APS tubes for blockages (none) and concluded that it must be a weak fan.

I took the top bearing off the fan and cleaned everything up and sprayed a bit of WD40 onto the shaft near the far bearing that I couldn't get to (due to rusty non-metric grub screw holding impeller onto shaft). Now the fan didn't want to spin at all. I dismantled the fan again (breaking what looked to be a graphite washer in the process), flushed out everything with WD40 and then put a drop of engine oil on the shaft and reassembled. Whoosh! the fan ran at a much higher speed, generating lots of air flow, and more importantly a differential pressure of about 10 mm H2O for the APS.

So boiler is now working, and replacement fan is on order. Wife is warm. Everybody happy!

Paul H
February 2013

Could someone, that has done the capacitor replacement confirm the values of C4, 6 and 7.

On my board I have the following:
C4 = 22uF 63V (can type)
C6 = 4.7uF 63V (radial)
C7 = 10uF 63V (radial)

I have tested my C7 and that measures 5uF, so that is definitely suspect, the other 2 measureok.

January 2013

been reading all of this has nobody ever thought to just tap the plastic outter casing while its on it 15 priming cycle, you should then get the click, then ignition ten heat. really simple.

November 2012

its the pcb board which is basically the brains of the boiler mine was struggling to ignite then finally it wud fire up after around 10 attempts until it finally gave up this weekend. Not a cheap fix was £263 but is working tip top now. Hope this helps.

October 2012

I know nothing about Boilers and have been reading these comments , I have Prima F series boiler that actually ignites for a minute or two then dies, it makes clicking noises after that and not much else anyone any ideas what the problem is ? cheers

July 2012

Hi guys - sadly this isn't working for me - can anyone help please?! I am reasonably competent at electronics and have replaced all three troublesome caps and checked the PCB closely for other issues but all seems well. Still no starting up though. When boiler is turned on I get the fan whirring and if I turn the boiler heat control know slowly from 0 to max I can hear one relay/switch click followed by another but nothing else happens. Boiler has been working fine for the three years we have lived here and was serviced a few weeks ago but worked fine non-stop since the service up to now.

March 2012

I had all sorts of intermittent problems: failing to ignite, cycling through and playing dead. Thanks to Leon Funnell Jan 2009. I pulled out the PCB and broken solder joints all over the place. Resoldered all the bad joints and it fired up straight away. Now running like a dream.

February 2012

Thanks Guys I had the same problem with my potterton with temp dropping on initial start up. thanks a lot for the advice......jimk

February 2012

Thank you for the information on the capacitor replacement. I suffered the same symptoms as most others on this forum, so visited Maplins this afternoon and replaced the three capacitors. Wonderful!! Fixed straight away, thank you. Coldest night of the year last night and it wouldn't start this morning, just kept on cycling through with no ignition. Let's hope everything will be good tomorrow morning. Thanks again for the advice. Much appreciated.

Norman H
February 2012

I have a Prima 50 F boiler with the same problems with not firing. If I tap the front of the boiler it fires up but intermittently doesnt cut back in. I would like to try the three capacitors but from the posts I cant be sure which three to replace. Does anyone have the maplin reference or the spec of the capacitors.

Any help much appreciated


February 2012

Can anyone tell me the specs for the 3 capacitors on the PCB (for Prima 100F) that people have been changing and resolvig start up problems with?

July 2011

Had the same problem on my boiler for ages, fan running but not sparking up. Changed the three capacitors today for under £15 including buying the soldering iron and solder. Boiler now runs like new, sparks up first time and runs quieter than before. Thanks for the advice.

June 2011

I have just done the 3 capacitor fix and low and behold my profile prima 60E has the fan working, calls for heat but no spark, I lit it with a match and it runs fine. Has anyone else come across this.

March 2011

I had been experiencing the same start up problems on my Profile 60E for a couple of months. It took less than an hour to change the three capacitors, and the problem is now fixed - just like that! Fantastic advice Mr. Berry and thanks for sharing it on the forum. Excellent!!!

Guy Martin
March 2011

Just done the 3 capacitor fix and low and behold my profile prima 30E started first time acid test will be tomorrow morning on a cold start, anyways just wanna thank all on this forum you just saved me 150+VAT (that I was struggling to save.)

March 2011

Hi Guys
Just turned on hot water and boiler fan just cycled. Boiler wouldn't fire. Took a look at these posts and tried the hairdryer. Fired after 30 seconds. Will be taking atrip to maplins for the capacitors.

brian gilchriest
March 2011

I'd just like to say a big thank you to everyone who contributed to the info here. I replaced the capacitors on my control board and all is now working fine. It cost less than a pound to fix - amazing!

February 2011

Hi, Symptoms the same as all previous posts. I obtained all three capacitors from Maplin for the grand total of £0.74, these were very easy to replace and boiler fired first time. The best bit I saved a £75 call out fee plus a quoted price of £175 for a new control board, all plus VAT.
So a very big thank you for the excellent advice.

Chingford Boiler
February 2011

I have a 22 year old Potterton 60e which was giving the same problem i.e. taking several attempts before firing. I tried the usual things: door seal, checking the air pressure valve and re-soldering all the joints on the PCB. I found this web-site and immediately went to Maplins for the three condensers. They only stock the radial type now which wasn't a problem as there was plenty of room on the pcb. Problem solved. The boiler is working fine now and firing every time - not bad for a total cost of 63 pence in 22 years.

February 2011

Potterton Prima F60 not fireing up. had an ex British Gas man round who said it was the fan, he ordered one (£180) fitted it, the fan came on and the boiler tried to start but just keep cycling through. He then decided it must be the PCB and said that would be another £125. Told him I had seen some where that did exchange ones for £30 and I would get one and fit myself.
I then looked at postings about this fault and tried the Hair Drier trick and it worked at treat. Thanks for that we now have a warm house after 3days with no heating. Will get the 3 caps from maplins and repair myself.
Again thanks for the tips

Happy & Warm Andy
February 2011

Hi, many thanks to this post!! Eventhough I hooked the pcb up to an oscilloscope , it took me all day to cop to Google here ( the C7 ,22uF 63V was my culprit) . When this is dud it makes the 48V "dc" chatter like hell . The 48Vdc is below the Neutral rail and this cap being dud lends for a large spike back "up" to the zero. The relays then can not "click" , in the "meanwhile" of the pressure switch changing over from "white to brown" to the then "brown" carrying the mains via the RL1's contacts to the pressure switch"s "Orange" and back to the pcb and on to the gas valves etc. Both relays have to "pick" (energise) before the flue/fan develops the air pressure. I reckon that "cold" temperatures would "stiffen" the diaphragm of the pressure switch and make it slower to flip it's microswitch , which should be a good thing except that the capacitor has dried out with the normal running heat in it's usual life so that it's value is way off. Heating it back up, as I read in some posts, is then making matters worse in the longterm , but would work as a quick remedy, but not for long, i reckon.
I went through virtually every other component in the pcb ,unluckily, beforehand, so there are still loads of other possibilities for someone else's snag as several could give similar upshots.
C ya

January 2011

if you the three wires are becoming live off the pressure switch when the fan runs .It Then points to the pcb.if any of the three wires from pressure switch are dead when the fan runs it is then the pressure switch.

January 2011

Hi, i have an intermittent prob on a Potterton Profile e40, unsure if PCB also: sometimes when boiler lit you can hear - i think the main gas valve - cluncking on & off (like relay chatter) & as it does the main gas burner flame jumps up & down as if gas supply being interrupted.
Gas valve or PCB any ideas please??

David w
January 2011

I was very VERY sceptictical about the hair dryer thing, but it DID work on my potterton netaheat profile 50e. I think whats happenning is that, in very cold weather, you have a power cut, turn the boiler off etc, and when the boiler stops heating, the fan keeps running for a while, drawing in cold air and moisture.This then freezes in the air pressure circuit, stopping the rest of the ignition process. (I might be wrong, any thoughts?) Thankyou to whoever it was that suggested it. If you are a woman, hairdryer will be on hand, blokey, borrow one from your mum! I am soooo happy it worked. That may be why there have been so many posts over the last three or four weeks. very cold intake on the flue, freezes the air pressure lines or switch, even though it is inside.
I hope this helps someone else. You all stay safe and have a good christmas and new year, and love to everyone you hold dear!

December 2010

Wooohooooooo!! same problem with mine. Of course the boiler failed on Friday night just before Christmas on the coldest night of the year so far.... No-one can come and help and we have so much snow that i cant get to the suppliers to buy a new control box. The trick with the hair drier has got the thing going for now though, so i am warm again! Thanks guys

keith jecks
December 2010


possible short term fix

not sure if will help any one or how safe it is but....

our Potterton Prima F60 had been cycling for ages but it never really gave us much of a problem. Then during that last really cold snap we came home to find no heating or hot water.
The Prima was cycling away but not firing.
Tried hard to find a gas engineer to come & fix but all busy for week or more....must re-train as Gas man..

Anyway, for why i do not know, i felt it would not fire coz the boiler seemed stone cold so....
I lowered the plastic front cover & fanned the componet area of the boiler with a hair dryer on hot setting.

BINGO ! after about 30 seconds of heat it fired
and continued to fire till the wife turned the room stat right down for overnite.
Next morning no heat, so tried hair dryer again & click
fired up again....
Don't know if this will work for anyone else but as a get out of jail on a freezing night till you can fix properly it must be worth a try.

We have now gone 2 weeks or more & have not used the dryer more than about 6 times, it still cycles but now does at least fire on it's own again.

Will get it fixed when I can find my soldering iron...

Dryer fanned close to bottom of open boiler, seems to take between 30 secs & 1 minute or so to work.
I assume the heat must be affecting the componets or joints in some way.

Good luck...

December 2010

my 40f was showing the same symptoms,just cycling and not firing,it had a new pcb 4 years ago and my gas plumber initially suspected it was the pcb again but i was not so sure.he replaced it and the problem was not fixed and actually got worse.he noticed the flue had collapsed and replaced that(£29) and finally diagnosed the air pressure switch(£41) which has cured the problem. i carefully opened the original pcb and all the mentioned capacitors looked fine so it went back on and saved me £180...

November 2010

I'm glad I found this post. Changed the 3 capacitors and it all works again, starts first time everytime. It's been playing up for a few years and as the fault was intermittent beat all the Corgi/Gas Safe people that looked at it. To be fair they said they would change the board but for less than £1, fault is fixed.

I noticed that C6 seems to be a smaller device than the designer planned for and that C7 can be replaced with either an axial or radial capacitor.

Maplin's high temp 22uF capacitor is only rated at 50v, so I used their standard 100v (VH29G) items for C4 & C7.

Thanks again!

Charles F
October 2010

since my last post in april 2009, my boiler has been working fine untill this week, then the pilot light was working ok but not firing up, this resulted in luke warm radiators, i again removed the circuit board again and found another dry joint on the air pressure sensor pin next to the previously repaired pin, i re-soldered and re-fitted the board, yet again everythings working fine..

andy colsell
October 2010

Ah yes, but if the fan did run in reverse surely the flue air pressure sensed by the tubes would preclude firing?

If it was wired up wrong, you'd notice the second you began the start sequence, as long as you hadn't replaced the front cover. There would be a draught into the room.

October 2010

Potterton Prima F fan.
Be very, very careful about the replacement fan "going backwards" this might blow products of combustion in to the room, ie carbon monoxide when boiler running.
Further investigation recommended.

October 2010

Me again - (previous post) in case anyone's following this.

Replaced the fan assembly with ease. It seperates from the duct which sits on top of the heat exchanger, and from the flue elbow on top of the fan unit. Both joints have a special gasket.

Unscrewing the two large wingnuts/studs at the front/top of the heat exchanger allows the whole lot to come out in one piece (with the air pipes removed and fan cables detached first), as the flue elbow wasn't fastened into the flue itself. Alternately, a thumbscrew detaches the flue elbow from the top of the fan unit, and two screws disconnect the fan unit from the duct.

On my replacement fan it wasn't clear which of the power supply leads went to which terminal, so I guessed, on the basis that if wrong, the fan would go backwards.

That sorted the fan out, but firing up was still intermittent. On one occassion I got it to fire by hitting the PCB (with a plastic spoon) but this may have been co-incidence. I was resigned to having to do something with the PCB, so followed the excellent advice in earlier posts and replaced the 3 capacitors plus refreshed all the solder joints.

For an inexperienced solderer like myself one issue is the close proximity of some of the terminals on the PCB. I accidentally soldered two together, but when melted, the solder can be pursuaded to detach easily.

Reassembled, and it seems to work fine, so thanks for all previous advice.

Regards - Greener

October 2010

I was aware the fan and the PCB are the two likely weak spots on these, and I'm getting the intermittent symptoms discussed in depth above.

However, my fan spindle bearing at the fan end appears to have failed, resulting in the motor shaking about on its rubber mountings and the fan spindle visibly vibrating up to a millimeter while running.

Would it be reasonable to assume this is slowing the fan down, changing the measured air pressure in the flue elbow, and therefore interrupting the firing sequence?

I have no choice but to replace the fan anyway, and if misbehavior continues I'll suspect the PCB of course.

Does the fan + its circular casing + motor detach from the ducting that sits atop the heat exchanger?

September 2010

I was aware the fan and the PCB are the two likely weak spots on these, and I'm getting the intermittent symptoms discussed in depth above.

However, my fan spindle bearing at the fan end appears to have failed, resulting in the motor shaking about on its rubber mountings and the fan spindle visibly vibrating up to a millimeter while running.

Would it be reasonable to assume this is slowing the fan down, changing the measured air pressure in the flue elbow, and therefore interrupting the firing sequence?

I have no choice but to replace the fan anyway, and if misbehavior continues I'll suspect the PCB of course.

Does the fan + its circular casing + motor detach from the ducting that sits atop the heat exchanger?

September 2010

Ok I feel a bit stupid now. The way the diode is mounted you cant see that it has a leading "B" stamped on it - so the code is actually BZW03C51 - indeed a 51V Zener. I am off to Maplins in the morning!

July 2010

I have repaired my PCB once before by re-soldering the dry joints. A new problem has developed now. The Fan spins up but no start. The troubleshooting checklist says to check if there is 240v between pins 4-6 and there is. It says you should then replace the PCB.

I replaced the three caps, and also the 100k resistor with a 2W jobbie. The boiler is still doing the same thing. The only other component it could be is Z3, which I think is a Zener diode. It has the markings ZW03C51. I cannot find this anywhere on google, and they don't correspond to any guides on zener diodes. I believe the last three digits mean its a 51V Zener. If this is the case, it is definately at fault as I am measuring 250V across it when the fan spins up, not 51V as it should read if it was working properly.

Can anyone verify this is indeed a 51V Zener? I am loathed to replace it with the wrong one in case this brings new problems!

July 2010

hi thanks for the great info my 16 year old prima f40 wasnt firing up no fan nothing happening, took out the board there was no solder on one of the pins on the longer plastic plug next to the fan plug.resoldered but changed the 3 capacitors while the board was out now firing up straight away thanks guys,also noticed some burn marks beside z3 but just left that one cheers saved me a few quid for the grand total of 74p.

June 2010

easy just buy my brand new 1 still in box never opened £250 trust me £150 down the drain on worchester combi boiler still not working properly see details on e bay deals.0208 or phone me on 07805570058

May 2010

Just replaced the three capacitors on my prima f pcb. Thanks for info and maplins nos

May 2010

I had the same problem of ignition the black control box a knock got it working and therefore located fault.
Its up to you what you do know, I initially took out the control box and resoldered the joints and it worked ok for a year.
Last week it did it again and I have now replaced the control box with a 30 pound exchange unit and all ok.

March 2010

I have had this problem but before I saw this thread I got an engineer to replace the board.

It has now developed a different fault.

The pilot ignites but the main burners don't come on. The pumps seem to be running OK.

If I switch it off and leave it for a few minutes it work for a few minutes then stops again.

Any ideas?

February 2010

I also managed to get mine working with the 3 capacitor replacement. Whilst the board was out, took the time to also give the 2 relays on the board a contact clean. You have to gently prize off the black cover with a small flat-balde screwdriver and squeeze it to remove. Then used a fine emery paper strip (or 240 grit sandpaper - nothing coarser though!) - about 5mm wide - to gently push between the contacts on both sides. Removed some carbonisation which could form as components fail and cause some arcing.

Thanks once again to all.

February 2010

Potterton Profile - about 10years old - intermittent firing - pressure switch could be heard clicking in and out - so suspected the Controller board. Removed the 3 capacitors as per previous posts - they all looked OK and behaved like capacitors should when tested on a small multimeter - put them back, soldering carefully - same problem as before - so made the trip to Maplins - 2x 22uF 63v and one 4.7uF 63v - soldered them in instead and it works perfectly! Magic. The capacitors are obviously breaking down at the 50v circuit voltage - so a higher voltage rating would be a better long-life design solution. The resistor R15 between the relay and the pressure switch connector block was looking seriously overheated - so changed that as well. Black-brown-yellow = 100K-ohms, looks as tho' it may have 240 across it - so with i=E/R that makes 2.4mA or 567mW i.e. over 1/2 watt - no wonder the little 1/10 watt component looked frazzled. Its a pity these circuit designers don't do their job properly - or perhaps it is deliberate built-in obsolescence. If it was fitted to a Toyota, we'd have a global product recall and all get our money back.

Roger Forshaw, Romsey Hants
February 2010

Note to mr "gas safe engineer" - When capacitors are at end of life they can either go short circuit, open circuit, implode, explode etc. ie they are unpredictable. If the capacitors failed one way the boiler might stop working but if they fail in the opposite direction, the pcb/boiler might still think everything is fine and continue to run when it isn't.

If peoples are replacing components on a like for like basis they are actually keeping their boiler and its safety measures in good shape. It would only become dangerous if the pcb's were altered from the original design or safety systems were bypassed to make the boiler work again.

The thing to make sure is that you change components on a like for like basis ie of the same spec/ratings/type and dont start bypassing stuff.

Now for my question - does the boiler produce a spark but it just doesn't light up or do the rest of you not even get to the sparking stage?

February 2010

Brilliant folks - found this, 3 new caps later at less than 50p and it's good as new.

Thank you for sharing!!!!

January 2010

hello all, im a heating engineer with lots of expierence fault finding on lots of gas appliances. No one should be replacing components on printed circit boards unless you can test the board to the approriote regulations. your boiler might seem to be working fine but your safty devices might not be. if your flame rectification isnt working you will end up with a dangerous situation. only qualified gas engineers should replace pcbs so they can test the safe operation of the boiler afterwards. saving money is always a good thing but dont put yourselves and your families in danger! I have had to replace a potterton profile pcb with is basically the same as a prima because the flame rectification wasnt working but the boiled worked fine in normal operation.

gas safe engineer
January 2010

Hi Guys,
Just got a registered gas supplier to fix mine. €287 + Vat + labour. How stupid do I feel? A few capacitors would have saved over €400.

Cahal Finbarr Murphy
January 2010

I have replaced the C4, C6 & C7 capacitors also found two dry joints, my Potterton Prima "F" 100
is now firing up on first time.
the advice from Tony, Phil and others experience is spot on, what a saving not having to replace the whole PCB which could have cost £118 plus VAT !!
I'm grateful to everyone who has taken the time to offer their advice here.
fantastic service which I will recommend to friends

"Dreamcatcher" Canvey Island Essex
January 2010

Replaced the 3 capacitors with high temp radial types (Maplins 48pence total) and now fires first time. Note: On my Prima f40 the circuit board accepts either axial or radial capacitors.

January 2010

Hi Guys
thanks for info above - so useful
my 60F prima was going all the way thru to pilot lighing stage but wouldnt fire on some occasions
Even found that by flexing the boiler thermostat control it would sometimes light
As so many have suggested I desided to investigate dry joints on PCB
I removed control board (not difficult) and with an eye glass checked eack soldered terminal in turn
Spotted 2 ternials where there were cracks around the soldered ridge leaving a crack between it and the pin to which it should have been attached
Turned out the pin that was cracked was the one which fed the white wire going to the main gas valve solenoid
Resoledered both pins
Re-assembed - problem solved
So happy - cost nowt except lots of time investgating cause
best wishes Paul

Paul Jones
January 2010

Worked a treat, thanks

Eric W
January 2010

Horsham Paul; the value of R5 is 4M7 +/-10%

Stew Shields
January 2010

i'd just like to say thanks for the info on this forum. my prima 50f has been playing up for a while now and this week stopped altogether. i had the same arcing on the pcb and using the info here i soldered the one dry joint and its now working perfectly. i had already purchased and fitted a new thermostat to no avail but it was only £17 so not the end of the world. a damned site cheaper than £150 for a new board which i would probably had to have bought if i hadn't found this site.

thanks guys

January 2010

Does the spark generator next to the pilot spark when you switch the boiler on. when this is about to go it starks working intermitantly. that why you waited a small amount of time and then a long time before the boiler fired.

butt plumber
December 2009

Tony Taunton; Thanks a million. Same problem, same fix. Thanks too to Jon for the Maplin part numbers.
Does seem to be witness heat marks around component Z3, but Maplin guy didn't know what this was (and I sure don't)!
Happy family now (until the next time it breaks). But at 16 years old it will need replacing sooner rather than later.

December 2009

December 16 2009
After a year of cold starting problems.
Just replaced the 3 capacitors listed below and re soldered diode Z3, put back together and 'BINGO' boiler started first time.
Thanks to all the information provided by others was able to buy the parts from Maplins for a total of 83p!
Once again thank you all for the excellent information.

December 2009

I followed your thread which exactly mirrors my own experience and I PURCHASED THE CAPS FROM mAPLINS FOR 72P! hOWEVER THE RESISTOR AT R5 on the PCB is cracked and needs replacing. I cannot read the value so cant replace it :-( Did anyone make a note of the component values?

horsham paul
December 2009

Yep, a spot of solder fixed mine too, after a couple of years of never knowing whether it was going to light up or not. It took a while to get the PCB out but once I did sure enough there was what looked like a dry joint. Slapped on some solder, put everyting back together and it it seems to be working, even when I replaced the bottom cover, which previously made burner cut out. Did not even need to replace any components. V v chuffed. Many thanks to all contributors below.

Jim S
November 2009

hi guys i think it's great that you're all fixing things instead of just replacing them BUT it's actually against 'Gas Safe' regulations to fiddle or alter any gas fittings or parts. So be careful.

Gary H
October 2009

Hi I have the same problem with my boiler and would like to get it resolved. However I dont know anything about boilers the HT spark generator (the black square box on the pcb. Resoldered etc.
I am nervous about calling someone in as it will cost lots of money and it might not be sorted out. What is the best way to do this? Should i be able to do it myself? Would I need an electrician? Please help

October 2009

having read all the threads relating to the potterton prima f40 boiler and recongnising all the problems to be the same as mine, i followed the advice and purchased the three components listed as possible faults, i removed the circuit board (a fairly straight forward process) and found a dry joint on the air pressure circuit, i re soldered and re-fitted without replacing the new parts, he presto!!!! it works
so, i would like to say a big thankyou to other users of this page for saving me calling a plumber when a eletrical engineer would have been more apt!
regards andy colsell

andy colsell
April 2009

Another successful fix by changing the capacitors. Beware, PCB tracks lift very easily.

dan M
March 2009

changed capacitors now working ok, did have a problem with the tracking coming away but gently scrapped away the coating and soldered it onto the exposed section all's fine thanks for the info

March 2009

My Potterton Prima F boiler has recently been taking more and more attempts to fire up. The relay clicks, the fan whirrs for a few seconds, and just when you think it might light, the relay clicks off again, wait 10 secs (ish) and repeat - for anything up to 2 hours!

I've just been quoted £160 for the PCB, which prompted an internet search. Will be off to Maplins tomorrow. Watch this space.

Cheers for your posts.

Steve Powell
February 2009

I've had the problem described for ages, and it recently went much worse. I got a start some times by lightly tapping the 407677 cover.

Phoned our local trade supplier - he was out of stock due to "selling out already today". Quote £150+ plus VAT!
If you don't fancy replacing the capacitors yourself, look on eBay and a couple of companies will sell guaranteed replacement boards, with a deposit system for them sending the replacement down to you first.

So guess what? I'll try the capacitor replacement & examine for bad soldering route first, hoping that our nearest Maplins has the capacitors in stock.

Tony Burgess
February 2009

Another vote of thanks here for the tip about capacitors. I've just changed the 3 capacitors on my in-law's Potterton Profile boiler PCB and it's now working fine.

February 2009

My board has dry solder joints all over it! some kind of cowboys build on this board! Mine has been "clicking" on and off for months and it turns out its because the pins on the front connectors were all badly soldered and arcing. When I took the board out to re-solder these connections I disloged other badly soldered components and it wouldnt work at all! Here was me thinking i ruined it, but I went over every joint and now it works fine! Rule of thumb - check the solder joints and re-do them all just in case!...

Leon Funnell
January 2009

Guys Thanks a Lot. You saved me a lot of money. I changed the 22 uF 63V Axial case (Maplin AT04E) only and that did the tric for my boiler. I remember the same thing happened 3 years agoo and I paid 100£ in Labour charges and 150£ for a new PCB, now I think they must have just changed the damn cap.
Anyhow thankyou all.

January 2009

Just found this thread on google....

I have a problem where the boiler will not ignite at all.

I removed the PCB and saw that the big black box on the top right which has on the PCB "SPARK" on the other side it has a thick layer of solder on one side of the silver contact. Also what worries me is that on the green pcb there is some black smudge which indicate that maybe this black switch maybe faulty.

Is it still worth giving the capiciators a go?

Or is this the problem only when you actually have the boiler sparking.

In my case I have no spark at did know what could be the problem....

December 2008

Many thanks for the electrolytic tip. Our Prima F has been slow to start for about 18 months and finally refused completely on the coldest night of the year so far. Less than a quid at Maplin for the three caps, 15 minutes with the iron and it's now firing first time every time. A much more satisfying solution than £100 plus VAT for a replacement 407677 board. I used the Maplin part numbers provided by Jon below and they were an ideal fit:

22uF 63V Radial case (Maplin VH28F)
4.7uF 63V Radial case (Maplin VH19V)
22 uF 63V Axial case (Maplin AT04E)

December 2008

welldone! bingo the capacitors did it.Thanks

December 2008


Just wanted to say thanks for the capacitor fix solution. My F40 has gone through fan cycling thing since we bought our house over 2 years ago. I just thought it was some kind primer cycle and ignored it. Anyway, we discovered we had cold water yesterday and today I have hot water and a boiler that fires up first time, every time (so far...) as a result of the answers posted here so once again thank you.

For what it's worth I think only the 22uF axial capacitor had failed (The one with a pin at both ends) but I changed all 3 anyway and refreshed all the solder joints on the board as a matter for the course.

Parts came from my local Maplin store:
22uF 63V Radial case (Maplin VH28F)
4.7uF 63V Radial case (Maplin VH19V)
22 uF 63V Axial case (Maplin AT04E)

September 2008

Well done Tony,

The capacitor fix sorted it out. Been having an intermittent problem for a couple of years and a couple of months ago it got to the stage where it would cycle for hours without firing. Replaced the components as per Tony and what a relief to see the bolier fire first time when I switched it on. The money saved will probably mean that we can just about afford a holiday that hung in the balance. Cheers mate.

August 2008

Tony from Tauton spot on again, 65p from Maplins changed all 3 capacitors and guess what! it fires 1st time every time. many thanks

August 2008

Many thanks to Tony Taunton. I spent a long time trying to fix an intermittent fault very similar to the ones detailed. It eventually then came on solidly and changing the capacitors at a grand cost of 33 pence for all three from Maplin fixed the problem, whereas a new board would have cost about £100.

George Tod
June 2008


christian Ruddock
April 2008

Well done Tony.......
Top post as the electrolytic capacitors were indeed the cuplprit on my potterton profile pcb and your post has just saved me in the region of £200 or more.
Cheers, I owe you a beer mate!

March 2008

HI, I am having the exact same problem, only this time the fan is not kicking in at all, dont know whether to change fan, or try the pcb, both expensive to replace, might be cheaper to replace boiler in the long run, any help greately appreciated!

March 2008

Tony Taunton has got this one right. i had the same problem and found a dry joint on the HT spark generator (the black square box on the pcb. Resoldered the joint and bingo - all is working.

November 2007

Tony Taunton's answer fixed it for me, on a 12 year old Potterton Prima F 40 boiler with a 407677 PCB. I've been putting up with these symptoms for over a year and replacing the 3 electrolytic capacitors fixed the fan cycling on start up, for grand total of 77p and a few minutes with a soldering iron. Thanks Tony!
I initially thought the white wire from the PCB to the spark electrode (a lone wire at the back of the board) was permanently attached to the PCB, but it just pulls off sideways.

October 2007

I had this problem, and spent a long time experimenting with various ideas, but the cycling on and off happened everytime that the boiler started up from cold. Sometimes it would cycle the fan on and off for 40 minutes before it finally started. I took the "black box" out and examined it. I clipped the tops off the relays and cleaned the contacts with paper: still the same. I then examined the underside of the PCB, and found several dry (ie badly soldered) joints. This didn't fix it. I then replaced all the electrolytic capacitors on the PCB, total cost £1.50, and Bingo!! it all worked fine. The electrolytic capacitors have a "+" marked on the board, and you must observe this. C4 and C7 were 22uF at 63v and C6 was 4.7 at 63v (but check on your board) The 63v is important don't get any lower voltage. Get high temperature (105deg) if you can, as they get hot, and that's what shortens their life. One of mine was oozing out at the bottom. You can buy capacitors at Maplins or CPC via the internet. This will probably fix most boiler control boards: the electrolytics will always be the first thing to fail on any bit of kit that gets hot.

Tony Taunton
November 2006

agreed pcb

steve b
February 2006

pcb 100%

February 2006

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