Potterton Profile boiler fault

Arrived home in the early hours of this morning to a cold house. Boiler not working. Much as I would have liked to curl up in bed, I set about opening the boiler. It was stuck at the point in the ignition sequence where the fan is running, but no ignition spark. Looking back at my home automation logs, it had run for a short time when it was switched on remotely a few hours earlier. The logs showed the main burner cycling off and on repeatedly within a second. I have seen this before when the relay which feeds back the main burner state to my home automation suffers dirt on the contacts - it's a mains relay with coil in parallel with the main burner solenoid, but the contacts are not designed for ultra low power signal switching. They don't self-clean at such low current and need manual cleaning every few years.
Anyway, working through the ignition sequence, I find the air pressure switch is not detecting a pressure differential, even though the fan is running fine. I disconnect the tubes and operate it by mouth, and then the boiler ignites and fires up, until I release it from my mouth obviously. So the sensor works, but is maybe drifted out of spec. I open the boiler casing looking for leaks, but the seal is tight all the way around. Remove the fan and look into the air inlet chamber - all clear - can shine a torch out through the outer flue pipe and see the neighbour fence. Can't check the inner exhaust without going outside with a ladder, and it's pitch dark and pissing with rain, but that's probably OK too. I dive onto the web to find a next-day supplier for a new air pressure switch, but just to rule out anything else, I momentarily bypass it.
Boiler works, for a minute, but then the main burner starts cutting in and out. So this is what I saw on the home automation logs, and not just dirty relay contacts in my signalling relay. It cuts out for a short enough time that the main burner doesn't have time to go out, indeed flame barely noticably drops before it comes on again (initially I didn't see this and thought it was the pilot solenoid dropping out, until I put a test meter on it). This seemed like a component heating up with a bad joint.
Now I'm theorising two faults. As someone who teaches fault finding, I know this is incredibly unlikely and it almost always means the root cause has not been correctly identified. However, I took out the circuit board, and there was a power resistor there which had done the usual desolding itself over ~25 years. Proudly took it to the electronics workbench and resoldered it, confident it would fix the second problem.
It didn't. I had temporarily replaced the control board without its cover, and I now noticed something else - in the momentary solenoid dropouts, the ignition spark was triggered (flash visible from the gas breakdown device in the ignitor circuit.
So, was I now looking at a false flame failure detection followed by instant reignition (except burner never actually gets anywhere near going out)? Still worrying about the two separate faults - probably not actually identified the real root cause.
Gave up and went to bed (nearly 4am).
This morning, I tried it again and ran it long enough to get enough hot water for a shower with the air pressure sensor bypassed. An order for a replacement sensor was just waiting for me to hit return, but the two faults were still bugging me. What could link the two problems? What about reduced air flow through the boiler? Fan spun freely and was making same sound it has for last 15 years, so I didn't suspect that. However, lack of air flow might cause flames to not play on the flame failure (and ignition) electrode. The one thing I couldn't check last night was the exhaust flue because it was dark. Walked around to take a look - a bird's arse was sticking out, jammed in the grille. How it got in that far in I can't imagine, but on pulling it out and reconnecting the air pressure sensor, everything works properly.
So there was just one fault, when it was correctly root caused.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Andrew Gabriel wrote:

[big snip]

Interesting story, glad you found it in the end.
Chris
--
Chris J Dixon Nottingham UK
snipped-for-privacy@cdixon.me.uk
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Agreed, thanks for posting that Andrew. But inquiring minds want to know, what became of the bird? Gassed to death?
--
(\_/)
(='.'=) Bunny says: Windows 10? Nein danke!
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I would imagine temperature would have got it first, if it was still alive at that point. The boiler flue gas (on this conventional boiler) runs at around 200-250C.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 20/11/2015 19:36, Andrew Gabriel wrote:

Just needed a nice plate of chips to go with it then ;-)
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Friday, November 20, 2015 at 10:13:26 PM UTC, John Rumm wrote:

========================================\

========================================/
The usual fault on this model is the fan cycling on and off every 10-15 sec onds. This can go away after some minutes and then the boiler lights normal ly. The fault is due to something in the control module that fails to respo nd to the changeover of the air pressure sensor.
Anyway it really was a blocked flue and the air flow was insufficient to mo ve the APS. In my experience 1-2 mbar is the typical differential pressure generated by the correct air-flow on most boilers of this sort (with a fixe d speed fan and no permanent pilot).
Sometimes dirt can accumulate on the pressure tube inlets near the fan. Mak ing the pitot-static differential pressure disappear as both tubes become effectively pitot tubes.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 19 Nov 2015 14:07:31 +0000, Andrew Gabriel wrote:

====snipped interesting tale of boiler woes===

I just bet the next time you have boiler trouble, the exhaust flue will be the first thing you check, come hell or high water. After all, if one goofy bird can do a "911" on the exhaust vent, who's to say another one won't go and do exactly the same at a later date? :-)
--
Johnny B Good

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 19/11/15 14:07, Andrew Gabriel wrote:

Around 5 years ago our Worcester-Bosch boiler (non-condensing - I can't remember the model) started cutting out as soon as it was warm. Once cold again, it would operate for a couple of minutes, then turn itself off. Our trusty boiler serviceman - who was excellent - replaced the FFD, then the pressure sensor, and finally the circuit board, all of which failed to cure the fault. A phone call he had with W-B technical support also failed to come up with a resolution.
Now clutching at straws, he happened to remove the silicon tube to the pressure sensor, and on the side normally hidden from view, found a hairline split in the end of it, barely 5 mm long,. Cutting that off and replacing the tube immediately cured the problem. It appeared that while cold, the diameter of the pressure sensor metal tube on which the silicon tubing fitted was low enough to keep the hairline split closed. However, once it had heated up, the hairline split opened enough (and it could only have been microns) to allow the pressure to drop and the boiler to be shut off .
--

Jeff

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Saturday, November 21, 2015 at 8:35:57 AM UTC, Jeff Layman wrote:

That was probably only one of the effects but it tipped the balance. A major effect is that as the boiler warms up so do the flue gases. These reduce in density a bit and therefore the pressure developed for the sensor. The sensor has quite a bit of hysteresis so the reduction in pressure rarely causes the boiler to stop. Your story does show the way that hidden small faults can produce symptoms that are misleading.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 21/11/2015 10:05, snipped-for-privacy@makewrite.demon.co.uk wrote:

A mate had an Ideal Mexico RS that showed similar sounding problem - it would run from cold, but then shut off after a few mins, then run for a few mins each hour.
In that particular case (i.e. simple boiler, no fan), a quick check following through all the control demands from the stat to the boiler, showed it still had mains on the gas valve even when it went out. It seemed like the valve did not like being warm! Swapped that out and it was fine thereafter. (until the casework rusted through and it developed a CO leak a couple of years later anyway)
--
Cheers,

John.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 14:07 19 Nov 2015, Andrew Gabriel wrote:

That account is interesting because I too have a Potterton Profile and, over time, it has had its ups and downs. I get the impression from conversations that it was somewhat out of date when I had it installed about 20 years ago. Something or another to do with the way its made (cast iron, could it be?).
I was gearing myself up to replace it but it seems to soldier on and I'm not looking to spend money unless there's a good reason.
--
pamela

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.