Is this boiler "fault" normal - video

[chrisw99]

Hi all,

I've got BG homecare and called them out for a fault with the boiler, he ended up telling me there was a blockage in the gravity hot water flow/return causing the boiler to get very hot at the flow output location and cut out.

So the boiler always lights from cold, but then if I try and turn it on within an hour or two of it having been on previously, this happens... (video taken through the observation window at night so the flame can be seen)

He then went on to say it's "probably" caused by the overheating flow, but until we've spent £700 on a powerflush he won't diagnose further.

So, is what is happening in the video normal under those circumstances i.e. lights for 1 second, goes out for a few secs, lights for 1 second, goes out for a few secs, about 30 times before lighting permanently. The engineer said the boiler is still too hot so it's deciding to switch off.

I'm a computer engineer, and the logical part of me tells me that won't be right, why would the boiler even be attempting to light if it was too hot, to then immediately cut out. Surely it would just monitor the temperature and only attempt to light once it's cooled. Or is it a case of the left hand not knowing what the right hand is doing, the ignition PCB decides it's time for heat, lights, and then the aquastat PCB tells it to cut out.

Also, with the above logic, every time it lights for 1 second, it is getting it a little bit hotter, so after 3 minutes as shown in the video it's got a lot hotter, so why does it then light permanently. It goes against what the BG chap said about it cutting out because its too hot. When I took the video, it had been about an hour since it was last on properly and all the pipes felt cold to me.

BG have left it saying we need a £700 powerflush or a new boiler. I do agree there's a blockage in the flow/return to the cylinder, but before I devote my attention and money to trying to clear that I would like a second opinion that it will then sort out the boiler fault. Are there any other situations that would cause the symptoms in the video?

Thanks

 

[oldbuffer]

Typically "cycling" faults, where the boiler fires, goes out, fires again etc. are caused by circulation problems.

Boiler heats water, water should lose heat during circulation, boiler checks difference between flow and return temperatures. If temperatures too close (or flow temperature too high) boiler shuts down. "Call for heat" still present, so tries to fire up, measures temperature (differential), too hot / too little difference, so shuts down.

Sorting out circulation problem very likely to sort out boiler issue, but no guarantee. However, until you sort circulation issue further remote diagnosis not very reliable.

Need to know boiler make and model, type of system (sealed, open vented) to get further.

 

[Dan Robinson]

Blimey... All that and no mention of the boiler model .

You could have a localised blockage where the feed joins the return.

Don't try and apply IT logic to heating.

 

[chrisw99]

Ok thanks. You'd think it would measure the temperature differential first before deciding to light!

It's a Halstead Best 50, nearly all internal parts replaced in the last 1-3 years (thanks to the £99 excess for BG homecare call out and broken gas valve last year, they changed everything else first!) so should be as good as new. Open vented, old fashioned gravity hot water flow and return. No valves, no cylinder stats, pure gravity.

Also - had another fault a couple of weeks back, complete failure to light, BG found a broken ignition cable and patched it up with tape (would have been nice to get a new ignition probe for my £99 rather than a bit of tape) and also did the annual service as it was due soon. It may be coincidence, but I wasn't aware of the cycling issue before this. Having said that, you don't notice it with the CH on as everything works perfectly then, it's only now we've got it on hot water only it's become very noticeable.

Will look into the circulation problem, does sound like it needs sorting first!

 

[Dan Robinson]

You'd think it would measure the temperature differential first before deciding to light!

It's a Halstead Best 50,

It is a very poor, and simple boiler. It doesn't measure the delta.

 

[DP]

One does not need to be a computer wiz kid to know there has to be a balance in the system.

In a heating system heat generated by the source has to be used by the load. End of
The pcb will maintain reasonable balance by modulating the burner but if the heat generation is too rapid, over heat lockout will take place. If the temperature output rises beyond the parameters set on the boiler, the boiler goes to sleep for a preset time to resume cycle when time elapses.

Perhaps you have a problem with your installation and boiler is reacting to that issue

 

[chrisw99]

Oh the final thing I forgot to add, the BG engineer added a half a bottle of Fernox F5 into a radiator and said it may sort out the issue, 50/50 if it does or not.

 

[chrisw99]

It is a very poor, and simple boiler. It doesn't measure the delta.

Ok. A question from my first post still confuses me, if it's cutting out due to the temperature, why does it then eventually fire up permanently, because each of those 30 attempts has been getting it slightly hotter each time.

 

[Dan Robinson]

You'll have to wait for my crystal ball to come back from the calibration company.

 

[Corgigrouch GTi]

So it only does this little on off routine when calling to heat the cylinder which is on gravity? And it only started after the gas valve was replaced?
Here's a thought.. from cold, the water takes a little time to start circulating, so switches on an off till the system starts to flow.. then all is happy till it all cools down again..

 

[chrisw99]

So it only does this little on off routine when calling to heat the cylinder which is on gravity? And it only started after the gas valve was replaced?
Here's a thought.. from cold, the water takes a little time to start circulating, so switches on an off till the system starts to flow.. then all is happy till it all cools down again..

Not quite. The gas valve was replaced last year and worked fine all through winter, the recent fix was simply patching the ignition electrodes cable and doing the annual service, and I only noticed it since then doesn't mean it wasn't doing it before that.

And it works absolutely fine from cold, always fires first time. It's only if I call for heat within an hour or two of the boiler last being on that it does it.

 

[chrisw99]

Probably helps if I detail the exact sequence of events leading to the video.

1. Boiler hadn't been on for 12 hours so all nice and cold
2. Turned on hot water only at 8pm
3. Boiler fired up, fired for 2-3 minutes until boiling/kettling noises could be heard then cut out
4. Felt the HW flow pipe where it comes out the boiler. After a few seconds this became too hot to touch
5. Five minutes of nothing happening, but I could feel the HW pipe slowly getting hot along it's length
6. Boiler fired up normally, again till boiling noises then cut out.
7. Five minutes of nothing happening, by this time the HW pipe had finally got too hot to touch up at the cylinder
8. Boiler fired up normally again.
9. Five more minutes, then it does the cycling in the video but about 4-5 times then fires permanently till kettling
10. Turned it off
11. Waited 2 hrs, pipework had cooled down a lot. Turned boiler on. Took the video above.

So if it is temperature related, I am confused as to why it would fire properly at point 6 above (when everything was too hot to touch), but not at point 11 when it had cooled down a lot.

 

[Agile]

I am surprised that as a computer engineer, in spite of being advised above, that you have done all that observation without considering the flow temperature from the boiler.

Quite simply it will light if the flow temp is below the set value and will turn off if the flow temp is higher.


I also believe that computer processors will also shut down it their core temperatures are too high!

Tony

 

[chrisw99]

I am surprised that as a computer engineer, in spite of being advised above, that you have done all that observation without considering the flow temperature from the boiler.

Quite simply it will light if the flow temp is below the set value and will turn off if the flow temp is higher.

Tony,
I've been saying the opposite. e.g. at point 6 above it lights fine when the flow temp is far too hot to touch. Then 2 hrs later when it's all cooled down it fails to light, taking 30 attempts. Each of those 30 attempts get it a little bit hotter (as can be felt on the pipe which is lukewarm and then gets warmer gradually as it keeps trying to light) and then it does light permanently.

By your logic, it will light if flow temp < X degrees so why do I get

is temp < X. Cut out
is temp < X. Cut out.
30 times
then is temp < X. Stays lit.

When temp X has been rising steadily during the 30 failed attempts?

 

[ianmcd]

Does it only do it on HW or does it do it with the CH on too ? I certainly do not think that a powerflush has anything to do with it.