Combi Boiler Gurus - what's wrong with my Logic+ 35

[jpaul]

So my heating engineer is having problems diagnosing a boiler problem, has anyone had similar symptoms with a Logic+ combi boiler ?

Below is the video (sorry it is on its side - bad production standards), what happens is
- turn hot water tap on (in kitchen adjacent to boiler)
- boiler try to ignite 0:13 (blue light comes on) and in doing so makes a noise like digeridoo, or a pining moose
- it fails to ignite with messages of ignition cutout, check other appliances
- it tries again at 0:30

what would be your informed opinion on cause ?


admittedly have been unable to get the boiler to fail when he visits, and it does not leave any kind of numeric error code (not very high tech)
I have consulted the manual which has some fault finding instructions eg p57, but I am not qualified/permitted to execute them

 

[jpaul]

Thanks for your analysis

common problem with combi boilers when it get's cold.

problem has occurred when it is cold - that's true

Does it only do this when in dhw mode and is fine for heating ?

problem seem to occur most when you use combi function and heating is running and then pull dhw (which I understand implicates plate 2 plate hex)

*Trap, condensate blockage.
*Gas pressure.
*Flue integrity.
Heat exchanger not clogged, clean it
Jet and Venturi clean.
Burner assembly tight and secure.

Thanks, it is maintained, and they had checked *'d items, but not yet any of last 3 afaik.
They ran it for quite a while on their 2nd consecutive daily visit and it had seemed OK, but inevitably in the evening it failed again

 

[jpaul]

OK, so the ignition and detection probes have been changed, problem recurred; but after he reseated the electrical connections touch-wood the problem could not be reproduced,
so a squirt of switch cleaner might have been order of the day.

All this stuff though, is really something you shouldn't be doing yourself, but that's a different matter

I wasn't touching it myself, just back streetseat driving ... it is always interesting to know how they work and likely failure mechansims so that you make sure you disclose the important
symptoms, to the doctor, before you freeze to death .... like funeral directors (unlike doctors) it is a career with guaranteed employment too. ?

...so some soak testing over the weekend.

edit : no not fixed - trying to pull hot water after using heating even after a reset the burner fails to re-light <<ignition -lockout, call installer>>> loads of gurgling noise, which is apparently condensate, why is it audible though. ...
...so wait 30mins+, for it to cool down or condensate to drain away ???? and then try it again

 

[jpaul]

What I posted earlier today, did your man test/check any of it?

If you want to test my theory yourself then disconnect the white pipe from under your boiler and put a bucket/bowl under to catch the condensate. This way you can see the condensate dripping away, and the pipework is taken out of the equation. See if it makes any difference to the lockouts. The trap will have its quantity of condensate held, so no fumes will leak out.

Thanks,
No - he just changed the ignition and detection probes, and we were then conned by the fact that it seemed to work, but that was probably because he had emptied the trap inside,
so the condensate evacuation problem was avoided temporarily.

Unfortunately condensate pipe is on a tight bend - but anyway (it's a rental proprty) I can't touch it

Where does the condensate terminate too? Is it to an internal drain pipe (prone to blocking from sink fat etc) or externally (prone to freezing)? As flyingfish mentioned you/repair man need to find out whether it is blocked internally to the boiler or in the external condensate pipework.

internal into the soil pipe somewhere out of site, beyond adjacent sink trap.
But frankly no evacuation problem on the sink, and no fat disposed off, so the condensate pipe has created its own blockage.

 

[jpaul]

update - so different plumber visited and has given a plausible solution ?

the comndensate trap needed refilling (with water) after the yearly maintenance of the boiler, when it had been removed and washed out;
without refill, can get an airlock which prevents it subsequently working effectively and evacuating condensate.

 

[jpaul]

the problem has not recurred over 24 hours, but OK, maybe issue is not really fixed.

you know more than I do, and indeed I could not find anything on the web to substantiate his claim,
only references to some boilers having

Even if the boiler uses the much cleverer auto-syphon system you may still need to increase the pipe size and/or lag unless the external run is short and near vertical. Most of the 5 or 6 winters up to January 2013 had prolonged, very cold spells. Increasing the pipe size and lagging the pipe make sense!

but I could not see what the implication of auto-siphon might be

some maybe relevent wiki info

A siphon used in an automatic unattended device needs to be able to function reliably without failure. This is different from the common demonstration self-starting siphons in that there are ways the siphon can fail to function which require manual intervention to return to normal surge flow operation.

The most common failure is for the liquid to dribble out slowly, matching the rate that the container is filling, and the siphon enters an undesired steady-state condition. Preventing dribbling typically involves pneumatic principles to trap one or more large air bubbles in various pipes, which are sealed by water traps. This method can fail if it can't start working intermittently without water already present in parts of the mechanism, and which will not be filled if the mechanism starts from a dry state.

A second problem is that the trapped air pockets will shrink over time if the siphon is not operating due to no inflow. The air in pockets is absorbed by the liquid, which pulls liquid up into the piping until the air pocket disappears, and can cause activation of water flow outside the normal range of operating when the storage tank is not full, leading to loss of the liquid seal in lower parts of the mechanism.

 

[jpaul]

Sounds like condensate drain blockage. Does the condensate pipe run outside at all?

Thnaks, runs exclusively inside the house joining the kitchen sink waste after its trap.

Plumber did check by pouring water into it that, there was no apparent obstruction, equally no flow problems ever observed on the sink