Baxi duotec GA 33 E53

[helplessowner]

1st post here, long time lurker. So I know that you cannot give gas-safe advice on the forum.
Long introduction before I get to the question - sorry.
Background: park home (basically a wooden bungalow) with above boiler installed (yes, I now know it is oversized) 10 years old.
Vertical flue (very short vertical flue).
Condensate pipe drops straight down under the home to join horizontal main waste pipe.
Runs on mains gas (NOT LPG). We have a water softener (also 10 years, also serviced each year).
Serviced every year under Baxi's own (now very expensive) maintenance plan.
Serviced in May, working all OK (electrode and seal replaced as part of service).
Next day, tried hot water - E133, Would not reset.
Since then, Baxi have been out 5 times (all under the service plan).
Tech 1 calibrated, said all OK, but it failed within 20 minutes, first time we turned on a hot tap (E133).
Tech 2 checked condensate pipe (from boiler end) said was clear, adjusted gas/air mix which removed E133, and it worked fine for 8 days, then started making SOMETIMES (not always) extremely strange and worrying rumbling and bangs on ignition before settling down and working quietly.
No error codes.
Hubby went up ladder to look at flue, pluming, no visible signs of any blockage.
Tech 3 managed to replicate the noise but could not find out why. Changed sensor and fan. Checked condensate pipe, said was clear.
Checked flue (from the boiler end) and said was fine.
Calibrated but would not complete (tried 3 times).
On last attempt E53 showed up. Tech said that was not possible, but left it anyway.
Next morning - E53 and lock out.
Tech 4 replaced gas valve and PCB, but turned boiler off as unsafe (not burning properly) while waiting for heat exchanger.
Tech no 6 replaced heat exchanger and was here for a very long time (about 3 hours) recalibrating and checking everything. Left it working very quietly.
That was 8 days ago - and guess what? E53.
It runs on either heating or hot water for about 10-15 minutes, all seems OK. Then flame symbol goes off and shortly after E53 appears on display.
BUT - turn off heating/hot water (whatever was running) E53 clears from display and then immediately turn on hot water and it fires up and runs for another 10 minutes or so before back to E53.
So - Question - finally:
Is it possible for either a flue to be intermittently blocked (one minute it is, next minute it isn't)
Or can E53 mean something else?
Calling Baxi out again tomorrow, but wondering whether to give up and start looking for an installer to replace.
Is this going to be fixable?

 

[CBW]

According to a Baxi website, E53 points to the condensate pipe being frozen, but could be blocked?

 

[helplessowner]

Thank you - indeed it does (another Baxi website also says possible combustion error..)
But - IF it is the condensate pipe, 2 techs have checked it and said it is clear, and surely if it was the condensate blocked or very slow draining, then the boiler would not immediately clear the E53 as soon as the heating turned off and the water turned on?
This is what is baffling me (and, it seems, the Baxi techs) - can a blockage in either the flue or condensate pipe be hear one minute (second) and gone the next? Anybody know this?
No criticism to any of the techs who have come out, as they have all been determined to be the one who finds and fixes whatever the problem is - but I am now getting paranoid about whether the boiler will work when we next use it.

 

[gas112]

e53 long shot it could be the actual trap faulty as opposed to the condense pipe to the waste .
one thing to try is remove the actual condense pipe from rubber fitting at bottom of boiler and let it drip into a container see what it does .

 

[helplessowner]

Thanks. We'll certainly ask the next Tech (no 6) to check the condensate trap again - has been checked twice and was OK. Getting the pipe under the boiler to a bucket may be interesting , as - like so many - when the boiler was installed by the home manufacturer most of the pipework is surrounded by cupboards, and there is nowhere underneath to balance a bucket.
Just for the record - Techs 3,4 and 5 had the boiler running full blast for calibrating for 15 minutes or so (Techs 3 and 4 did this 3 times as it would not complete calibration, overheating). Tech 5 ran the boiler after replacing the heat exchanger, and then ran a very long calibration (with all hot taps on), then after calbibration left it running while he packed up all his tools, finished his coffee, and ate the toast we made him - with no errors.
Worked perfectly for several days.
But now we get error 53 after 10 minutes or so, turn off whatever is running, turn on a hot tap and it fires up immediately and runs for another 10 minutes. No funny noises. or odd gurgling sounds, just click -burner off, pump runs, E53.
Sigh. Better stock up the tea, coffee and biscuits.
(I am really not looking forward to the next gas bill, or the next water bill (on a meter).)
The irritating thing is - everything was fine until it was serviced. (yes, I know it is just co-incidence - it happens).