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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3629
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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I have a glow worm 24ci on hot water demand it randomly throws an F19(thermistor not conne

Customer Question

I have a glow worm 24ci on hot water demand it randomly throws an F19(thermistor not connected to pipe).
Have replaced both thermistor and board but still same. Diverter works as CH flow stays cold.
This came up after replacing the display interface board.
Help please
Submitted: 2 years ago.
Category: UK Plumbing
Expert:  Andrew Smith replied 2 years ago.
It could be a poor wiring connection, try taking both ends off and cleaning and springing them out to make a better connection.If it is an external thermistor make sure there is a good clean connection with some heat sync paste.
Customer: replied 2 years ago.
Hi Andrew,Thank you for your reply.I have checked all the wiring connections from all the sensors going to pcb(s), and also interconnections between pcb boards. I have no voltage drops (same voltages both ends ie at sensor(s) and at the pcb). I used a handheld oscilloscope and di a wiggle test but no glitches.I've measured thermistor voltage signal output (with the hand held oscilloscope) coming back from the thermistor via the 2 blue wires. These plug on to the pcb board via a spring connector. I did a wiggle test here on all wires / sensors including the thermistor, the signals are clean and voltages steady without any noise (or any ac spikes).Yes it is an external thermistor (ntc 10k at room temp), clipped onto copper pipe near the diverter. I have removed the pipe (the other end of pipe goes onto heat exchanger) and cleaned it internally as it was coated with magnetite and slowing down heat transfer rate to thermistor. I used silicone jelly as I did not have heat sink compound available. I will source heat transfer compound to try.The following puzzle me;
When I disconnect the wire from the thermistor I get an F6 code.
When I unclip the thermistor with wires still connected the boiler stops after 15-30 seconds. I cannot reproduce an F19 here either. I have tried reproducing the F19 code with other sensors too but nothing.I do not get an F19 code when the CH is on. F19 is triggered only when I use the hot water.
It also seems to take longer to go into F19 if the CH is on when I turn the hot water on.I suspect, as F19 only started, since I replaced the user interface board part no.(###) ###-####(which is also configurable for several other boilers via 2x com switches), may have too sensitive a tolerance setting with regard to the heat transfer rate on the hot water side only, and possibly less sensitive on the CH side. If so is there a work around to test this out.Many thanks,
Expert:  Andrew Smith replied 2 years ago.
If your model has an identical hw and ch thermistor you may be able to swap them out and see how the boiler behaves
Customer: replied 2 years ago.
Hello Andrew,One thermistor only on this one common to both hw & ch.
Yesterday I relocated the thermistor as near to the boiler main heat exchanger (not the hw plate to plate) as I can.As always no F19 on ch.
And since relocating thermistor no F19 on hw.Can it be thermistor slow response speed (thermistor slower due to build up inside pipe. Plus the new control board thinks the thermistor is not clipped on pipe due to a narrower time window / stricter tolerance programmed on the new control board)?No issues with ch? Perhaps as the ch naturally takes a lot longer to get to temperature the board is happy with thermistor feedback speed.Many thanks,
Customer: replied 2 years ago.
Hi Andrew,I have just had F19 come up on hw since relocating thermistor yesterday.Many thanks,
Expert:  Andrew Smith replied 2 years ago.
ok, so either cabling or PCB at fault.Were there jumpers to swap over from the old PCB?
Customer: replied 2 years ago.
Hi Andrew,My apologies for my delay and not replying to you sooner.The main pcb was replaced 3 years ago and it was different to the original pcb which came with the boiler.
Yes there were jumpers with new replacement pcb but no alterations were required.
And this worked fine with the original 'LCD display / Control interface pcb' which came with the boiler.F19 (on hw only) started after replacing the original 'LCD display / Control interface' pcb which went blank.
The replacement lcd pcb for the original is different and is configurable for a number of other glow worm models by way of using 2x Com switches to select your model. Com1 and Com2 to select the required model. I've double checked my selection settings are correct for 24 ci model.I also went through the burner pressure setting procedure using the 2 potentiometers on the board P1 and P2 as per instructions provided. I did notice that the burner pressure is at the lower end of the quoted mb range at around 3 mb.On further investigation I discovered that the gas supply pipe to the boiler is via a 15mm pipe. The boiler gas input is a larger bore.The original installer used a fitting to convert from 15mm to the larger size. I think the boiler output has suffered as a result but never had an F19 issue. I also manually throttled the gas supply shut off valve to reduce gas flow but still no F19 generated.The main pcb was suspected too and I did replace it but the replacement also gave an F19.
However on a few occasions (hw only) I've noticed that instead of the usual F19, the display flashes red and the pressure reading displayed on the lcd will blink too. No fault code in manual is indicated for this.The pressure is well within the quoted range at 1.3 bar, and again no such problem on ch side.I have all but given up now. I am now trying to source a working second hand original LCD Display / Control Interface pcb but no luck yet.Many thanks,

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