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My combi boiler’s buner keeps firing up in fixed 4 minutes…

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Hello,My combi boiler’s buner keeps...
Hello,My combi boiler’s buner keeps firing up in fixed 4 minutes interval when there is no HW and CH demand. Please bear with me with the long description below.I moved into my current house in this summer. At that moment, the HW of the boiler (Worcester 24i RSF Combination Boiler) was working intermittently. After checkinig by following the installation manual, I replaced the PCB with a NEW one and the HW resumed working well.In this winter, both the CH and HW work well. However, I recently noticed the following sequence of behaviour of the boiler when there is no HW and CH (i.e all the TRV in the house are off) demand:
- Burner fires up for 18-20 secounds
- Burner stops the fire and rest for exactly 4 minutes
- Burner fires up again for 18-20 seconds.
The above 4-min-cycle repeats for the whole day when there is no HW and CH demand. It stops when I switches the boiler to HW only mode.I checked the installation manual and from there I got that the burner fires when
1) The CH programmer is on and the room theromostat is on
2) Internal Autofrostat demand
3) HW demandSo, I suspected that the problem might come from the “Internal Autofrostat demand” which fires up the burner at once after the 4 minutes anti-cycle delay. However, the boiler is inside the utility room which has a temperature simlar to that of the kitchen and it is always well above 10°C. I checked the resistance of the Thermistor (i.e. the central heating sensor) and it is within the range of 690 and 33k ohms (as per the installation manual) when it is either hot or in room temperature.
I then suspected that it was the problem of the PCB which calls for Internal Autofrostat demand even when the water is well above 4°C. I replaced the PCB with a refurbished one but the problem still occurred (CH and HW worked normally with this refurbished PCB). I returned that one and got another refurbished PCB but the problem still occurred. I then wonder whether the problem is caused by a faulty PCB.I get stucked now. Any advice on the cause of the problem?Thank you.
Submitted: 5 months ago.Category: UK Plumbing
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Answered in 1 hour by:
11/15/2017
Plumber: Andrew Smith, Engineer replied 5 months ago
Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3,767
Experience: Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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Are you getting warm water flow through the heating pipework?
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Customer reply replied 5 months ago
No. When that 4-minutes cycle occurs while there is no HW or CH demand, the heating pipework is cold.
Plumber: Andrew Smith, Engineer replied 5 months ago
Ok.
If the heat is staying inside the boiler tben perhaps there is a hot water preheat on your model?
Can you find the GC no. To identify the full model (and build age)
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Customer reply replied 5 months ago
My boiler, as per the manual, has no preheat function. I found that there are 2 GC numbers shown in the inside plate of the boiler (please see the attached photo). I also attached the installation manual of the boiler for your reference.
I checked the pipework outside the boiler again tonight. It is still cold even the 4-mins-no-demand cycle runs for more than an hour. I believe the pump keeps circulating the CH water within the pipework inside the boiler via the by-pass pipe. The pipework inside the boiler and the pump is super hot.
Plumber: Andrew Smith, Engineer replied 5 months ago
Ok.
Your boiler is old in design - no divertor valve and a twin pass heat exchanger.
If the fault was on the ch side tben the pump would run.
This would suggest the hw side and the hw flow switch.Have you checked to see if it is 'made' continually? This would mean the boiler would fire all the time but keep shutting down on overheat (then cooling and restarting)
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Customer reply replied 5 months ago
Do you mean whether the sparking fires continually? No, it doesn't. The burner fires up for 20 seconds and then the fire is out (and no sparking) for 4 minutes, then the fire lights up again for 20 seconds.I am sorry that I don't quite follow what you wrote:
"If the fault was on the ch side tben the pump would run.
This would suggest the hw side and the hw flow switch."
What is the implication of this?
Plumber: Andrew Smith, Engineer replied 5 months ago
Has the hw flow switch been looked at?
I think the boiler thinks the tap is running and fires up. Then when the heat has nowhere to go it overheard and shuts down. Then cools down and repeats.
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Customer reply replied 5 months ago
However, the problem does not occur when the programmer is switched to "0" (i.e. no central heating) or when the CH water temperature is turned to the minimum. If the boiler thinks that the tap is running and fires up, it would keep on doing this even when the programmer is switched to "0" or the CH temp. is turned to minimum. Right?
Plumber: Andrew Smith, Engineer replied 5 months ago
Correct.
It's difficult if not stood in front if it with a multimeter.
This boiler has no bypass.
When in hw mode the flow switch fires the boiler and heats the water through one set of pipes in the heat exchanger.
When ch is called for then the pump starts and the water is heated through 2 other pipes in the heat exchanger.
Are you able to do Check O on page 33?
The only thing that can fire the boiler without the pump coming on is the hw flow switch
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Customer reply replied 5 months ago
I will do the flow switch check by following the procedure "O" on page 33 tonight. On the other hand,
I found a fixed by-pass in figure 2 and figure 36. Is it different from the bypass you mentioned.
On the other hand, when this 4-minutes-cycle-with-no-demand is running, the hw pipe inside the boiler is cold while the ch pipe (including the bypass pipe) inside the boiler and the pump is super hot. I believe the pump is running when this 4-minutes-cycle-with-no-demand problem occurs.
Plumber: Andrew Smith, Engineer replied 5 months ago
Ok.
Still strange then.
If the pump is running then try removing the frost stat and seeing how it behaves?
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Customer reply replied 5 months ago
34;removing the frost stat"? I don't know where it is. The manual mentioned the "built in frost thermostat" in page 17 (last paragraph) but didn't show it in any of the diagram or the parts list. I thought that it is something in the PCB that works with the
"central heating sensor". That was why I checked the "central heating sensor" and found that it is fine and also replaced the PCB twice but the problem is still there.
Plumber: Andrew Smith, Engineer replied 5 months ago
It looks like the case, the only thing left to check is the wiring harness between the stat and PCB if you have checked both?
Uncommon but possible.
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Customer reply replied 5 months ago
I have checked the flow switch. It works ok. I checked again the CH and HW sensor on the PCB connector end, they also give the expected resistance.
I found the following sequence when the 4-minutes-cycle-with-no-demand is interrupted by HW demand:
- burner fires for 20 seconds
- the fire stop, then within 4 minutes, turns on the hot tap
- burner fires up at once to provide HW. When the hot tap is closed, burner keeps on firing for 20 seconds more.
- then the fire stops. After 4 minutes, the burner fires up again and the 4-minutes-cycle-with-no-demand repeats.If the cause of the problem could not be identified, my next bet is to order a NEW PCB and give it the last chance (even my current PCB is less than 6 months old and I already tried 2 refurbished PCBs)..
Plumber: Andrew Smith, Engineer replied 5 months ago
Sorry, but that would be my guess too or a wiring harness fault?
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Customer reply replied 5 months ago
Hello,I have replaced the PCB with a NEW one but exactly the same problem still occurs. The previous expert mentioned that it could be wiring harness fault. I tried disconnecting the CH sensor and the HW sensor. The LED flashes when either one of them is disconnected. So, the problem would not be the wiring harness of these 2 sensor. How can I find which wiring is creating the problem? Or, do you have any idea why the problem occurs? Thank you.
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