Hi Folks, I have trawled the forum for an answer to my problem, but can't find the exact fit. CH works fine. HW always works when the CH is running. When CH is off (or not demanded) the HW works when it feels like it. Once it is running it stays that way. When it doesn't work and I open a hw tap, the boiler sounds like it is starting (i.e. the fan and pump run) but the burner doesn't fire. The DHW flow switch is OK as it starts the fan and pump and is new. Any idea why the burner won't light before I call out an engineer? One thing I checked was the DHW thermistor and this reads open circuit (should be 3k hot to 12k cold, according to the manual). A month ago I had a completely non functioning boiler and the engineer changed the PCB and a thermistror (don't know which one).
Alpha CB28 Boiler - intermittent HW
- Thread starter Silverdog
- Start date
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if it is reading open circuit you need to change it, common problem not expensive and an easy enough job
Are you saying this is the cause of the burner not firing?
yes if the DHW thermistor is open circuit then the burner will not fire
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Thanks. You say it is an easy job to replace, can you tell me the basic steps to isolate the area from water leakage during replacement?
Just had another look as the HW is currently working. Ran with the sensor disconnected and the pump ran but not the fan and the burner didn't light (as you correctly stated). Checked the sensor and it was 20K. Put it back and ran hot water and once hot the sensor read 3.5K. I can only assume that it is intermittently going open circuit, or it exceeds the resistance whereby boiler will fire up (although at 20K, outside the specified limits, it fires). Or the connection to the sensor was dirty and pulling it on and off has cleaned it. What exactly does this sensor do? The X models apparently don't have this. Could I just stick a resistor in place, or is this an overtemp safety measure?
Info on how to change the sensor without flooding the room would be appreciated!
Another update : now really confused. Checked hot water again (2 hours later) and the burner didn't fire up. The confusing bit is that both fan and pump ran (and I know the fan won't run if the HW sensor is disconnected or open circuit). So I don't see how the problem can be the sensor. Anyway I thought I'd grab my meter and check the sensor resistance. Dropped the front panel and checked it still wasn't working by running the hot tap and guess what - the boiler fired up! Now I guess there is the possibility that the wiring disturbed by dropping the panel is going intermittent somewhere.
Just had another look as the HW is currently working. Ran with the sensor disconnected and the pump ran but not the fan and the burner didn't light (as you correctly stated). Checked the sensor and it was 20K. Put it back and ran hot water and once hot the sensor read 3.5K. I can only assume that it is intermittently going open circuit, or it exceeds the resistance whereby boiler will fire up (although at 20K, outside the specified limits, it fires). Or the connection to the sensor was dirty and pulling it on and off has cleaned it. What exactly does this sensor do? The X models apparently don't have this. Could I just stick a resistor in place, or is this an overtemp safety measure?
Info on how to change the sensor without flooding the room would be appreciated!
Another update : now really confused. Checked hot water again (2 hours later) and the burner didn't fire up. The confusing bit is that both fan and pump ran (and I know the fan won't run if the HW sensor is disconnected or open circuit). So I don't see how the problem can be the sensor. Anyway I thought I'd grab my meter and check the sensor resistance. Dropped the front panel and checked it still wasn't working by running the hot tap and guess what - the boiler fired up! Now I guess there is the possibility that the wiring disturbed by dropping the panel is going intermittent somewhere.
OK, have now had the DHW diverter valve and the DHW thermistor changed. The DHW diverter assy was leaking, so it needed changing anyway. To start with I thought the problem of intermittent HW was cured - but no. Every so often the boiler fails to fire up (as before). Fan and pump are running. If I switch the CH on so it is demanding heat, then the boiler fires up and I get hot water.
The engineer who replaced the DHW diverter said it might be the air pressure switch, but wouldn't that affect the CH as well?
As I previously stated, the PCB was replaced a few months ago by another engineer. Are there any settings on the PCB (P1 - DHW set?) that might affect the HW control that he might not be aware of? Also the HW flow switch has been replaced recently.
The only other thing I can think of is a dodgy connection in the wiring. Any help would be most welcome
The engineer who replaced the DHW diverter said it might be the air pressure switch, but wouldn't that affect the CH as well?
As I previously stated, the PCB was replaced a few months ago by another engineer. Are there any settings on the PCB (P1 - DHW set?) that might affect the HW control that he might not be aware of? Also the HW flow switch has been replaced recently.
The only other thing I can think of is a dodgy connection in the wiring. Any help would be most welcome
That engineer has been there, we have not!
Diagnosing faults is done at the boiler and involves testing the circuits and functions.
"Thinking" it may be an APS is not diagnosis, it is merely guessing!
A proper engineer tests the APS !
Tony
Diagnosing faults is done at the boiler and involves testing the circuits and functions.
"Thinking" it may be an APS is not diagnosis, it is merely guessing!
A proper engineer tests the APS !
Tony
Yes, it turned out to be the air-switch in the end, which I replaced easily. A local plumber recommended this as a possible cause. Hope this solves your problem.
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