Agile said:
Baxi 105e Combi Saga Continues
- Thread starter Jennywrens
- Start date
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DHW takes priority, the boiler is sticking in DHW mode and this is causing the boiler to o/heat.
If this is the case the pressure differential valve (flow switch) needs investigating/replacing prt 248734. To test this valve remove the top microswitch (black rectangular box with black and white wire) the plunger pin should move in and out approx 15-20mm, as the tap is turned off the pin should quickly retract.
If this is the case the pressure differential valve (flow switch) needs investigating/replacing prt 248734. To test this valve remove the top microswitch (black rectangular box with black and white wire) the plunger pin should move in and out approx 15-20mm, as the tap is turned off the pin should quickly retract.
baxpoti said:
Thankyou - is it still replacing the sensors aswell?
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baxpoti said:
I don't pick the new one up until tomorrow - we know for sure that this will be the first time it has been repalced as we put this boiler in new three years ago.
It isn't the flow switch as this has been tested and works fine.
We have just taken out the DHW sensor and the heating works a treat.
I have an identical example of this problem occurring in my 3 year old 105E instant. DHW and CH appear to work OK for most of the time but the boiler often "Sticks" in DHW mode , no flame, pump running continuously with DHW LED solid ON. Temperature LEDS are fullscale indicating 80 Degrees +. Boiler appears never to come out of preheat mode and can stay like this for 20 to 30 minutes, it then clears when pre heat water cools.
If Hot water tap turned on it corrects to normal sequence ie preheats water for around 30 secs ( DHW LED pulsing) then turns off flame LED and DHW LED. Boiler emits a noise rather akin to a landing jet decreasing in pitch just before this sticking happens, presumably the air supply fan?
Side effect is that CH doesnt activate, even when demanded, since DHW has priority.
I have not removed the front to examine any further as yet but am interested in obtaining any leads as to possible causes. What normally terminates the pre-heat sequence, I guess pre-heated water up to temperature and pump completed any cool-down time ( if needed?)
Another possibility could be boiler over-temp sensor cutting in until cooled by incoming cold water when tap turned on.
If Hot water tap turned on it corrects to normal sequence ie preheats water for around 30 secs ( DHW LED pulsing) then turns off flame LED and DHW LED. Boiler emits a noise rather akin to a landing jet decreasing in pitch just before this sticking happens, presumably the air supply fan?
Side effect is that CH doesnt activate, even when demanded, since DHW has priority.
I have not removed the front to examine any further as yet but am interested in obtaining any leads as to possible causes. What normally terminates the pre-heat sequence, I guess pre-heated water up to temperature and pump completed any cool-down time ( if needed?)
Another possibility could be boiler over-temp sensor cutting in until cooled by incoming cold water when tap turned on.
Yes, I saw that post earlier but was left a little confused. Does this imply that the boiler is working with NO DHW sensor or that a non-working sensor has been replaced?
Noticed today that when boiler is "stuck" in DHW mode with no flame any increase in the pre-heat temperature knob setting retriggers a new Pre-heat cycle OK just as opening up a hot DHW tap does.
Noticed today that when boiler is "stuck" in DHW mode with no flame any increase in the pre-heat temperature knob setting retriggers a new Pre-heat cycle OK just as opening up a hot DHW tap does.
OK. We are back to square one.
What I meant to explain more clearly was, that we took out the DHW sensor completely and hey presto, the heating came on.
This was just a temporary measure as we were cold. If we required hot water, we would put the sensor back in, then take out again after use.
We have today replaced the sensors for new (both) and nothing has changed.
It started off ok, with the heating coming back on, but this evening after using DHW (sensor in), it refused to switch back to heating, so I have taken the sensor out again.
Your joking?
Your taking the sensor out???
Putting the sensor back in for hot water????
I don't think so.
You need a new PCB/brain
No, I am not joking, this is the only way we can have heating on. Obviously this is only temporary until we can find a fix.
I wonder if you can help - the PCB we originally ordered and tried (part number 248731), would not fit. Were we informed correctly that this part is now only made by siemens? - Do you know the part number we should try please?
This is correct for an instant
Hi Tony
What we have to do, is use a pair of small pliers, gently grip the black plastic part that sits at the end of the wiring, and pull it out away from the actual sensor. The sensor remains in the boiler, but the wiring and connector are loose. As soon as we take this connector out, the CH neon lights immediately and we have heat.
Ridiculous I know, but we found this by trial and error and at least for the time being gives us heat.
I am concerned about replacing the PCB as we already bought one, and my partner said that it was not the same and would not fit. I am wondering if I was given the incorrect part, although I gave them the specific part number.
I am not a gas service engineer but do have some experience of complex control systems in industry so these comments are a bit theoretical. It seems to me that the problem with this boiler appears to be in the relationship between the DHW differential pressure operated 3W diverter valve and it's associated sensing microswitch. Contamination of the DHW pressure sensing spindle which operates both the microswitch and the 3W diverter valve can confuse the PCB logic. That is... the spindle sticks when returning from DHW mode, the DHW microswitch stays operated and the diverter valve is not fully returned to CH mode. Suspect that removal of the temperature sensor is kicking the PCB logic out of it's "stuck" state and restarts the CH cycle even though the diverter valve is not fully returned to CH mode. I have no idea what happens with a valve that is part way operated, suppose primary water goes into both DHW heat exchanger and CH system simultaneously.
Suspect the only cure is to replace or cure the jamming of the 3W Diverter Valve and DHW Flow switch assembly. My experience of electronics has shown that modern electronic PCB's are rarely the cause of the problem unless they have been connected to a faulty output load or subjected to over-voltage inputs, fairly unlikely in this case since the only PCB output is to the gas valve which is working OK. Moving mechanical parts are usually the culprit in these cases or broken rubber seals/ diaphrams. Operating the 3W valve directly from the DHW pressure sensor without using a motor obviously looked a money saving option but is possibly not man enough for the job when parts age a bit.
Hope these suggestions offer some food for thought from the guys who have real experience ...
Suspect the only cure is to replace or cure the jamming of the 3W Diverter Valve and DHW Flow switch assembly. My experience of electronics has shown that modern electronic PCB's are rarely the cause of the problem unless they have been connected to a faulty output load or subjected to over-voltage inputs, fairly unlikely in this case since the only PCB output is to the gas valve which is working OK. Moving mechanical parts are usually the culprit in these cases or broken rubber seals/ diaphrams. Operating the 3W valve directly from the DHW pressure sensor without using a motor obviously looked a money saving option but is possibly not man enough for the job when parts age a bit.
Hope these suggestions offer some food for thought from the guys who have real experience ...
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