Worcester Bosch Combi 240 BF - central heating, delayed ignition

[jaw]

Hi

I visited a customer today to carry out the annual service on their WB 240 bf boiler. During checks I noticed that when you set demand for the central heating the boiler will not ignite for between 1min 58secs and 2mins 06 secs, the pilot light still remains on though. External heating controllers and boiler were set to max and the boiler was tested from cold due to the mild weather. I refered to the MI's and followed the fault list. Upon carrying out the tests the WB diagnosis was to replace the gas valve. How can this be so when the boiler operates perfectly when a water demand is created regardless of whether it is in water or water / heating mode. I contacted WB technical who agreed with this. The only thing they could offer is that the CH thermistor was faulty, even though it didn't appear in their original fault list. As asked, I tested the thermistor for resistance but it read OL, I thought I'd cracked it even though I wasn't totally convinced. I replaced the thermistor and hey presto, still no joy. Just out of curiosity I decided to check the resistance of the new thermistor but again this was OL. I have a feeling that the PCB may be at fault, but before I invest money into an ancient boiler I just wanted to ask the thoughts of you guys on here.

Your thoughts are much appreciated, Thanks in advance

 

[dancinplumba]

If it fires up after the delay why worry ?

 

[zeebeedee]

Hi

I visited a customer today to carry out the annual service on their WB 240 bf boiler. During checks I noticed that when you set demand for the central heating the boiler will not ignite for between 1min 58secs and 2mins 06 secs, the pilot light still remains on though. External heating controllers and boiler were set to max and the boiler was tested from cold due to the mild weather. I refered to the MI's and followed the fault list. Upon carrying out the tests the WB diagnosis was to replace the gas valve. How can this be so when the boiler operates perfectly when a water demand is created regardless of whether it is in water or water / heating mode. I contacted WB technical who agreed with this. The only thing they could offer is that the CH thermistor was faulty, even though it didn't appear in their original fault list. As asked, I tested the thermistor for resistance but it read OL, I thought I'd cracked it even though I wasn't totally convinced. I replaced the thermistor and hey presto, still no joy. Just out of curiosity I decided to check the resistance of the new thermistor but again this was OL. I have a feeling that the PCB may be at fault, but before I invest money into an ancient boiler I just wanted to ask the thoughts of you guys on here.

Your thoughts are much appreciated, Thanks in advance

If the new stats reading open circuit as well Id be testing my equipment, if all is good I'd suspect a faulty stat from the suppliers

 

[jaw]

Surely it needs to be firing up as soon as there is demand for the central heating, not after 2 mins

 

[jaw]

Thanks ZeeBeeDee

This did cross my mind and I have requested a new thermistor from my supplier today. Have also tested my multi meter and this is fine. Not convinced by the thermistor being at fault as I once had a similar problem on a Baxi which turned out to be the PCB

 

[zeebeedee]

Surely it needs to be firing up as soon as there is demand for the central heating, not after 2 mins

Some boilers take a while to respond. I'm racking my brains here to think of one that took ages to change from hw to ch I came across the other week.

 

[zeebeedee]

Thanks ZeeBeeDee

This did cross my mind and I have requested a new thermistor from my supplier today. Have also tested my multi meter and this is fine. Not convinced by the thermistor being at fault as I once had a similar problem on a Baxi which turned out to be the PCB

Stats shouldn't read open circuit. Did you disconnect at least one wire before testing?

 

[dancinplumba]

Has it ever been a problem to your client ?

 

[zeebeedee]

Do these have a pump overrun stat? I know the 230 rsf does and this can cause similar issues.

 

[jaw]

Hi ZeeBeeDee

I've tested thermistor via white wires from board and across its terminals. Also, yes it does have a pump overun but the problem is there when the boiler is first switched on from cold. Water working fine, regardless of mode

Hi Dancinplumba

No it hasn't caused a problem to the customer as they won't notice due to it only being a 2 minute delay. It doesn't make it right though

 

[zeebeedee]

You should really test all stats at the stat with at least one wire disconnected. You could get a false reading from the board or a dodgy wire!

 

[jaw]

Like I say I have also tested across the thermistor terminals. New thermistor should be here tomorrow so will pop back in and re-test the one I fitted. Will report back

 

[Derek Hardman]

This boiler does commonly have a 2 minute delay, particularly when switching over if memory serves. It is a gas valve fault, eventually the solenoid will start buzzing... just checked historic service note!

 

[zeebeedee]

Like I say I have also tested across the thermistor terminals. New thermistor should be here tomorrow so will pop back in and re-test the one I fitted. Will report back

Ill get me coat!

 

[jaw]

Thanks Derek
The problem happens even when the central heating is started first when the boiler is cold and no switch over from water as taken place

 

[jaw]

Ill get me coat!

I don't get what you mean

 

[Ollieg]

I would ignore this fault personally. It's not right but causing no harm.
Anyway.
Does the boiler have different ignition rates for htg/HW? If so could be solenoid related.

 

[vern]

Ok I will bore you the driver board in heating mode has a 2 min delay for low to high ramp up from 2.8 Mbar to 14.7 Mbar max( it can be set anywhere inbetween these values to suit system load, rads etc via the rear mounted pentiometer)

if you have changed the thermistor, it will be either the driver board (PCB) or the modireg on the gas valve as it is is perm pilot it will light after 2 mins at the higher value ( this would not happen on auto ignition model and would lock out)

To eliminate the modireg being at fault test as follows, place in DHW MODE, wind down gradually the hot tap, observing your manometer on the gas valve test point, the pressure should fall within the readings above, if the burner goes out there is little or no modulation so the modireg is faulty( this assumes you don't let the DHW flow rate get too low or the micro switch will drop out on the changeover valve also cutting out the burner)

if you get modulation the driver board is faulty.......sorry zzzzzzzzzz I know

 

[zeebeedee]

Ok I will bore you the driver board in heating mode has a 2 min delay for low to high ramp up from 2.8 Mbar to 14.7 Mbar max( it can be set anywhere inbetween these values to suit system load, rads etc via the rear mounted pentiometer)

if you have changed the thermistor, it will be either the driver board (PCB) or the modireg on the gas valve as it is is perm pilot it will light after 2 mins at the higher value ( this would not happen on auto ignition model and would lock out)

To eliminate the modireg being at fault test as follows, place in DHW MODE, wind down gradually the hot tap, observing your manometer on the gas valve test point, the pressure should fall within the readings above, if the burner goes out there is little or no modulation so the modireg is faulty( this assumes you don't let the DHW flow rate get too low or the micro switch will drop out on the changeover valve also cutting out the burner)

if you get modulation the driver board is faulty.......sorry zzzzzzzzzz I know

Vern our very own resident breakdown genius! Once more i doff my cap!

 

[vern]

Vern our very own resident breakdown genius! Once more i doff my cap!

LOL Zeb.........

Jaw the parts are not cheap and with the boiler age make the customer aware of your findings by all means, but i would tell them to save thier pennies for a new boiler anyhow, nice to see you aint buried your head either.....goodman everydays a school day in this job.....

 

[diamondgas]

Nice one vern ... jaw, you can pull off one of the modureg leads and check the low fire burner pressure ... You can adjust the low setting at the modureg ... does sound like someone has been messing with it.... It'll most likely tell you in the mi's how to set the minimum pressure... IMHO

 

[Derek Hardman]

Its the solenoid Im sure, it could run like that fro years without a problem, but eventually it will start buzzing and then stop working.

 

[buckley plumb]

if the customer has not notice I would not worry don't think they would want to spend £200 on a new pcb for the loss of a 2min delay - I have learned to leave alone can open a can of worms for you and will they pay you for the time

 

[jaw]

Hi Buckley

I know what you are saying but this problem as cropped up during servicing the boiler. You can see whats coming next if I don't act or inform the customer, sods law says the problem will get worse and the customer will say "it was working fine until it was serviced"!

Customers, who'd have em!

 

[buckley plumb]

I would just note it on service sheet mate .there is nothing wrong with what you are doing but I have had cusdtomers not return to me as I have spent time like you have they don't see that you are putting it right they just see that it was an expensive service.