28CDi Air Pressure Fault & PCB Replacement

[JDA2012]

Hi all - first post here though I have visited numerous times in the past prior to registering today.

I have a problem with my 10-year-old Worcester 28CDi boiler - not the first problem but the first on which I need to ask particular questions.

I am well aware that any gas work is a big no-no so will refrain from asking questions in that area, however at the moment I am looking at a PCB replacement which should, I hope, be no problem, though I do note that the FAQ says that a PCB may qualify as "gas work"?

The present issue is that when either CH or HW demand is applied, the pump starts but the fan does not, and after 10-15 seconds the DHW indicator flashes rapidly.

Boiler was working fine until last week, and I have performed the following checks as per the service manual (Fail Point J) and other advice:

- Does fault occur in both CH and HW modes? Yes
- Does fan run before fault light? No
- Are APS terminals bridged when off? No
- Are APS terminals bridged when HW is requested? No
- Are APS terminals bridged when red tube is gently blown into? Yes
- Do APS wires have continuity? Yes
- Are APS tubes in good condition? Yes
- Is flue free of obstructions? Yes
- Is there 230VAC across L+N fan terminals when HW is requested? No

This appears to indicate a PCB fault, but does not seem to exclude something having occurred at the fan that has damaged the PCB in some way.

I am aware I can get a WB fixed-price repair for £250, or a BG one for £79/£189/£409 dependent on time taken to fix, but do not currently want to pay that much when I can pick up a new PCB for <~£100 on eBay, a working used one for <~£50, and am quite happy (indeed, enthusiastic) to fit it myself, though I do not wish to do so if unsafe/illegal.

I am fairly familiar with the interior of the boiler as I have done several other pieces of work on it (replaced DV diaphragm, which involves removing W-W HE and various other bits, replaced most fibre washers, fixed loose/exposed(!) room stat wiring left by previous occupant, replaced manual timer with digital one, drained, cleaned and refilled system), but thought I should check I am not missing anything as the PCB is more expensive that any other component I have replaced and obviously I cannot break the gas seal to check the fan directly, nor do I want to get into replacing the PCB if particular checks need to be done afterward for which I am not qualified.

Thanks for any assistance/advice - aware this may consist of "leave it alone"!

 

[picasso]

I wonder how many people have changed the PCB only to find the thermisters(or something else) is at fault.

 

[Agile]

How did you blow into APS tubes, check contacts and fan voltage and ensure flue was clear without opening the combustion area?

Whilst we don't advise on DIY repairs on gas or combustion aspects most will expand that to include safety devices constituting what the gas registrars call the "gas train" which includes all devices involved in burning the gas and monitoring the safe operation of the boiler!

My advice would be to get one of the local independent engineers like Dave or John.

Tony

 

[JDA2012]

The APS (and tubes) are outside the combustion chamber, and all contacts mentioned are on the exterior of the APS or the PCB itself (no voltage across the terminals to which the fan is connected - cannot check the fan itself as it is inside the CC). The flue has been checked from outside.

Absolutely hear what you are saying as regards both the above checks being somewhat incomplete without being able to get to the fan itself or inspect the other end of the flue, and also on the problem with technically leaving the gas alone whilst messing with all the bits that manage the flow.

At the risk of having missed something obvious, how would I get in touch with Dave or John, or are you just using examples and any local independent will do? Have one just around the corner but not sure if all are as dependable as each other so recommendations are very handy.

 

[Agile]

Have you measured the temperature sensor resistance?

I had wrongly quoted Norwich based engineers although I am sure there are several in Manch called Dave and John as well.

Properly checking fans and flues needs access to the combustion chamber although the pressure presented to the APS can still be measured on your particular boiler.

Tony

 

[JDA2012]

Thank you for the assistance. We do have plenty of Daves and Johns here also, that is true.

HW and CH sensors are both 17 kOhm, measured at the PCB when cold.

The sensors appear to be Honeywell 6655-90030004 variants, for which RT2 (expected resistance at current temperature) can be calculated using:

B=((T1*T2)/(T2-T1))*ln(RT1/RT2)

B=3694
T1=298.15K (25C)
RT1=12kOhm
T2=288.15K (15C - cold out there at present)
RT2=~18.5 kOhm

B is not a rigorous constant and so some discrepency is expected, and the parts have a 7.6% tolerance anyway, so overall I am happy that they are reading as expected.

I have also checked the overheat sensor, which has a resistance of 16 Ohm. It appears to be a Honeywell 2455R variant which as far as I can tell should have a resistance of 24 Ohm and should open when an overheat is detected. Again, the reading does not appear abnormal.

 

[Agile]

Where did you find this overheat sensor?

Why do you think the resistance should be 24 ohms?

I think it should be less than one ohm!

Tony

 

[JDA2012]

Cannot access the sensor itself (appears to be at the top of the CC), but it is wired into the PCB, so I again checked it from there, and the part number is listed in the service manual.

Resistance is definitely 16 Ohms (across the PCB terminals); I was going by the 240V/10A values listed in the datasheet due to lack of a stated resistance value but I suspect that as it seems to be more of a thermal on/off switch rather than a variable thermistor like the others that isn't relevant and I should just be looking for open/closed?

 

[Agile]

They are normally the Klixon type and the resistance should be well under one ohm even with the lead resistance in series.

I am not sure where they are connected in that boiler. You could try measuring the voltage across it and see if that's significant.

It may be directly in series with the gas valve in which case 16 ohms in series with a 2k solenoid is going to be negligible. But it also might be in a 24v circuit and that resistance might be having an effect on the circuit.

Tony

 

[JDA2012]

The sensor leads disappear into the CC near the top of the right side panel, however tracing them up them made me realise I had the wrong pair at the PCB to begin with Correct pair do indeed have resistance < 1 Ohm, in fact showing as zero on my meter, so no problems there either.

 

[JDA2012]

New sealed PCB located for less than £50, so gave that a go. Has fixed the blown power LED and is a much newer revision (interesting to compare), but results remain otherwise the same.

However, I have identified an error in the fault-finding flowchart in the service manual, which points at a PCB fault instead of (what is actually) a fan fault.

Specifically, the step instructs: "Remove the fan connector from board position ST1 and restart the boiler in DHW mode. Is there mains 230V across the centre (L) and left (N) tracks at ST1? Note: Take care not to short L to N."

This is correct for a 24CDi, however for a 28CDi centre is live only in CH mode. In DHW mode, right is live - in fact, a similar check in another location on the same flowchart acknowledges this.

Not best pleased at being sent down the wrong track by a dodgy manual ("Worcester Heat Systems cannot be held responsible for costs incurred by persons not deemed to be competent"), but glad the PCB was not expensive, and if I check the voltages with my newfound knowledge I get 243V across the relevant pair in either CH or DHW mode, which suggests that the fan is not responding correctly, so I'm back to calling a gas man out to get inside the CC...

 

[JDA2012]

Did one final check myself - hooked up u-gauge, turned on the boiler and observed no change at all in pressures at the APS tubes. Pretty certain it's the fan / venturi now, looking for a local gas-safe engineer to perform the necessary work.

 

[45yearsagasman]

I've never had a problem with WB fault finding procedures if followed correctly, but I can see how this might happen with inexperienced diyers, who then blame the flow charts.

 

[JDA2012]

New fan sourced - £55 - and fitted plus boiler serviced for a very reasonable price by a local chap called Patrick of Eureka Plumbing and Gas. Hot water now working, and he sorted the gas fire (which by all accounts was in an appaling state!) whilst here, very happy!

45yearsagasman - as I noted, for a 28CDi the centre contact is live only in CH mode. In DHW mode, the right contact is live instead so there will never be a voltage across the centre and left contacts. The instruction is unequivocally wrong in the print manual, and still wrong in the version available online. Of course, if you mean with experience comes the knowledge to know that it is wrong rather than blindly following it, I'm right with you!

 

[Agile]

I am glad that you have had it fixed.

Also that you are pleased with what Patrick has done for you. Its nice to have a happy customer occasionally instead of the stream of complaints we often see here.

Tony