Is my PCB broken ?

[nickgray]

I've got no heat or hot water from my Chaffateaux Britony Combi boiler.
I power it on & ask for HW or CH (same symptoms happen) Pump starts ok, but fan does not. Eventually it displays an error code meaning the "air pressure switch is not responding with the fan on ". But the fan is not on, so it wouldn't trigger the switch !

If I supply the fan with 240V manually (i.e. with an extension cable and reasonable care), then the air pressure switch responds and the boiler gets passed that.

But then...it doesn't power the piezo ignition. So then the boiler locks out as it knows the gas hasn't been lit

Both the fan and the ignition come from the PCB and neither seem to be working ok or quite as I expect - but then I'm not an expert.

I would be grateful for a bit of advice/experience on these things. PCBs are too expensive, and non-returnable if I buy one I don't need ! Any quick fixes to try ? I have a soldering iron if need be...

 

[breezer]

i am not a plumber or heating engineer, but since you have a soldering iron, you could try looking for dry joints (and of course you are going to note which plug / lead goes where first)

 

[ChrisR]

Usually a boiler looks to see that the air pressure switch is in its resting state before it turns the fan on. ie the switch is a changeover type. If it sees the switch is wrong, it stops there. A check on the signals back to the pcb would confirm.

Changing airflow parts is a corgi -bod's area

 

[COG]

if you have no powe to the fan on your ignition demand then its either a board fault, plug/lead fault, or the air pressure switch is stuck open.
you said you put 240 v to the fan and the air pressure switch "responds" do you mean it switches over ,proven with an AVO ?. If your air switch isnt sticking and the connections are all ok the it looks as if your board has had it.

 

[nickgray]

yep, it switches over when the fan runs. so if I power the fan (separately so it runs) and turn on the boiler then i get to a different fault. i don't get any ignition, and eventually the error code is 'no flame detected'.

I can hear a few relays (there are several close together) clicking over, so I guess its not the DC power (24V ?) for those which is at fault. I guess they are the most likely things to fail (being mechanical). I've given the board a couple of gentle taps with a screw driver handle in case anything was sticking !

A related question: will boiler spares providers let me take the PCB back if I find that's not the problem ?

By the way, the fault happened within 30 minutes. i.e. one morning we had heating and hot water, then 30 minutes after doing the washing up I was faced with a freezing cold shower.

 

[ChrisR]

"if I power the fan (separately so it runs) and turn on the boiler then i get to a different fault."

You can't say that! You are doing something never envisaged by the designer - no knowing what it would do. The fan would never be on without the boiler being on!

How do you know the fan switch switches over when the fan runs? Identify the fan connections on the pcb. You would expect a voltage to the common connection on the fan switch, which would come back to the pocb with the fan off, and change to another connection to the pcb when the fan starts. It's a very quick job to identify where the fault lies if you have the Manufacturer's Instructions. I believe they're downloadable.

All the tapping and listening to clicks in the world is no substitute for logic.

 

[galwag]

A related question: will boiler spares providers let me take the PCB back if I find that's not the problem ?

unfortunatley not
If the retailer takes it back after it has been fitted and resells it as new he is effectively selling second hand components and im sure that cant be legal.

Theres also a lot of devious people out there as I have found out on one occasion. When opening the box of what I thought would contain a brand new pcb only to discover a god knows how old burnt out pcb someone had passed back to supplier.

 

[nickgray]

the reason for powering the fan manually was to see whether it was just the control of the fan which was faulty.... and to be sure that the pressure sensor & circuits were actually OK as suspected.

there's no problem at all giving the fan the voltage it is rated at, whether that is from a PCB or directly. its a fair way of testing it in isolation.

of course i wasn't planning to run it like this for real - just a diagnosis check to see 'what if' the fan was running -- does the boiler fire, what happens then ? etc. The fault codes (i.e. the LEDs on the panel) translate to 'the fan is running, the sensor is not responding'. But if the PCB is not sending power to the fan, it would suggest the PCB is broken.....and is that it -would everything be ok if the PCB was giving the fan 240V ?

So: the sensor and fan are ok, but the board isn't sending 240V so the fan doesn't start, and the pressure switch doesn't close. How does the fan get powered ? Oh, a relay is closed using 24V DC, connecting the live wire of the fan. Is that relay OK ? Is the resistance of the coil correct (yes). Is the coil getting 24V DC ? No. So some part of the PCB which I expect to close that relay isn't working as expected (designed)

As for logic, I have carefully followed Chaff's diagnosis flowchart, whereby the path ends up as 'pcb fault'. I was just throwing this open for comment to people who know a lot more about heating systems than me. Any other thoughts are very much welcome.

 

[breezer]

i should still go for dry joint

 

[ChrisR]

"of course i wasn't planning to run it like this for real - just a diagnosis check to see 'what if' the fan was running -- does the boiler fire,

You shouldn't expect the boiler to fire. You're still missing the point. The problem isn't what happens when the fan runs, it is what happens when the fan isn't running. Expect the pcb to read the switch status before it applies power to the fan. If the switch status reads wrong AT THAT POINT it will not supply power to the fan.

Electronics generally works on "state machines". The status of the boiler moves through a sequence like a computer program executes instructions, branches etc. They work sequentially not just combinationally. If you try to start at a point in the middle of the sequence, by applying power to the fan
"'what if' the fan was running -- does the boiler fire, what happens then ? etc"
at the wrong point in the sequence, the sequence/program won't run properly , so you will NOT be able to look for faults. You have proved the fan but NOT the switch OR the pcb.

Don't take the fault messages or lights as gospel - they are only pointers in practice.
A meter costing a few quid, ana little more knowledge, would enable you to resolve the fault. As things are you are proving that you are not competent to work on the appliance, which makes your work illegal as well as dangerous.

 

[nickgray]

As I said originally

If I supply the fan with 240V manually[...]then the air pressure switch responds and the boiler gets passed that.

by 'responds' I meant it switches over. having two states, one without positive air pressure and one with the air pressure. So it starts off in the no pressure state, and when the fan is on, it switches over.

So: The air pressure switch is in the correct state to start with. The fan works but is not turned on by the PCB. If the air pressure switch were to see positive air pressure due to the fan being on then it would respond in the correct way.

If the PCB does check the switch state it would find it OK. And probing the board with a DVM shows this is the case.

 

[ChrisR]

Nick you're still missing the same point.

 

[nickgray]

Expect the pcb to read the switch status before it applies power to the fan. If the switch status reads wrong AT THAT POINT it will not supply power to the fan.

It 'reads right' at that point.

 

[nickgray]

If anyone's interested...........
After a few more measurements on the board (e.g. 3V across the fan power relay instead of 24) came to the conclusion it was knackered. Replaced it for 100 quid (inc VAT) and everything is OK now.
The layout of the new PCB has quite a few changes since the version for my boiler, (including funky light display when drawing HW instead of no LEDs at all). Perhaps there was a problem with the earlier boards and its improved now.