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I though we'd do a little bit on sequence of operation today.
Just one thing to bear in mind guys, this is a more modern pre-mix appliance, and this boiler has 28 different fault finding codes, okay. This is old atmospheric, and you know in a lot of cases these are the type of boilers that you're working on because these type of boilers you've made manufacturers put these long warranties on there, so you know at the end of the day, a lot of boilers that you attend are of the older type. So we'll address you know, later technologies, we move on, but one of the things we need to understand is sequence.
This has got 28 fault finding codes. This one, the old atmospheric and they're all the same, the top line depicts power, and the bottom line when flashing, it's gone to overheat for some reason, and if the bottom line stays solid, then it's gone to flame failure. Well sorry guys but yeah these are handy to have all these fault finding codes, but the whistles and bells, at the end of the day, you know, you're the engineer guys, you're out there actually fixing it, and yeah they're a good handy guide but you know, train the trainer again and let's be engineers.
There's only two failures on any gas boiler, whether it be a pre-mix or whether it be an atmospheric, and one of those failures is attributed to an overheat situation, and the other is attributed to flame failure. What we must get into as mindset guys, is understanding the sequence, okay, now understand the sequence and understand the standard of a gas boiler.
The sequence on gas boilers is virtually the same because they all go through the same test styles to get to the end sequence. If we switch a boiler on, then the first component to energise is the pump, and the pump comes direct from the circuit board in older technology like this. If we look at, if we move on, some boilers may have some device like this, they may have a pump proving switch, where if the pump's not energised then the sequence can't continue. But let's look at boilers that you're working on without flow switches and without, if you like, a pump proving switch like so.
We switch this type of appliance on and the pump immediately energises, and the pump comes direct from the circuit board. Now a regular component to be sold as a PCB, but what you must appreciate, with all manufacturers PCBs are pretty much bullet proof, so the only time I will actually tell you there's a problem with the PCB is if there's no power to the pump, the first line in the sequence, okay. Now the standard says then, once the pump is energised, the standards is asking for the circuit board behind the panel to search for the APS, to search for the Air Pressure Switch. Now this is very important guys, because this is where you can actually buy a PCB or necessary.
I'll just swing around to this flip chart, okay, and what we've got here, this is an old type APS. Now on very old appliances, we had three taps, and on those three taps there was three wires. One of those was common, one of them was normally open, and the other was normally closed, okay? So all of those three taps were actually used. As time went on and things moved on, we found that we had two wires, but three taps, and one the taps was covered with a red or an orange rubber boot. And then as time went on, we found two taps and the third tap had disappeared. Well it hadn't disappeared, it became a flat piece of metal on the APS, on the Air Pressure Switch, because that flat piece of metal was still a usable part of the Air Pressure Switch, and that usable part of the Air Pressure Switch was the normally closed position guys, okay, so remember we've now got the sequence of operation the pump is running, then the standard says that the circuit board must search for the Air Pressure Switch to see if it's in place before ignition can continue, and ignition continue it needs the fan to run, okay.
Does it search from common to normally open, or does it search from common to normally closed? Well think about it [inaudible 00:04:40] guys. If it searched between common and normally open, it would mean that you could link an Air Pressure Switch out and walk away, but basically you're leaving a bomb. So the standard says that we don't search between common and normally open, we search between common and normally closed. So once it sees that there's continuity between common and normally closed, it knows that the Air Pressure Switch is there guys, don't it? And when it knows that the Air Pressure Switch is there, then it allows the fan to energise. So that is the thing that will throw you off track, where you would buy a PCB. It's not the PCB. Just remember what I was saying to you, PCBs are bulletproof guys, all manufacturers, okay.
Right,