Gas Boiler simply stopped working - ideas?

[Phillip Brown]

I appreciate it is the bank holiday, but as my house is rather cold, I hope you can respond before then :) I recently fitted a reconditioned Potterton Netaheat Fan Unit after the old one gave up the ghost, and it has be running well until I turned the thermostat down for one night whilst I was away.

When I returned on Sunday, the thermostat had turned itself on as the house was below 21-degrees, but the boiler wouldn't turn on. Nothing I could do would make the boiler start up again. I have tried:

Replacing the thermostat Testing the manual control of the motorised valve Turning off/on all the switches on the timer Checking the fuses to the main system and on the boiler's PCB

When the thermostat turns on, the valve opens and the pump starts, however the boiler just does not ignite. It also does not make the 'humming noise' that you get for 10 seconds before ignition, which I assume is the fan?

Is it possible the fan could be faulty, or is there some other possibility between the timer (which works as it too turns the pump and valve on and off) and the boiler?

Many thanks in advance of your prompt reply.

[John Stumbles]

Is there mains getting to the boiler? Sometimes there is just one mains cable to the boiler which is made live when there's demand via the programmer + room 'stat + motorised valve, sometimes there is a live feed via a fused spur and a separate switched live via prog + stat + valve. Either way if the live is getting switched on and off with the room stat + valve (test with a neon screwdriver) but the fan's not starting it suggests the fan is kaput, but if there's no switched live you've got an electrical problem.

[Phillip Brown]

Thanks John, there is no other fuse, the cable chain is as follows AFAIK:

Mains box with fuse > Wall thermostat > Timer / Pump / Valve > Boiler

When I turn on the timer with the thermostat up, it starts the valve opening and pump running. I can see the cable running out of the timer which goes all the way in to the boiler.

What part of the cable would I prod with a neon screwdriver to check the live is getting to the boiler?

Could it not be the PCB on the boiler itself?

Thanks again.

[BigWallop]

If you can't hear the flue fan running in the boiler, then I'd be tempted to say it is that which is at fault, again. You say the new fan was a re-conditioned one, but you didn't say what made the old fan go faulty. You may have fitted the new fan unit to a faulty boiler. What was the symptoms of the old fan unit failing? Did you get any sort of guarantee with the re-conditioned fan? Did you test the circuit that supplies the fan from the boiler?

[Phillip Brown]

It was the original fan, so over a decade old, and was just groaning and turning very slowly - basically 'old age'. Boiler had been serviced a week before that by British Gas.

I'd have to check but I'm sure I do. It was from CET Ltd. who seem quite good.

Nope, that's the next step I guess. Can you confirm if I simply use a neon screwdriver on the red/brown cable that goes into the fan to check this? Then if it is live but the fan is dead, then the fan is... well, dead I assume. I'm quite clever, me. ;)

[BigWallop]

The fan is a mains voltage appliance, so a good test to see if the fan is working properly, is to fit a length of cable to it and plug it in to a power outlet. If it don't work then, it's dead. This lets you rule out the fan completely and you can concentrate on the boiler supply side of things. Once you know that part, get back to us and we'll try to talk you through other things to test for.

Placing a mains tester screw driver on the connector terminals is not a fair test I'm afraid. You really need a test meter to make sure you are getting all the connections you need too, to make the fan and things work properly. The break in circuit may be on the negative side of the supply, which a mains tester screw driver won't show.

[John]

Why? did you by any chance test it and prove it faulty or just make a guess? Seriously what is your level of technical expertise in tracing a possible fault?

I can't recall if this boiler has an overheat thermostat which requires manually resetting. If it has find it and test it for being open circuit.

[Phillip Brown]

Forgot to mention, the thermostat had been behaving strangely for a week beforehand. E.g. the required temperature was set at 24-degress, the room temperature was 25, yet the thermostat had not clicked 'off'. It is a Drayton Digistat 3 and previous to this worked fine for a year. My logical conclusion was therefore that the thermostat had given up the ghost and was failing to turn on my heating system.

Could you point me in the direction of what to look for? I see nothing in the manual about this.

In the past, when the water ran dry in my system, it did shut itself down, but seemed to come back on after an hour, of its own accord.

Thanks for the ongoing advice.

[Lurch]

On Mon, 03 Jan 2005 15:43:28 GMT, "Phillip Brown" strung together this:

Christ! How accurate do you want it? I'd call that reasonable, look at the instructions and you'll see what the olerances are. I'd allow it to be more than 1degC out. Did you measure the exact temperature on the exact point on the stat that measures temperature, or did you think it would measure the temperature on the other side of the room?

[Phillip Brown]

Thanks for the compliment however my name's not Christ, but that aside there's another misunderstanding. Normally I set the stat to 24. When the temperature display *ON THE STAT* reaches 24, the stat turns itself off -- it knows it has achieved its goal.

Lately however, the display *on the stat* was showing 25+, even though the desired temperature was set to only 24. It was continuing to heat the house beyond what it was set to do. This had never happened in over a year I had it, so I understandably diagnosed a problem with the stat.

I did not measure the actual air temperature separately!

[Lee]

Ours tends to do this when the batteries need changing...

Lee

[John]

Beware if the fan has three connections (high speed, low speed and neutral

Once more for good luck - beware if it a two speed fan (see above)

Try "testing" at both sides of the connections. The Neutral should not light the neon if present. If its not then you will get a light

[Phillip Brown]

That was my first hunch too, so I changed them but no different. Also the battery indicator wasn't flashing so they were apparently still good. Thanks anyway.

[Lurch]

On Mon, 03 Jan 2005 17:15:04 GMT, "Phillip Brown" strung together this:

Ah, that would appear to make sense.

[Toby Sleigh]

The basic unit does not come with a overheat thermostat. Potterton sell an add on overheat thermostat kit for sealed systems.

BTW my Potterton Neataheat 16/22 packed up on Boxing day, similar symptoms except that the fan started then nothing. Removed control PCB and found a couple of blown tracks. Soldered wire links across break, no probs since.

Toby

[Phillip Brown]

Must be a special 'break around Christmas time' setting they all have.

My fan isn't turning however, which indicates to me that the PCB is the 2nd in line, fan being 1st in line. I.e. if the fan doesn't turn, then the PCB can't be the fault?

[BigWallop]

How did you test the fan, Philip? Did you actually remove the fan to do the testing?

[Phillip Brown]

Simpler than that, I can't hear it. Normally it hums quite audibly. I'm pretty sure it's the fan that hums anyway, and now no noise from it.

However tomorrow I will buy a neon screwdriver and test for 230v across the fan coil. Can people here confirm that is correct and safe?

[Lurch]

On Mon, 03 Jan 2005 22:34:56 GMT, "Phillip Brown" strung together this:

It's not, I can test your fan from here as accurately as you can do it with a neon screwdriver. You can't test across anything with a neon screwdriver, you'll strtuggle to test anything reliably.

If you're going to test it buy a decent multimeter. You can get a suitable one for about £20-30. Alternatively, leave it well alone and give someone a call.

[John Stumbles]

(You have checked that the fan's not just jammed by something, haven't you?)

Yup, that'll give you a reasonable indication. Make sure you know how to hold it to get it to light up: there's a little contact on the part of the screwdriver you hold which you have to be touching for it to light up when the tip of the 'driver contacts mains (I know this sounds like an apprentice booby trap but it's not: the current required to make the neon light up has to go travel through your body to earth, but it's so minute you can rarely even feel a tingle from it). Check it on parts of the system you know should be live e.g. the thermostat and programmer, then check the live terminal where the supply cable enters the boiler. Check also that the neutral terminal is _not_ live (in case the connection has come adrift on that side).

WRT Lurch's comment on neon testers and multimeters, note that both neons and modern digital multimeters have such high input impedances that they will give unreliable indications on some sorts of faults e.g. where a connection is basically broken but still capable of passing or picking up the minute amount of current required to light a neon or register on a meter. For these situations a simple 2-probe mains tester with LEDs (or as a DIY alternative a low power mains bulb such as a fridge or oven bulb or small candle bulb connected to 2 probes) will give a more reliable indication.