POT (potentiometer) - it's purpose?

[staffordshireterrier]

thanks for that paul .... i get it, much helped

I know some thermistors can be PTC - the single pin ones...?

On the whole it's all shifted to NTC, not shure why there was a good reason for it though.

Just as they shifted to flame rectification (which means the ac voltage applied across the flame will only satisfy the safety checks of the pcb if it is converted to DC, a flame apparently acts as a diode [1/2 wave rectifier]) Previous to this they just looked for a voltage, but it was found that if the sense electrode was shorted or leant over to or tracked to earth it would inform the pcb there was a flame when there wasn't. flame rectification will only take place in the presense of a flame.

As for thermocouples, less and less about. Things are ramping ahead in the complexity stakes..

yeah i did hear about flame rectification being changed due to the ac / dc problem.

i know thermocouples are old, but still quite a few about, mine is as well!! the old type with boiler stat, gas vale, thermocouple and that's it. just a s/l from cyl stat/3 port mid to activate it

 

[Paul Barker]

Paul, what do you mean in this case you will never have a failure with this pot?

That is my experience

when the ntc's fail, are the most common to fail wet pocket types due to sludge etc sticking to them?

that does happen, but it is a fragile little gimo which just gets intermittently a poor conductor as the wires connected to the gizmo inside the metal case do the job badly over their life. there are some really bullet proof ones and some notorious ones. Dodgy hot water on a vokera Linea change the ntc in every case. In an ALpha clean the plate heat exchanger in every case. you will learn over the years, long slow learning curve of what happens to which boilers.

what are the fibre washers for? wet pocket thermistors?

Many mating surfaces inside a boiler which will leeak of not renewed after disturbing.

do you mean most common faults are flow switches'/diverter valves, aps's, blocked flues, faulty fans, etc??

yes, but trying not to tell the public everything go to ocombustion chamber for more friendly replies, trying to code the replies a little to make them hard for the truly ignoprant to grasp.

 

[staffordshireterrier]

thanks a lot for your help paul, i really appreciate it.

so when you first start doing boiler breakdown's it takes a lot longer - until you get to know the boilers more/common faults.... and then you gain the experience?

one other q - are parts hard to change on a boiler? i have never changed a heat ex / diverter valve / prv / gas valve etc - are they fiddly/tricky? any tips?


is there any other ways i can get to learn more - books/dvds/websites etc?

 

[ChrisR]

There's more available in the Combustion Chamber, go for it.

Don't take all Paul says as absolute gospel - but broadly!

Some boilers are certainly pigs to work on - there's no quick way to find out,and it all depends where they've been installed.

 

[Paul Barker]

I am talking to you on a level above the entry level. quite advanced stuff, but you will become a very good breakdown man if you get on combustion chamber and seek advice loads of very competent guys share really good trade tips.

You basically have to just get started and work your way up. To start with it is difficult to change parts and dangerous. So you will get better or flood someone then get better. You will certainly get wet a few times and you might have the odd explosive ignition. One important lesson is not to imediately relight a boiler after a failure until you have had a look for a cause. Fronts of boilers and primary flues have been known to take a blast from the clueless have a go heros..

 

[staffordshireterrier]

Thanks for all your help

How do I join the combustion chamber as I am not gas safe reg at the mo ... acs reg in ccn1, cen1, ckr1, wat1, htr1 and cpa1.
paul, when you say it is difficult to change parts, do you mean as in fiddly?
and do you mean dangerous as in water-wise?

 

[Paul Barker]

I expect the moderators will offer you a way, whoever they are, if you show them your certs.

water damage is the most common risk.

 

[Paul Barker]

thanks for that paul .... i get it, much helped

I know some thermistors can be PTC - the single pin ones...?

On the whole it's all shifted to NTC, not shure why there was a good reason for it though.

Just as they shifted to flame rectification (which means the ac voltage applied across the flame will only satisfy the safety checks of the pcb if it is converted to DC, a flame apparently acts as a diode [1/2 wave rectifier]) Previous to this they just looked for a voltage, but it was found that if the sense electrode was shorted or leant over to or tracked to earth it would inform the pcb there was a flame when there wasn't. flame rectification will only take place in the presense of a flame.

As for thermocouples, less and less about. Things are ramping ahead in the complexity stakes..

yeah i did hear about flame rectification being changed due to the ac / dc problem.

i know thermocouples are old, but still quite a few about, mine is as well!! the old type with boiler stat, gas vale, thermocouple and that's it. just a s/l from cyl stat/3 port mid to activate it

You are a lot brighter than the average plumber whats' the story.

 

[staffordshireterrier]

ok thanks. i'll register.

i've got another q but i'll start a fresh post

 

[staffordshireterrier]

thanks for that paul .... i get it, much helped

I know some thermistors can be PTC - the single pin ones...?

On the whole it's all shifted to NTC, not shure why there was a good reason for it though.

Just as they shifted to flame rectification (which means the ac voltage applied across the flame will only satisfy the safety checks of the pcb if it is converted to DC, a flame apparently acts as a diode [1/2 wave rectifier]) Previous to this they just looked for a voltage, but it was found that if the sense electrode was shorted or leant over to or tracked to earth it would inform the pcb there was a flame when there wasn't. flame rectification will only take place in the presense of a flame.

As for thermocouples, less and less about. Things are ramping ahead in the complexity stakes..

yeah i did hear about flame rectification being changed due to the ac / dc problem.

i know thermocouples are old, but still quite a few about, mine is as well!! the old type with boiler stat, gas vale, thermocouple and that's it. just a s/l from cyl stat/3 port mid to activate it

You are a lot brighter than the average plumber whats' the story.

thanks for that i jus worked with a local plumber, went to college for about 3 years, learned as much as i could on my own, did jobs in my house, families/mates houses. leanred a lot of experience too that way.

it's just hard getting into boile rbreakdown / repair's. I've got a glow worm and a ravenheat course (1 day each - fre of charge_ in october, which i'll be attending, have studied a few books, dvds about combi repairs.

 

[mysteryman]

Yes, there are negative and positive temperature coefficient thermistors [NTC & PTC. The major supplier of solar controls has just changed from one to the other.]

A potentiometer is strictly a potential [or voltage] divider. It is a means of producing a variable voltage from a fixed supply. This voltage is compared with the voltage coming through the thermistor, and the difference controls [eventually, via chips etc] the gas valve.

I hope you go far with your career - diagnostics can be very interesting. You can't really generalise about common faults across all boilers, but the most important thing, as stated in these threads so often, is the quality of the original installation work. System cleanliness is all.

 

[ChrisR]

If you've studied what books there are and done a couple of courses, you're way ahead of the rest.
What the books don't tend to explain though is how different things get when the boiler's been in a few years. Little grub screws go rusty, brass screw heads break, things start leaking, get covered in every sort of goo and crud, and so on.
How would you like to just pop this apart... ?

 

[DP]

Vokera Excel that has been neglected. Not seen one poor as this.

Good picture Chris.

I am surprise our Tony has not posted. I bet he use to make pots size of double decker buses when he was with BBC

 

[D_Hailsham]

Ohm was an old bloke and since he discovered some stuff about electrons flowing around a conductor his name was attributed to the laws of electricity just like there are laws of gravity.

And what about Alessandro Giuseppe Antonio Anastasio Volta and André-Marie Ampère?

 

[Agile]

If you are not gas reg then how are you doing cp12s?

You should be going out with a competent boiler engineer. You are asking too advanced questions for your needs now. Most boiler repairers dont know exactly how a pot works just how to test them.

The interesting pots in the old BBC were the faders!

They were actually make before break switches with perhaps 32 positions and resistors to give a constant impedance of 600 ohms using a 't' circuit. Not so big but beautifully made.

Some had a switch to start a playback or on air sign when they were lifted.

Tony