By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Forum House Ad Zone

How does this flame failure device work?

All Topics | Latest Posts

Search for:  in Thread Title in  
old mart07/04/2023 17:56:51
4655 forum posts
304 photos

I recovered this from the bin in the garage, a left over from boiler servicing. It is a mystery to me as to how it works. The two white electrodes are the spark igniters, but the blue one which detects the flame has only one electrical connection._igp3136.jpg

Robert Atkinson 207/04/2023 18:08:15
avatar
1891 forum posts
37 photos

A flame is ionized and thus electrically conductive. The circuit connected to the probe detects the conductivity.

Fulmen07/04/2023 18:14:13
avatar
120 forum posts
11 photos

The detector could use one of the spark electrodes as common ground. Or more likely use the burner as the other electrode. The difference in temperature could in theory form a crude diode, this would make it possible to detect short circuits and leak currents.

old mart07/04/2023 18:50:45
4655 forum posts
304 photos

Thanks, Robert, that makes perfect sense to me.

 The traditional type of flame failure device relied on a thermocouple providing power to a magnet in most older boilers and also the burners and oven of my gas cooker.

Edited By old mart on 07/04/2023 18:53:56

Howard Lewis07/04/2023 20:45:09
7227 forum posts
21 photos

They are often referred to as "Thermocouples" so implying that at the other end of the lead the control unit detects a volyage and allows the main valve to open to supply gas to the burners.

Howard

not done it yet08/04/2023 08:26:07
7517 forum posts
20 photos

Our boiler has just a single electrode which provides the ignition spark and detects the pilot flame. Advances in electronics, I suppose - but our boiler is over 15 years old.smiley

John Doe 208/04/2023 09:21:33
avatar
441 forum posts
29 photos

I have just repaired the central heating system I recently "inherited", (and which had 6 separate faults),

During my investigations and fault finding; I learned that a flame is conductive and can be used to detect the presence or absence of a flame.

In the case of my old Potterton Netaheat 80E, nobody would give out any information, so I traced through the PCB and found that the flame sense circuit uses the ignitor probe itself to detect the flame and hold a relay over, which enables the gas valve. The ignitor is literally just a metal probe, not a thermocouple in this model.

I tried to find the fault on the board, but could not obtain a circuit diagram and ran out of time during the coldest part of the year so far, (isn't it always when the heating fails?), and bought a new PCB. Cost me £82.

I have no doubt that had I been clueless and had to call a plumber, they would have sold me a new boiler and a new pump etc.....plus labour costing probably the best part of £2,000.

 

Edited By John Doe 2 on 08/04/2023 09:23:18

Clive Steer08/04/2023 10:51:22
227 forum posts
4 photos

Having designed gas boiler ignition controllers the flame failure detector uses the ionisation effect of a flame which acts like a diode. So an AC voltage is applied to the flame probe and the resultant DC voltage operates a transistor to allow the main burner gas valve to open. The controllers have to be designed so that a single circuit component failure will either shut down or prevent start up of a boiler.

Most modern domestic boilers are fan assisted so the controller first runs the fan for a time in order to clear any combustion products or gas from a previous start attempt and then tests for a change of state of the microswitch on a diaphragm air pressure sensor. If this proves OK the controller opens the pilot flame valve and begins spark ignition whilst detecting for pilot flame presence. If this is proved the controller opens the main burner gas valve and the main burner is ignited by the pilot flame. If pilot flame ignition isn't proved within something like 5 seconds the controller may immediately go to lockout and display a warning, usually a red LED, or repeat the sequence, usually no more than two times before going to lockout.

Older boiler designs used a continuous pilot flame which used a thermocouple to detect flame and hold the pilot gas valve open. Since this type of boiler contained no electronics the thermocouple although only producing a few millivolts could generate a reasonable amount of current. The coil of the pilot valve was just a few turns of thick wire and the valve would be opened manually to close the magnetic circuit. So the current from the thermocouple was insufficient to pull the valve in but sufficient to hold the valve open so providing fail safe control. If the thermocouple went open circuit, which was the most common failure mode, the pilot gas valve would close. An interlock within the valve block would prevent the main burner valve being opened if the pilot wasn't operated.

CS

Clive Steer08/04/2023 10:58:07
227 forum posts
4 photos

I forgot to mention that it is now illegal for anyone to service a gas appliance that are not trained and certificated to do so. However like many thing " for the guidance of wise men (women) and the obedience of fools".

CS

Andy Ash08/04/2023 11:33:13
159 forum posts
36 photos

I have a Potterton Netaheat that I have been "nurturing" for three decades, and I can confidently say that the boiler control lasts for fifteen years before it needs fixing. I conducted the fix twice in the same way, and it worked both times. I do now have a spare controller which I swap in when it fails so I have time to fix the old one and keep the newer one at "low hours".

Like John I reverse engineered the controller, and made all those discoveries. I've included the schematic I extracted below. I never thought anyone else might benefit from it, so I never posted it.

The main problem is that C2, which is an electrolytic capacitor, dries out. The flame sensor circuit is very sensitive, and it depends on a smooth DC rail from the mains to work properly. When the capacitor dries out, the supply has too much ripple and the circuit can't see the tiny (around 1nA) flame current.

The world is going to the wall, unfortunately, as we all know. I've been looked at getting my trusty boiler working from a battery and an inverter. As we know security of power provision is looking a little feeble over the coming few years.

With this boiler control the flame sensor depends on an earth connection and it needs significant capacity. If you run the boiler from an inverter you will find that the flame sensor fails even though the pilot is alight. To resolve this issue when using a battery inverter you will need to do two things.

1) Ensure that the boiler earth is connected to the physical earth for your property.

2) Connect the inverter neutral output to the earth that goes to the boiler.

If you do these two things, then a Potterton Netaheat can operate from a battery inverter, even when there is no electrical supply to your home. In these circumstances, the law may not need to get in the way of your probable need for basic hygiene.

boilercontrol_page1_sml.jpg

boilercontrol_page2_sml.jpg

Edited By Andy Ash on 08/04/2023 11:33:33

John Doe 209/04/2023 10:04:10
avatar
441 forum posts
29 photos

Andy, that is a brilliant and very nicely drawn circuit diagram ! Much better than my spidery and confused efforts !

I could have done with yours about 6 weeks ago to fix my own system, but nobody will allow that information, even though it is just electronics, and not touching the gas side at all.

I don't quite understand where Q1, Q2, Q3, get their other supply rail from? The mains neutral feed is one side and presumably 39V is created across C2 by Z1, but the circuit seems one legged to me ? Where is the current return path from the collector's side ? I think that is where I got confused with the circuit and was forced to give up. 

I think your inductor, labelled (?), in series with the spark transformer primary is actually a spark gap. I think C8 charges up until it can jump the spark gap which sends a pulse through the transformer.

Ah well, at least I can fix the old board now to keep as a spare..

 

Edited By John Doe 2 on 09/04/2023 10:32:08

Robert Atkinson 209/04/2023 11:53:28
avatar
1891 forum posts
37 photos

The supply to the transistors is is via diode D1. They are running from a negative supply with respect to ground. The mains neutral (connected to PNP transistor emitters) is connected to earth at the mains supply.

Robert.

John Doe 209/04/2023 19:46:40
avatar
441 forum posts
29 photos

Ah, sorry, me being dim. I assumed "FL" meant flame probe. It means fused live. Makes perfect sense now. 

Edited By John Doe 2 on 09/04/2023 19:51:20

Nigel Graham 227/04/2023 22:34:13
3293 forum posts
112 photos

Even the gas-fitters find it difficult to obtain information.

A lot of the boiler manufacturers will refuse to allow anyone - qualified or not - to repair the electronics, only to replace the whole board at £(several hundred) a time. So they do not issue manuals and components.

if you wonder why plumbers, electricians, etc., seem to charge so much for so "little" try thinking how you would run a business, or how much you really cost to employ. Not your Gross Pay, your charge-rate. I think mine was something like twice GP.

If they are GasSafe, HETAS, FENSA, etc., registered it costs them a considerable sum of money annually, in fees and lost income, to maintain. A big company like British Gas can pick up the tab but a sole trader has either to shoulder the burden from his own pay or spread it around his invoices, within "Labour Charges". Money for old rope for the accreditation companies!

When I had the front of my home re-pointed recently I was surprised not by how much but by how little, relatively, it cost, with the scaffolding hire as well.

Fred Jolly 127/04/2023 22:53:43
5 forum posts
Posted by John Doe 2 on 08/04/2023 09:21:33:

I have just repaired the central heating system I recently "inherited", (and which had 6 separate faults),

During my investigations and fault finding; I learned that a flame is conductive and can be used to detect the presence or absence of a flame.

In the case of my old Potterton Netaheat 80E, nobody would give out any information, so I traced through the PCB and found that the flame sense circuit uses the ignitor probe itself to detect the flame and hold a relay over, which enables the gas valve. The ignitor is literally just a metal probe, not a thermocouple in this model.

I tried to find the fault on the board, but could not obtain a circuit diagram and ran out of time during the coldest part of the year so far, (isn't it always when the heating fails?), and bought a new PCB. Cost me £82.

I have no doubt that had I been clueless and had to call a plumber, they would have sold me a new boiler and a new pump etc.....plus labour costing probably the best part of £2,000.

 

Edited By John Doe 2 on 08/04/2023 09:23:18

I cut my eye teeth on Netaheats and the Abergas HTM boilers as a gas service technician in the Early 1980's. 90% of circuit board faults were the main relay failing. The Abergas was the first Domestic unit to use the flame rectification system in the mid to late 1970's . Common though when instant shut off was required for large industrial burners and mid range package systems they could singe ones eyebrows drastically if they took more than a millisecond to detect a flame outage. Now, flame rectification detection is probably the most common flame failure device used in room sealed chambers

Edited By Fred Jolly 1 on 27/04/2023 22:54:55

Fred Jolly 127/04/2023 23:01:57
5 forum posts
Posted by Clive Steer on 08/04/2023 10:58:07:

I forgot to mention that it is now illegal for anyone to service a gas appliance that are not trained and certificated to do so. However like many thing " for the guidance of wise men (women) and the obedience of fools".

CS

Incorrect. The gas safety regulations state that one must be competant to work on gas fittings (this includes appliances) competency would be decided by a court of law in extreme circumstances.

One only needs to be a certified and registered if doing such work in the course of trade or buisiness

John Doe 228/04/2023 11:02:18
avatar
441 forum posts
29 photos
Posted by Nigel Graham 2 on 27/04/2023 22:34:13:

Even the gas-fitters find it difficult to obtain information.

A lot of the boiler manufacturers will refuse to allow anyone - qualified or not - to repair the electronics, only to replace the whole board at £(several hundred) a time. So they do not issue manuals and components.

if you wonder why plumbers, electricians, etc., seem to charge so much for so "little" try thinking how you would run a business, or how much you really cost to employ. Not your Gross Pay, your charge-rate. I think mine was something like twice GP.

If they are GasSafe, HETAS, FENSA, etc., registered it costs them a considerable sum of money annually, in fees and lost income, to maintain.......

That's a fair comment.

My beef is telling even trained and qualified electronics engineers, that we may not legally work on a control board.

If there is a minor fault on a board, the "Gas" technician puts in a whole new board at enormous cost, when the fault might have only been a £4 relay or a 40p capacitor for example. Fine, the Gas technician does not have the time or the skills and knowledge to fault find a PCB and solder in new components, but those of us who do are prevented by the manufacturers not releasing the information and forcing us to pay huge sums.

My other beef is that in my experience of the last three houses we have owned; the central heating systems have been installed in a very slap-dash way, with very poor pipe runs, in the sense of restricted flow, and components thrown in as quickly as possible, rather than being logically laid out, and no thought given to maintenance of, or replacing components. Floor joists weakened by installers cutting them in the wrong places for pipes. Also the use of push-fit water pipe couplings, which only have a life of around 20 years owing to the O rings failing. Very quick to install but when they start to leak and water comes through your lounge or kitchen ceiling, meaning you have to tear out the plaster board, fix the leak, and then replace, re-plaster and redecorate............

I don't mind paying high prices for good quality, but I object to high prices for terrible quality or terrible work.

I just know that had I been a little old lady with no technical knowledge when our boiler failed; the gas technician would have pretty much forced me to have a new boiler supplied and fitted for about £2,000 because "the old one was beyond repair".......They typically add a 60% surcharge on new items they supply too.

Nope, it is all working beautifully now - much better than when we moved in, and the cost of components was about £85.

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Sign up to our Newsletter

Sign up to our newsletter and get a free digital issue.

You can unsubscribe at anytime. View our privacy policy at www.mortons.co.uk/privacy

Latest Forum Posts
Support Our Partners
cowells
Sarik
MERIDIENNE EXHIBITIONS LTD
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest

 

Donate

donate