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

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.

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

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.

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

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.

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.

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

I know this thread is over a year old but just wanted to convey my thanks to Andy Ash for the diagrams.

I also have a 30+ year old Netaheat that has had occasional controller problems.

Apart from the dry joints that were reflowed many years ago, one infrequent issue is with the pilot ignition.
The fan starts, pressure is detected and the pilot valve operates but no spark.
You can see this from the fact that the gas discharge device on the controller does not spark.

The cure for me is to brush all the collected dust etc from both sides of the board around R14 and C8.
I think either the gas discharge component is ageing or the dust build up is slightly conductive meaning that C8 never gets charged to the breakdown voltage. You don’t need much conductivity around a 22M resistor to make a difference.

This boiler has out lived one fan, two pumps and three main controllers (one of which was a recent Wiser which went pop (literally) one morning).

We’ve even had the gravity hot water replaced by a Megaflow. It was only after the Wiser was installed that I discovered the safety motorised valve in line with the heat exchanger on the cylinder was not electrically connected!

Although working on gas appliances may not be illegal if there were a “incident”, proving you had the required competence, in court, may be very costly. If you thought a gas fitters’ hourly rate was high wait till you have to engage a Barrister.

Re-soldering dry joints in a controller, which is a fairly common fault due to the thermal cycling they experience, is probably OK and low risk. However replacing any components associated with the “safety” circuit is more problematic as replacements must match the original exactly. Some components used by controller manufactures were specially made or selected for their designed function. For instance relays used on controllers may have their pull in and drop out currents different to those of normal commercially available types to ensure that they could be considered fail safe. Similarly their contact size and spacing may be larger to ensure the contacts can’t weld. Without reverse engineering the circuit and determining what components are involved in the safety circuit replacing any component may produce subtle function changes. It isn’t always about what the circuit should do but what it mustn’t do that is more important and how does one test for that. Those who have designed “fail safe ” products will know how a special mindset is needed plus exhaustive testing by someone not associated with the design phase.

CS

On 6 July 2024 at 08:52 eeeps Said:

…
I assume that the resultant gas escape is not considered unsafe because the fan is running and forcing the limited unburnt fuel out of the boiler.
By extension, it can therefore be assumed that the ignition circuit is not a safely critical area and thus can be worked on by DIY ‘specialists’.

 

 

That’s two ‘assumes’ both of which could be wrong!   Years ago I did a full course on the danger of assumptions,  where the point was rammed home by telling us to always render ‘assume’ as ‘Ass of You and Me’.   Anyone stating an assumption was sent away to clarify it, either by confirming the assumption was a fact, or, if that was impossible, by recognising it as risk.   Risks are managed, whereas the failure of an assumption comes as a surprise.  Also told to be suspicious of ‘facts’, because humans often confuse them with opinions, beliefs, misunderstandings, folklore, fake news, and lazy jumping to conclusions.  Many things claimed to be facts are assumptions.

Here’s another likely assumption.  When eeeps boiler was designed, it was for a given service life, after which it was expected to be replaced.   The service life is governed by wear and tear on the mechanicals,  corrosion, ageing of the electronics, condition of the burner, cost-cutting compromises and other characteristics of the system.    Likely a commercial boiler is safe only on the likelihood that  it will be replaced as soon as the cost of a commercial repair is uneconomic.   All bets are off in the very unusual event that an increasing risky boiler will be kept going well beyond it’s service life with patch repairs.

Another consideration is efficiency.  Old boilers weren’t particularly efficient when new, and efficiency tends to decline with age.   Given the ever rising cost of gas, it’s likely that eeep would have saved money by fitting a modern boiler 15 years ago…

Keeping stuff going by repair and restoration is great,  but keep an eye on the big picture.    As a whole, is the device being mended still safe?  Is the work worthwhile.   With something like a Myford lathe the answer is almost certainly yes. A 30 year old gas boiler is in a different league: Carbon Monoxide is extremely poisonous;  there’s a serious gas leak and explosion risk;   the control system is moderately complex, and includes safety critical factors.  This is all in addition to the usual plumbing and electrical hazards.

Dave

A lot of points covered in this revival
1/ it is interesting that unlike certain electrical work (part P), gas work does not require any notification. This means there is no record of the work.
2/ One ssue not mentioned if you DIY and something goes wrong is that your insurance may be invalidated.
3/ The spark gap breakdown voltage may increase with age due to a number of mechanisims. These include electrode erosion and decay of any radioactive material that is sometimes added to stabilise the the breakdown.
4/ I agree with Clive on parts subsitution. You don’t know the full reasons for component selection especially safety components. As an example I once designed a controller for a set of 3 halogen lamps (used for a non-lighting application). The mains power was switchd by a relay. As many will know filament lamps have a high initial (inrush) current. Having 3 halogen lamps in parallel does not help. I selected a particular version of a standard size relay. This had siver cadmium contacts specifically designed for lamp loads (today I’d use a solid state relay) and cost about three times a standard model. The only difference was one letter in the part number and a amber rather than clear cover. It was a standard part of many of the products the company made. The PCBs where assembled in house. As there was a relay and software (PIC) control I included a thermal switch on the lamp housing to prevent overheating in the case of failure. After a couple of years we got a few failures on new machines. The first were fixed by the field service technicians but when they failed again I was asked to look. As soon as I saw the PCB the problem was revealed, the relay had a clear cover. Production had decided to do a component cost review and decided to fit the “identical” cheaper relay.  The subsequent rework and replacement programme, we had machines all over the world, cost a lot more than the saving on relays.

The use of two components when one would give the same nominal “headline” value can be for a number of reasons including increasing a secondary parameter such s power or voltage or for reliability / safety / regulatory reasons. For the latter three you can’t tell by looking at the component datasheet. Only the designer knows for sure. Even fitting a “better” component can affect them.
Again a practical example. Forty years ago I designed a conditioning circuit for an aircraft fuel gauging system. As it had a connection inside the fuel tank it had to meet fuel tank non-ignition / intrinsic safety requirements. Normally compliance would be shown by testing but test house availblity and first flight dates did not align. So I suggested showing compliance by design and analysis to a (non-aviation) intrinsic safety standard. Much to the surprise of the “old hands” the regulator acceped this approach. The circuit ran off 28VDC and the input circuit was a small fuse followed by a 33 Ohm power resistor and a 36V 10W zener diode (spike supression). At a nominal supply voltage of 28V even a short circuit of the diode would only result in 0.85 amps flowing. However the fuse was rated at 3.15A. The circuit also had two resistors in series with the connection to the fuel tank each would limit the current to less than the minimun ignition current. Two were required to meet the requirement that no two failures could cause excess current to flow (there is a lot more detail to this). All went well and the aircraft was certified. A few years later, after I’d left the company (it had changed ownership) they contacted me to ask for a change to the fuel gauge calibration as they had changed the wing structure. I said OK but needed a spare conditioner and the test box to validate the change. When I opened the conditioner I noticed that it had a 0.5A glass fuse fitted. On questioning they said their sub-contract assembly house for the conditioner had noticed that the 3.15A fuse would not open even on a short circuit and the unit only drew 0.25A they suggested a change to 0.5A. They obviously had not been given the safety analysis. The only reason for the fuse was to meet a requirement in the intrinsic safety standard for a “sand filled ceramic bodied” fuse approved to a specific standard. The lowest rating allowed by that standard was 3.15A so I had to use that. By changing it they had technically broken the safety of the system. In reality the system was safe, the specific fuse was required becuase the non-aircraft standard considered that the DC supply might be accidentally connected to the high energy 240V mains supply. Obviously this does not exist on the aircraft. The aircraft in question was the NAC-6 FieldMaster / FireMaster.

Robert.