Boiler repair

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Wiki editors note:

Author and Copyright - much of this document was originally prepared by Ed Sirett (owner of London based property maintenance company Make Do & Mend, and also professional gas fitter. The material was originally hosted on his web site, however since that appears to no longer be active, and the original licence terms permitted copying with certain limitations, it seems sensible archive sections of it here.

A great deal of the material which is not specifically associated with boilers has been drawn extensively, with permission, from Samuel M. Goldwasser's FAQs. As they are found on which is associated with the USENET news group (also known as s.e.r). I openly acknowledge that his FAQs are a great inspiration to me and a significant encouragement to create this document.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

  1. .This notice is included in its entirety at the beginning.
  2. .There is no charge except to cover the costs of copying.


I will not be responsible for damage to heating systems, house, boiler, your ego, blown parts, local or regional power cuts, death (by carbon monoxide or worse), explosions (unintended or otherwise) or any personal injury that may result from the use of this material. In fact, in the UK no one is permitted to work on a gas appliance unless they are competent to do so. See the Gas Ftting FAQ. If a manufacturer states anything in their manuals contrary to this FAQ then the manufacturers instructions are to be followed and not the advice given here whether explicit or implicit.


This document attempts to answer some questions that are frequently raised on uk.d-i-y. It may answer some questions better by giving a fuller context to the answer than can be done easily on line. By giving descriptions of certain boilers and the faults that can occur I hope to show some of the principles involved. If you are lucky the symptoms, cause and cure for the fault your boiler has might be covered explicitly.

I am aiming this at the do-it-yourself enthusiast, or perhaps someone who considers they have general technical ability to "have a go" but lacks the more detailed knowledge to make the repair in a timely and efficient way.

Mostly you will not need specialist tools just basic plumbing, electrical and general purpose tools will be sufficient in most cases. There are a few tools that you might need in some cases and I shall cover those in due course.

In order to get a broken boiler to work you need to find out what is wrong and be able to repair or replace the faulty part. To find out what is wrong you need to know how the boiler should work. This is one of the harder parts of the job because the reason you are now examining the broken boiler is due to it having ceased to work. You can only find out so much about how it should work by examining the broken model in front of you. Fortunately many boilers work much the same way as each other so a complete description of a similar boiler should help. In order to cover the range of technologies used I have chosen to describe three boilers models each with a significantly different level of technology.

You may well be reading this document out of desperation on a freezing winter's night. You might be resigned to your boiler having come to the end of its life but think you might yet be able to do “something”. You might simply want to be able to communicate better with any professional help you are going to engage. You might feel that fixing the boiler is a challenge and if asked why you would want to fix it you would respond “because it's broken”. Whatever your reason knowledge is potentially better than ignorance. Above all read and understand the Safety Guidelines and put them into practice.

Safety Guidelines:

I apologise that for some people this will be an example of the worse kind of nannying, all of us tend to cut corners when we get experienced. The circumstances of been cold and unwashed can however drive even the most cautious of us to take risks we might seriously regret later. Even grandma needs to revise her egg sucking skills now and again.

When it comes to how dangerous things are, as an appliance, the boiler not likely to be the most dangerous item in your house. Indeed that dubious honour goes to the microwave which has high voltage power electrics, it has the volts to not only drive the electricity through you but also to leap across an air gap to do so and the power to seriously burn you or electrocute you. However a boiler has the distinction of having the most ways of harming you and others, it is also the appliance that could cause the worst worst-case 'incident'. It can at least:

  • Electrocute you.
  • Explode due to the build up of gas
  • Explode due to the build up of steam
  • Poison you with Carbon Monoxide
  • Burn you
  • Scald you
  • Injure you on unfinished sharp edges
  • Harbour harmful dust

Safety Guidelines for High Voltage and/or Mains Powered Equipment.

Adapted and Translated to English from the guidelines written by 'Silicon' Sam Goldwasser in the Sci.Electronics.Repair FAQs

Normally, boilers are safely enclosed to prevent accidental contact with mains electricity. However, when troubleshooting, testing, making adjustments, and during repair procedures, the cabinet will likely be open an occasionally the mains supply will need to be switched on. Depending on overall conditions and your general state of health, there is a wide variation of voltage, current, and total energy levels that can kill. Normally if you have clean dry skin and are not otherwise in contact with a low resistance path to earth touching a conductor at 230 Vac will produce nothing worse than a painful tingle. That alone is bad enough, and since you may be someone who is exceptionally susceptible to an electric shock it could be very serious. However, when working on a boiler there is every chance that you may have damp hands. The boiler is almost entirely constructed of metal parts which are deliberately well earthed, a real hazard is therefore present in that you could receive a lethal shock.

Ignition circuits generate high voltages (5-10 kV) whilst the energy available on each pulse is not in the lethal range some modern boilers issue a stream of pulses at 25Hz or more. There is a possibility that the total energy could be quite harmful it will certainly be very painful. Even if you survive an electric shock the violence with which you may involuntarily move your limbs could easily do you some serious injury and or significant damage.

In addition, read the relevant sections of the service manual for your particular unit for additional electrical safety considerations as well as non-electrical hazards like dust, heat and fire.

General Safety Advice.

The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of boilers can be both rewarding and economical. Just be sure that it is also safe both for yourself and everyone else.

  • Don't work alone - in the event of an emergency another person's presence may be essential.
  • Always keep one hand in your pocket when anywhere around a mains powered line wire or high voltage system.
  • Wear rubber bottom shoes or trainers. An insulated floor is better than metal or bare concrete but this may be outside of your control. A rubber mat should be an acceptable substitute but a carpet, no matter how thick, may not be a particularly good insulator.
  • Wear eye protection - large plastic lensed eyeglasses or safety goggles.
  • Avoid wearing any jewellery or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.
  • Clear your work area of unnecessary items that can interfere with your methodical and focussed approach. A quick look can have the nasty habit of turning into an epic saga with boiler diagnosis.
  • Have a fire extinguisher suitable for electrical fires in a readily accessible location should something burst into flames.
  • Make sure you know where and how to turn the gas supply off before you need to do so, both on the appliance and for whole the dwelling.
  • Use a dust mask when cleaning inside the combustion chamber, flue, burner or draught diverter. On a very old appliance there may be asbestos fibre seals particularly between the top of the heat exchanger and the cowl that collects the flue gases.
  • Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or connect temporary wires to reach cramped locations or difficult to access locations.
  • Occasionally you must probe live wires, put electrical tape over all but the last 3mm of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip, connect or otherwise secure the second lead of the probe or tester to the neutral supply so that you need to only probe with one hand.
  • Perform as many tests as possible with power off and the equipment unplugged. For example, an overheat cut-out could be tested with a basic continuity tester or multimeter.
  • Boilers really should not be supplied from circuits which are protected by an RCD (Residual Current Device) due the possibility of nuisance trips being generated by the boiler. Also a boiler whose supply is subject to nuisance tripping could lose its ability to protect an unoccupied building from frost. However, with no RCD there is no second line of defence against a serious electric shock. Take on board the stuff about safety as if you had never heard it before. If your boiler is supplied via a 13A plug and socket you could add a plug in RCD just for live troubleshooting.
  • Modern boilers contain PCBs which contain static sensitive components. An anti-static wrist strap can be used, however, holding the board by the edge and first placing your other hand on any metal part of the boiler should be sufficient.
  • Don't attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity.
  • Do not smoke, you already know why.
  • Never assume anything about any wiring colours either inside the boiler or even in the fixed wiring. When it comes to heating controls all the bets are off, even the green/yellow wires may have been 'borrowed' by some cowboy who wanted an extra signal wire and was too mean or lazy to buy and use the right stuff. Note that black wires are officially, with the new harmonized wiring colours, line conductors. Boiler internal wiring colours vary from psychedelic schemes to the “Ford Model T” approach (any colour as long as it's black) .
  • Finally, never assume anything without checking it out for yourself! Don't take shortcuts!

Testing after you have completed electrical repairs.

Most boiler service manuals will ask you to perform commissioning checks after making a repair. Most boiler commissioning instructions will ask you to perform the basic checks that the boiler is correctly connected to its electrical supply.

  • Earth continuity. The easiest way to show that this is correct is to test that the metal casing of the boiler is a good connection to the supply earth, the metal screws of a nearby electrical socket or switch should do. Ideally you need a proper continuity meter that can show you the resistance whilst actually sending a significant current of 200mA around the circuit. The resistance must be less than 1 Ohm.
  • Resistance to Earth. With the supply isolated and the boiler switched on you should test the resistance of the mains input connection to the cabinet At least 2M Ohms is needed do not test with more than 500Vdc. A low resistance to earth could be due to a faulty repair. The circulation pump would be the most likely component to show a problem, also the gas valve especially if you have sprayed it with a lot of leak detector trying to trace a gas leak.
  • Polarity. Confirm the polarity is with regard to line and neutral is correct. Some boilers will not detect flames correctly if this is incorrect.
  • Fusing. Confirm that the fuse in the plug or supply connection unit is as stated by the manufacturers, invariably 3A.

Testing after you have completed work on gas carrying parts.

When you have reassembled any joint that carries gas, or re-closed any test point then you must check that the joint is not leaking. If the joint is upstream of the gas valve then you can check for leakage just as you would for the whole installation. See Gas Ftting FAQ. For downstream joints and for test points you should apply some leak detection fluid. There is a danger that you could spray this onto mains (or higher) voltage connections. For awkward joints apply the fluid to a small paint brush and then brush the fluid onto the joint. Note that a small leak will cause bubbles a very big leak might not.

Testing after you have completed work on water carrying parts.

Don't think you are out of the woods until the boiler has been run a full heat for half an hour. Soldered joints that have not been properly heated can hold when cold but give out when holding warm water. Small leaks (less than a drop every few seconds) can be sealed with a proprietary leak sealer, which may, unfortunately, also seal up an auto air vent.

Basic Troubleshooting

General notes on trouble shooting.

Hearsay and folklore sometimes indicate that you should replace a given part when certain symptoms occur, and in the case of frequent failures of such parts, this information might even be true. But that's no way to become a competent technician.

Troubleshooting is a special field of knowledge and has its own outlook on things. The device did work, after all. Just because you don't yet know why it's stopped working does not mean it has been subject to a paranormal event. Let's be frank it's broken, you're don't know why - yet. You're out of ideas; take a break for now; there's no need to assume the boiler has been hexed.

Learn to use a digital multimeter, and an analogue one as well; the latter is easily damaged if you don't know what you're doing. However an analogue meter can show a momentary effect perhaps due to a brief or infrequent loose connection which a DMM just won't register.

Some of My Rules of Troubleshooting

Much thanks to Silicon Sam for starting this list off.

Safety first - know the hazards associated with the equipment you are troubleshooting. Take all safety precautions. Expect the unexpected. Take your time.

Always think 'what if'. This applies both to the analytic procedures as well as to precautions with respect to probing the equipment. When probing, insulate all but the last 3mm of the probe tip to prevent costly shorts.

Learn from your mistakes. We all make mistakes - some of them can be quite costly. A simple problem can turn into an expensive one due to a slip of the probe or being over eager to try something before thinking it through. While stating that your experience in these endeavours is measured by the number of scars you have may be stretching the point, expect to make mistakes - we all can point to that disaster due to inexperience or carelessness. Just make it a point not to make the same mistake again.

Don't start with the combustion analyser, start with some analytical thinking. Many problems associated with boilers do not require the circuit diagram. The majority of problems are mechanical and can be dealt with using nothing more than a good set of hand tools; and your powers of observation (and a little experience). Your built in senses and that stuff between your ears represents the most important test equipment you have.

If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous and mostly non-productive (or possibly destructive).

Many problems have simple solutions. Don't immediately assume that your problem is some combination of several esoteric complex convoluted failures. Dirt and debris are frequently a major cause especially if the appliance has not been serviced. Electrical problems tend to be simple things like open connections or maybe a short to earth in the pump. Corrosion, leakage and lack of lubrication are frequent causes in older units.

Try to remember that the problems with the most catastrophic impact (like the whole things appears totally dead) usually have the simplest solutions. The kind of problems we would like to avoid at all costs are the ones that are intermittent or difficult to reproduce: overheat or flame failure lockouts that occur one or twice a month.

Don't blindly trust your instruments. If your get readings that don't make sense, you may be using your equipment in a way which is confusing it. In particular a DMM or a neon screwdriver can give very weird results especially if the are connected to a wire that is currently connected to nothing.

Realize that coincidences do happen but are relatively rare. Usually, there is a common cause. For example a combi boiler which has excessive radiator temperature and also scalding hot water could be due to the only temperature sensor having drifted out of spec. In other words, first look for a common root cause rather than trying to locate several simultaneous faults. Exceptions include times when something drastic has happened like a major leak or a small fire.

Confirm the problem before delving into deep diagnostics. It is amazing how many complaints turn out to be impossible to reproduce or are simple cockpit error. It also makes sense to identify exactly what is and is not working so that you will know whether some fault that just appeared was actually a pre-existing problem or was caused by your poking. List out, mentally at least, what all the problems are before you begin.

Whenever working, make copious notes and diagrams. You will be eternally grateful when the time comes to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not always. Apparently identical screws may be of differing lengths or have slightly different thread types. Little parts may fit in more than one place or orientation. Self tapping screws abound in boilers if possible try to be so organized that the same screw goes back in the same hole.

Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly.

Prepare your work area. Arrange for more lighting if there is not enough already available. Clear away the clutter and put down a sheet you may lose something in the sheet but it won't bounce and roll under the skirting board or through a crack in the floor

Some Quick Tips or Rules of Thumb

Problems that are erratic or intermittent - that come and go suddenly - are often due to bad electrical connections. Connectors that need to be cleaned and reseated.

Problems that are not present when cold but occur as soon as the boiler has warmed up a bit. These could be over enthusiastic temperature sensors. These could be marginal problems with air flow, when warm the air is less dense leading to a reduced air-mass flow rate perhaps by enough to stop operating a pressure operated switch. Heat can also affect PCBs and electronics.

Problems that result in a totally dead unit are generally power supply related. These are usually easy to fix. And often caused by either a blown fuse or the over-heat cut out has tripped (but to reset it you first need to find it). You also need to find the underlying cause that caused the problem.

Don't blame the boiler for any problem that is the fault of an external control. Make sure you understand the purpose and function of all the external controls and that they are asking the boiler to perform as they should.

The low tech boiler (see the section What class of boiler do I have?) has a permanent pilot light most pilot light problems are thermocouples or dirt in the pilot injector.

Combustion fan problems are usually due to gummed up grease. WD40 is not the answer but may let the boiler run (for up to 48 hours) until the replacement is obtained.

Boilers that light reliably but won't stay lit and then lockout are usually a problem with the flame sense electrode or lead.

Combination boilers which take, week by week, a bigger and bigger flow of hot water to get them to start are very likely due to a splitting water flow diaphragm.

Combination boilers which light correctly in response to hot water flow and the heating controls but don't do what you want have probably got a diverter valve problem.

Most problems can be traced down to at most three components, replacing all modules in desperation is both irrational and expensive.

Tools, Test Equipment, and Other Stuff

Hand Tools

Cheap tools are expensive! Why? Because they end up wrecking the job and waste many many times their cost “saving”.

Some basic hand tools.

  • Screwdrivers of all types and sizes including straight, Philips, Pozidrive and Torx. There is a difference between cross head screws learn to see the difference and used the right tool. The Pozidrive have 4 short diagonal scratches placed between the arms of the slots.
  • Hex key wrenches or hex drivers.
  • Pliers - long nose, round nose, curved. Both smooth and serrated types are useful.
  • Adjustable wrenches of varying sizes.
  • Pump pliers.
  • Wire cutters – end and side.
  • Wire strippers - fixed and adjustable.
  • A good torch or mains inspection lamp, not something that boosts that it has power 10,000,000,000 candles and can outshine the Bishop's Rock lighthouse. Just something that can reliably lighten a dark corner.
  • A dental mirror. When you are next at the dentist you may be surprised that s/he is only too happy to give you a handful of used ones, apparently they are only allowed a single use in dentistry and they are really well made and hold the mirror at just the perfect angle.
  • Magnetic and/or long-reach pick-up tool. Sooner or later you'll wish you had one.

Advice on reassembly

When reassembling the equipment make sure to route cables and other wiring such that they will not get pinched or snagged and possibly broken or have their insulation nicked or pierced and that they will not get caught in moving parts. Many parts will be very hot and so the wires may have been routed to avoid coming into contact with such heat. Replace any cable ties that were cut or removed during disassembly and add additional ones of your own if needed. Some electrical tape may sometimes come in handy to provide insulation insurance as well.

If you have been careful and organised about the screws and other small parts you should have none over or missing. Except for the ones that you dropped and which then rolled between the floor boards.

And on the still lighter side, from an IBM maintenance manual, circa 1925 (displayed in the Chicago Museum of Science & Industry):

"All parts should go together without forcing. You must remember that all the parts you are reassembling were disassembled by you. Therefore, if you can't get them together again, there must be a reason. By all means, do not use a hammer."

Test Equipment

  • A 0-30 mbar manometer or U – gauge. An electronic one is OTT and suffers from the same problems as a Digital Multimeter Meter (DMM), the simple water filled U-gauge can show a transient effects that would be nigh on impossible to see on a digital manometer. You can make you own with plastic tubing, rubber bands and bits of wood and calibrate it so that a difference of 98mm in the water level is 10 mbar. Alternatively you can pay £20 at a plumbers' merchant get one that works really well, has a moveable scale to let you zero the water level, and get a length of neoprene hose that fits the boilers test points.
  • A simple mains electrical probe. Given the conflicting results a DMM or neon screwdriver (spit) can give, the ideal tester is bulb holder holder with a 15W 230Vac pygmy bulb in it and a couple of leads on it.
  • A continuity tester. A simple battery & bulb unit is probably more helpful than a DMM. However a DMM with an audible continuity indicator can be excellent.
  • Some leak detection fluid, essentially soap solution which unlike washing-up detergent is formulated to be non-corrosive. Comes in a spray can or a bottle with a brush. It's needed to check for gas tightness of pressure test points after use.
  • Combustion Analyser. This won't be needed unless you need to set up a modern premix burner. If you need to do that you would almost certainly be better off getting a professional in. You won't buy the simplest model for under £250 and you may be able to get a heating engineer in for a minimum call out of £50.

Supplies and Parts

The good news is that if you have ever struggled trying to get the right spare part for your car or searched in vain for some weird alien looking rubber bit for your washing machine then things are nowhere near as bad for boilers.

Firstly, the manufacturers are obligated to hold spare parts for a lengthy period of time. I'm not sure just how long but it may be 15 years after the end of the production run. In practice you can still get spares for boilers that ceased production 20 years ago or more.

Secondly, the parts used are often generic and a particular part, say a gas valve made by Honeywell will have been used in a large range of makes and models so that its availability is further improved.

Thirdly, every boiler model has an identification number called the GCN (or Gas Council Number or G.C. No.) which uniquely identifies its maker, model and revision. With this number you will be able to order the right part. This must come as a welcome relief to anyone accustomed to queueing at the spares counter of a car dealership. To those not in the know this is an unpleasant subject, the story goes like this:

You arrive at the dealership, you find that all the parking is reserved for 'service customers only', so you have to park 250m away on the main road. The counter is unattended but there is a bell push nearby alongside which a tatty note has been sellotaped to the counter “Press bell once ONLY”. You press it the just the once and wait a while. Then you spend at least another 5 minutes deciding whether you should press the bell again or not. Eventually someone arrives either they are too young to hold a driving license or they look like they should have retired last year. Although the latter is somewhat hard of hearing and can't work the computer you are generally much better off with them than with the former. You then present your request for “The external door handle, drivers' door, late model Mk III Vauxhall Astra”. The young guy will spend two minutes on the computer, he then declares he can't find the part (on the computer) so he then asks the old guy who was waiting in the back to come out. The old guy will look through several large paper manuals then start asking was the car manufactured before or after 9:15 on 13th December 1997 in Germany or in Britain. You declare you have no idea, you say it's a late model, R reg and then produce the vehicle serial number, which after five minutes of hunting through some other manuals is declared to be a 'late model'. The old guy explains to the young one that what he needs to look for is a “OFSD DR OPRTNG LVR – BLK” . The young guy then says “Yes we have them in stock on the computer”. They both disappear the old guy returns 10 minutes later to explain that we only have them in stock for the passenger side. Do you want us to get one in? Yes, please you say. “That'll be £54.67+VAT, as we have to order it in, you will need to pay for it now.

At one of the local boiler spares shops in North London they have someone on the staff that can recognise the broken part you are carrying in as you get out of your van and then gets the new one from the back room before you enter the shop. It's mostly not as good as that, but the situation is better than finding parts for cars or washing machines.

Fourthly, you can often order the right part online. Try searching a few sites here are a few names to get you started: abcotuk, keeptheheaton, hrpc.

The Question: To Repair or Not to Repair

The question of whether to repair or replace the boiler is heavily stacked in favour of repairing. This is uniquely so relative to all other household consumer goods as the costs of replacement are very substantial indeed. Nevertheless there will come a time when a boiler whilst technically repairable will leave you with an old and possibly unreliable unit. Furthermore there has been such a transformation of the efficiency in the last 30 years that a new boiler can justify its replacement on gas savings alone. Diagnosis only costs you your time and effort, you may discover that the boiler can be fixed with little or no parts. You won't know until you try. Here are some reasons why you may wish to diagnose and repair a boiler.

  • For the challenge and rewards associated with success.
  • To save money.
  • To heat the house and water sooner rather than later.

However, there are situations where it probably isn't worth attempting to repair on your own (or possibly at all):

  • Any situation where safety, yours or others, would be compromised by the repair. For example, a boiler which has caught fire. Unless you can repair all the damage and find the underlying reason for the fire and fix that also. Even then there may be heat damaged components which appear to work but are going to cause another, and possibly bigger, incident.
  • If you really don't know what you are doing, leave it to a professional. They may or may not have more education than you, they almost certainly have more experience and they definitely have better insurance.
  • Finally, don't attempt to repair a piece of equipment for which you are not equipped in the tools or test equipment department and for which you do not have the correct replacement parts. Never ever ever defeat bypass or modify any of the safety controls in order to get the boiler to work.

Sources of Information

The manual

Anyone who has struggled to repair a piece of electronic equipment which you feel sure would be repairable if only you had the right information about how it works, diagrams and drawings will find that life is much easier with boilers. How many times have you looked at some product only to be told “No user serviceable parts inside”. You know full well that there are plenty of 'serviceable' parts in side if the user is you, if only the makers would give you some help. You are particular cheesed off with instructions which say here is the on/off switch.

With boilers the situation is much more encouraging. It may well have a user manual telling you to call your installer if there is a problem but it will also have a 'proper' installation and service manual. This manual may well have functional block diagrams, circuit diagrams, fault finding decision charts, lists of test voltages and pressures, instructions on how to replace major components and a check list of things to do to 'service' the appliance. It may well also have a list of the commonly replaced parts along with the part numbers and exploded diagrams.

But it's even better:

  • The manuals especially for newer models are often downloadable from the manufacturer's (or import agents) web site.
  • It is a legal requirement that the manuals are left with the customer after installation, a law can't make things happen but it does make at least a bit more likely.
  • The makers will send you out a manual free of charge if you ask them (they may be legally obligated to do this). I've never known any want to charge for this service.

Inside Cover of the Equipment

Even if you don't have the manual, on the older and simpler models, there may be a circuit diagram inside the cover. There will always be a rating plate this usually will give the GC number and also specify the maximum gas consumption it will also give the make and model. It may also give the burner pressure or even a range of them. Searching for Information from USENET Newsgroups

Google Groups (formerly includes a USENET newsgroup searching facility. It has been archiving newsgroup articles since March, 1995. By going to their web site, you can invoke a search of over 45,000 newsgroups (hundreds of GB of data!) for any set of words, names, or email addresses. Within *seconds*, they will provide a list of postings that satisfy your search criteria. Try using Google Groups at least once - you will be instantly hooked. :( Some of the relevant site URLs are:

Google Groups Homepage

Specifically for the uk.d-i-y newsgroup:

Articles on uk.d-i-y

This results in listing of threads by date. However, going through the Search page provides many many options to locate specific articles relevant to your problem or just your curiosity.

While postings typically drop off of your local server in a few weeks or less, Googlegroups maintains them indefinitely so that locating an entire thread becomes a trivial exercise in identifying a search string that will narrow down the postings to those relevant to your needs.

Posting to the uk.d-i-y Newsgroup

It was due to the large number of boiler queries that I wanted to write this FAQ. This was, in part, because really thorough answers would take too long to give to every question and in part because very similar questions come up day after day. On the uk.d-i-y news group you will usually get quite helpful replies, occasionally you will get ones that are, frankly, wrong. There is enough cross-checking that any gross errors in will be uncovered and corrected.

There is also generally no profit motive, well I actually get a small proportion of my work from contacts made through uk.d-i-y. However that is not my motivation.

Even if your ISP doesn't provide USENET newsgroups or allow posting for some reason, you can always access them (read, search, and post) via Google Groups. See Newsgroup access tips

No matter how you do it, however, here are some tips that will get you what you want without unnecessary flame wars:

Please read the on-line FAQs first. Your problem may be covered. Even if an exact solution is not provided there, the additional information may allow you to ask your questions more concisely and intelligently and therefore arrive at a solution more quickly.

The FAQs can be found at: Gas Ftting FAQ

  • Put the maker, model name and model number in the post. You know, some people must think that all boilers are made by one maker, who has one model and they all work the same way! However it's more likely they simply don't think.
  • If you don't put the make, model name and number in your post the first reply you will receive will be to provide it. Avoid this waste of bandwidth. For general questions, such info may be unnecessary, but it won't hurt.
  • Provide as much relevant information as possible. Ambiguity can lead to totally bogus advice. Such as people giving advice applicable to a combi boiler when the unit is not such. However don't exceed one screen-full of text as this becomes really off putting to read.
  • Don't just ask for repair tips - describe in chronological order what you have done so far in terms of diagnosis and tests performed but don't fill screen upon screen with details. People don't want to read a lot of filler. Include only the essentials.
  • Turn off any fancy formatting like HTML or WORD! Use plain ASCII text since everyone can read that, formatting adds NOTHING to the content, takes up space, and is very confusing for those people whose news readers cannot interpret it.
  • Take a bit of time to make sure what you have typed is legible. DO NOT use ALL CAPS - that is like shouting in cyberspace and a good way to be ignored or flamed.
  • A bit of courtesy won't hurt as well. Many people who reply won't care about the use of please, thank you, and any help much appreciated, but none of these will hurt and don't take much effort.
  • If a little diagram will help, provide it in ASCII if possible. ASCII takes up almost no space and everyone (with a fixed width font) can read it. Alternatively photographs and other digital files can be posted to public images hosting sites, and links placed in your post to the newsgroup.
  • Binary files, such as photos, may not be posted on the newsgroup directly.
  • You need to be patient. Not everyone sits at their computers all day. Some news servers may be days behind in their postings. If you truly get no replies of any kind (to the newsgroup or email) in a few days, repost your question with a note that it is a repeat. The net isn't perfect and due to finite disk space, many servers may miss postings or purge them after a day or less. Sometimes, your posting may not have made it out of the bowels of your computer system.
  • Don't ask for help on 25 problems in the same posting - that is taking advantage of the generosity and time of others. Dribble them out and reciprocate by replying to other people's problems as well if you can. If you act immature, you will end up in everyone's kill file.
  • Don't ask for help on problems that you could just as easily solve on your own by checking the manuals, a FAQ or a Web site that you should know about.
  • Don't ask for an email response. First of all, it is very impolite. uk.d-i-y was not created for your sole benefit. Many regular posters answer posts because we like to help people but at the same time we learn a lot from each other and also do not want to feel like we are being taken advantage of or taken for granted. We are not your private consulting service. In addition, others will know when an adequate response to your query has been provided and will not need to waste their time repeating the same information. And, everyone will learn something in the process.
  • More importantly for you, receiving replies via email will circumvent one of the most important functions of the newsgroup - cross-checking to locate errors in responses either because the responder didn't know what they were talking about or made an error in interpretation. Perhaps, they were just being a bozo and sent a totally bogus or even dangerous response. And, some people may have hidden agendas that aren't in your best interests. If that was the only reply, you would never know. While there is a lot of high quality information available via the Internet, there is also a lot of noise. Yes, you will need to read the newsgroup for a few days. That will be a small sacrifice and well worth the effort.
  • When making replies for clarification add your replies at the bottom of the previously posted text, either point by point or all at the end. Do not put your replies above the older material in the post this is called top posting and is particularly disliked on uk.d-i-y.
  • If your news feed is indeed poor - as many are - and you are honestly afraid of missing the responses, then phrase your request for an email reply in such a way that it doesn't sound like you are totally immature and lazy.

Another alternative is to search for replies at:

Google Groups

This service will enable you to search for only the postings you are interested in and seems to be pretty reliable. They subscribe to a half dozen news feeds just to avoid missing *your* postings!

Many people will send you a CC of their posting anyhow so avoid getting flamed for poor netiquette. However, take note below.

Don't accept the first response as the definitive word. Gather a few replies and follow-ups and then you will be able to make an evaluation of which to believe and act upon. Post a question for clarification, if needed.

In any case, once your problem has been resolved (or you have given up), it is polite to post a concise summary of the problem, suggestions, the solution or frustration.

Troubleshooting of Intermittent Problems

These are the ones everyone dreads - equipment that is temperamental, working or not working apparently depending only on its own mood. Behaviour may appear to be totally random but in most cases, there will be some correlation with physical, environmental, or external interference. Careful observation and perhaps a bit of detective work will ultimately allow a repair to be successful. Troubleshooting such problems is a primary cause of hair loss in engineers and technicians. However, with a methodical approach and patience, it should be possible to identify the cause and repair/replace misbehaving components.

Attacking intermittents

First, determine whether the problem is internal or external.

If internal, it may be physical, heat related, or mode related. Gentle whacking (yes, whacking is an acceptable diagnostic technique but don't go for the 12 pound hammer!), pressing, flexing, cable wiggling, etc., can and should be used in an attempt to confirm at least that there is a physical cause inside the unit. Doing these tests just as the problem comes or goes is the best time as whatever is marginal, will be most marginal then.

If the problem appears or disappears, or does both, over a period of time after the equipment is turned on, then temperature is almost certainly a factor as the circuit board and components expand, the air through the boiler becomes less dense which can alter things or the effect of the temperature is having an exaggerated effect somewhere.

The whacking, etc., can be done without taking the cover off the equipment and may or may not reveal anything. In either case, you will have to go inside. But if there is an effect, then you will know that the problem is inside and further tests will need to be done to identify the specific cause.

Once the cover is off, there still may be quite a challenge to find the specific solder connection or contact that needs attention. Knowing something about how the components relate to the symptoms will help narrow it down.

Inspection and Power Off Tests for Intermittent problems.

Assuming these don't help (or you consider letting someone else solve your problem to be cheating), a detailed visual inspection is the next step. This may be all it takes. With the unit unplugged or switched off at the supply, remove the cover.

First of all is to clean the whole boiler in accordance with the service instructions. Many problems will simply go away, at least for a while.

Start with all the wiring connectors. These are most likely to cause marginal solder connections to break apart due to thermal and physical stress. Hairline cracks at solder joints is a primary cause of electrical intermittents.

Make the inspection under a bright light. If your eye sight isn't perfect, use a good magnifier - these may be hairline cracks and their visibility may be obscured by reflections from the solder joint. Use a pointed stick to gently prod any suspicious looking pins to see if they move. Look for discoloured patches on the circuit board. Such discolouration isn't in itself a problem unless it is severe but indicates that hot components live there or nearby and bad solder joints are very likely.

Check for tan or brown glue on the top and bottom of the circuit boards. A rigid adhesive may be used to attach various components but some varieties decompose and become conductive with heat and age. Some very weird problems have been linked to decayed glue! So, carefully scraping it away and replacing it with non-acidic RTV Silicone or similar adhesive may be prudent. However, I don't know how to tell which types are a problem.

Check for loose or damaged cables.

Remove, inspect, clean (if necessary), and reseat all internal connectors. Even if they don't seem to be in an area of the circuitry that is relevant, they could be feeding a power or control signal. Check for discoloured or fatigued contacts as well as physical damage to the wires and improperly made crimps.

Where a problem is found, don't assume there is only one! In many cases, bad solder connections or bad crimps are caused by poor manufacturing process control and will be repeated in many locations. So, correct what was found and then continue to inspect the entire unit. Sometimes, manufacturing is so poor that resoldering the entire board is the only solution with any chance of long term success. Or replacement of the PCB preferably with a repaired and tested unit rather than an expensive new one that will likely have more of the original faults.

If a suspicious area is located, it may be possible to use a continuity tester between selected pins to determine if a connection is intermittent. To increase the chance of detecting a momentary change in resistance that may be too brief to register on a meter use a probe with an audible indicator.

Power On Tests for Intermittent problems.

If none of this produces a breakthrough, the next step is to power up the equipment. WARNING: Depending on the particular equipment, lethal voltages or other hazards may exist. Make sure you understand and follow what's in the Safety Guidelines.

Using an insulated stick, start gently prodding likely areas of the circuitry in an attempt to make the problem come and go. If you are successful, don't assume the journey is over! Pressing at a corner of the board may have an effect at the opposite side. In fact, you may find that pressing *anywhere* appears to have about the same effect. It may take some very very light tapping, flexing, etc., to locate the culprit. Until a physical cause if actually located (e.g., a visibly cracked solder joint where the pin can be see to move!), don't assume you're home and dry even if the problem appears to clear up. In fact, it is very common for an intermittent problem to go away as soon as troubleshooting begins not to reappear for several days or more - but it will reappear!

If the cause is heat related, no amount of prodding (or cursing) may result in the problem occurring with the cover off. In this case, a heat gun (or blow dryer) may be needed to carefully warm up selected areas of the circuitry in an effort to identify the culprit. A hair-dryer with no heat may be used for cooling. Or, "circuit chiller" or "cold spray" may be used for more aggressive spot cooling. However, simply removing the cover may have altered something physical.

The hardest intermittent problems to locate are those that occur infrequently and for only a short time with no chance of making a measurement.

Boiler-specific faults

This section deals with well known faults on specific makes and model of boiler. Please include any addition ones that you find in this section.

Keston C25 ignition failures
one common reason for this is the electrodes moving apart. Do not try and bend them back when cold. Someone here said that you can straighten an electrode when hot (hold in gas hob flame) but I've not tried this myself.
The biggest downside to Kestons seems to be parts availability

Ideal (Stelrad) Classic buzzing
fan, pcb (Stelrad 25b) or gas valve
* fans tend to buzz when the bearings have failed and it is no longer turning
* the gas valve tends to buzz because the relay on the pcb is buzzing because the drive circuit is faulty - not the most common problem with this pcb
* or you have an internal timer which has a faulty clutch - if so, is it losing time?
Ideal Response 120 losing water pressure
  1. "Is the heat exchanger leaking yet?"
  2. "How badly is it leaking now?"
Potterton Suprima locking out
PCB fault: replacement from CET Ltd
Note if Suprima has a red HT lead, it's a common reason for flame sense failure - it's carbon fibre, not wire and shouldn't be trusted

Potterton Combi 100 dead
Symptoms may include supply fuse/trip blown, burning smell and possibly burn marks or even hole in PCB
Modulation PCB fault: possibly replacement from CET Ltd
Baxi Barcelona
Ravenheat RS84F
  • short cycling: cause unknown Screwfix forum
  • thermistors "notorious" for unreliability
Ravenheat LS80
unreliable thermistors - can replace wet-pocket sensor p/n 0007TER05005/0 with clip-on p/n 0007son11010/0 (both cost over £20 + VAT!)
Icos / Isar / Istor boiler fault
fault not uncommon with this type of boiler gas valve.
l..f flashing
voltage at the gas valve on the conector (still connected) fine
negative vacuum on the venturi fine
connector is faulty: plug tight on gas valve pins but not making contact. Aparently this is common.
Icos / Isar / etc?
PCB fault which causes fuses to blow on supply cct (when DWH called for on Isar? also happens on Icos)
replace PCB module (complete with plastic shell).
Gloworm Fuelsaver F ... comes on and the flame kicks in for about 5-10 seconds and then goes out.
Fuelsaver - some have a problem - the fans have windings which are prone to overheating and a resettable thermal fuse which will cut out when the windings get hot (a matter of seconds to minutes) and close again when the windings cool down
Vokera Excell
Baxi Barcelona
Discussion: Intermittent Ignition Lockout on uk.d-i-y

Generic parts in boilers

Could do with info on what parts of what boilers are replaceable by standard or generic parts.

  • Diverter valve diaphragm repair kit from Banico (BES stock code 17115) is claimed to fit Giannoni and also Mutmacanica(?) valves. Checked OK for Worcester CDi.
  • Many boilers use standard Grundfos (or other) pumps and one can just replace the heads (though on some - such as Potterton Performas - you can't, even though they look from the outside like standard Grundfos heads).

Must-also-replace parts

Some repairs involve replacing various O-rings, fibre washers etc. Would be good to know which, and if generics are OK.

Info, manuals etc on-line



Worcester / Worcester-Bosch


See also

How boilers work