Worcester Bosch Boiler Repair, London

What the boiler is saying: it could not detect a flame through the ionisation probe, so it shut the gas off and stopped.

Read the number beside it. EA 227 means no flame was detected after ignition, so it never lit. EA 229 means a flame was established and then lost part way through. Same two letters, two different investigations.

What 227 usually is: the flame sensing electrode and its lead, carboned at the tip so the signal never reaches the board. Then the ignition lead, then the gas valve.

What 229 usually is: the gas to air ratio, the dynamic gas pressure at the appliance, or flue products recirculating into the air intake. Parts rarely fix a 229.

What it usually is not: the circuit board. It was doing its job by locking out rather than continuing to release gas into a boiler that was not burning it.

Check first: does anything else in the house run on gas? A hob that will not light points at the supply. In winter, check the plastic condensate pipe outside for ice.

The water in the primary circuit went past the limit the safety thermostat allows, so the burner shut down. On most Greenstar models this appears as E9 224.

What it usually is: circulation, not the burner. Sludge, a seized or closed valve, a tired pump or a pocket of trapped air. The boiler is producing heat faster than the system is carrying it away, so it cooks in one spot.

What it usually is not: the heat exchanger, which is what people fear when they hear overheat. It is the last thing to suspect, not the first.

Why swapping the thermostat rarely works: the limiter is the part reporting the problem. Replace it and leave the circulation alone and it will trip again, because it is still measuring the same water.

What helps most: not a part at all. Clearing the system of sludge addresses the cause where a component swap usually does not.

The board asks the fan for a speed and reads back what it actually achieved. When the two do not match closely enough, it stops.

What it usually is: the fan itself. The bearing wears, and on a Greenstar this almost always announces itself first. If the boiler has been noisier than it used to be for a few weeks, that is the same fault earlier on.

What it usually is not: only the fan. A flue that is partly blocked, or a terminal packed with leaves or nesting material, makes a healthy fan work against resistance the board reads as a fault. Checking the flue costs nothing and a fan does not.

Worth knowing: a fan that has been noisy and then goes quiet has not repaired itself. It has usually stopped.

Shown on the newer Greenstar i and 8000 range. Older models say nothing at all and simply refuse to fire, which is why people assume the boiler has died when the gauge is the whole story.

What you can safely do: top up through the filling loop until the gauge reads between 1 and 1.5 bar cold, then reset once. This is one of the few Worcester codes a householder can clear.

What it usually is not: a boiler fault. The boiler is reporting the system, not itself.

• Once or twice a year: normal. Sealed systems lose a little water.
• Every few weeks: a leak somewhere, often a radiator valve or a pinhole under a floor.
• Every few days: the expansion vessel has lost its air charge and is pushing water out of the pressure relief pipe on every cycle.

Usually shown as A1 281. The boiler asked the pump to move water and did not see the result it expected.

What it usually is: a pump seized after a long summer standing still, or a system with enough air in it that the pump is spinning in a void. The second is far more common in spring after nobody has bled a radiator since October.

What it usually is not: a reason to replace the whole pump body. On Greenstar models the head unbolts from the body, so the wet side of the system stays sealed and the job is shorter and cheaper than most people are quoted for.

Related: A1 and E9 often arrive within days of each other, because a pump that has stopped moving water is also the reason the water is overheating.

A condensing boiler makes about two litres of mildly acidic water a day and has to get rid of it. When it cannot, it stops.

What you can safely do: in cold weather, pour warm water along the plastic pipe that runs from the boiler to an outside drain. Warm, never boiling, which splits it.

What it usually is when the weather is mild: the siphon inside the boiler rather than the pipe outside. Worcester use a siphon that holds a small volume of water as a seal, and debris settles in the bottom of it over the years.

One thing to check: on some CDi models D5 is a service reminder rather than a fault, so it is worth confirming which your model does before anybody charges you to attend.

The opposite fault to EA. The board closed the gas valve and then carried on reading a flame signal, so it locked out.

What it usually is: moisture or condensation tracking across the electrode or its connector, giving a false reading, or the sensing lead sitting where it should not be. Occasionally the gas valve is passing.

What it usually is not: something to keep resetting. This is a gas safety lockout and the boiler is refusing to run for a reason worth respecting.

Why it matters more than the others: most codes mean the boiler stopped when it should have. This one means the boiler saw something it could not explain, which is a different order of problem and not one to sit on.

The shape of the code tells you which generation of Worcester you own, and it is the reason so many people look up a code and find an answer that does not fit their boiler.

Letters, or letters with a three digit number: EA, E9, C6, A1. This is the CDi Classic, the CDi Compact and the Greenstar i.

Bare numbers with a letter suffix, such as 227 V or 1017 W: the Greenstar 8000 range and the GB162. The number means something completely different here, so a generic list written for the older display will send you the wrong way.

No display, only a light: the Greenstar Ri and the older CDi Highflow report by flashing an indicator, and the pattern is the code.

T1 through T6: not a fault at all. Somebody has left the boiler in component test mode after a service.

The classic Worcester hot water fault, and it is not the heat exchanger. A combi has to know the instant a tap opens, and on most Greenstar models that job belongs to a small turbine in the cold inlet.

When the turbine sticks, the boiler either does not fire at all when you open a tap, or fires late and then drops out. What you feel at the tap is water that keeps changing its mind.

How to tell it apart from scale: a scaled plate heat exchanger gives you water that is steadily lukewarm and never gets properly hot. A sticking turbine gives you water that is properly hot some of the time. Steady and disappointing is one fault, erratic is the other.

Does it ever get properly hot, or never? Call 0203 868 6134.

The Greenstar Ri and the older CDi Highflow do not have a digital display. They report faults by flashing an indicator light in a pattern, and the pattern is the code.

This catches people out badly, because a boiler with no readout looks like a boiler that has simply died. It has not. It is telling you something in a format nobody remembers how to read.

What to do: count the flashes, note whether the light is red, green or amber and whether it flashes steadily or in bursts, and tell us. That description does the same job as a code.

Describe the light and we can usually narrow it down on the phone. Call 0203 868 6134.

Intermittent is the hardest kind of fault to get taken seriously, and on a Worcester it usually has a physical explanation rather than a mysterious one.

The two usual causes are the condensate siphon, which only stops the boiler once it has filled beyond a certain point, and an electrode that fails when cold and works once the boiler has warmed through. Both produce a fault that clears on reset and comes back later, which is exactly what makes people put up with it.

Worth doing: write down the date and time each time it happens before you reset it. A pattern of early mornings points at one cause, a pattern of every four or five days regardless of time points at the other. That note is worth more than any guess we could make.

Worcester sell their own controls alongside the boiler, and a good share of no heating call-outs on newer installations turn out to be the control rather than the appliance.

A wireless thermostat with a flat battery, a controller that has lost its pairing after a power cut, or a schedule that reset itself will all produce a boiler that is in perfect health and completely idle. There is no fault code because there is no fault.

Before you book anybody: turn the heating on at the boiler itself if your model allows it, or put the control into a constant or manual override. If the boiler fires, the problem is upstream of it and the repair is a great deal smaller than you feared.

Does it fire when you override the control? Call 0203 868 6134.

The number is the output in kilowatts. The letters are the type, and that is the part that decides which half of your system an engineer is looking at.

• i, as in Greenstar 30i: a combi. No cylinder, hot water made on demand.
• Si: a system boiler, built to run a separate hot water cylinder.
• Ri: heat only, usually a cylinder in the airing cupboard and a tank in the loft.
• CDi: the higher output range, sold as combi, system and regular versions.

If your badge carries Si or Ri you have stored hot water somewhere in the house, and the cylinder is as likely to be the fault as the boiler is.

Greenstar i and Greenstar Si Compact are on more London walls than anything else Worcester make, usually in the 25 to 30 kilowatt range and usually in a kitchen cupboard.

Greenstar CDi Classic is the older higher output range, and the 30CDi and 42CDi turn up constantly in larger houses. Parts are still current.

Greenstar 8000, sold as Life and Style, and Greenstar 4000 are the current generation. They report faults differently from everything above, which matters when you look up a code.

Greenstar Ri and CDi Highflow are the ones with no display. The Highflow has an internal store, so it behaves like neither a combi nor a system boiler and gets misdiagnosed as both.

Worcester guarantees run for a fixed term from the date of installation, and the term is routinely longer than people remember agreeing to. The condition is normally that the boiler was fitted by an accredited installer and has been serviced every year since.

Here is the part that catches people out: the guarantee is registered against the appliance, not against you. Buy a house with a five year old Greenstar in it and the remaining cover often comes with it, and almost nobody thinks to check.

You need the serial number off the data badge. Check before you spend anything, including with us. If you are inside the guarantee we will say so and stand down.

The sensor sits in a pocket on the cylinder and tells the boiler what the stored water is doing. When it fails, or simply works its way out of that pocket, the boiler stops heating the cylinder while the radiators carry on perfectly. That is why the fault gets blamed on the appliance rather than the tank. It is an inexpensive part and a short job, once somebody looks in the right place.

On a system with a cylinder, a two port or three port valve decides whether the boiler is heating radiators, heating the cylinder, or both. When the motor or the microswitch inside it fails you get heating with no hot water, hot water with no heating, or both running when you only asked for one. On most installations the head changes without draining the system down, so it is rarely the job people brace themselves for.

Most cylinders carry an immersion element as a backup for when the boiler is down. It scales, and a scaled element either trips its thermostat or stops heating altogether. It is worth testing before anybody condemns a cylinder, because an element costs a fraction of a tank. Large houses on stored hot water, of the kind we see across Dulwich, often have a scale reducer fitted at the same visit.

An unvented cylinder is a pressure vessel, and it carries an expansion vessel, a pressure reducing valve, an expansion relief valve and a temperature and pressure relief valve, all discharging through a tundish. Water running at the tundish is the system telling you one of them has failed. It is not something to leave, and it is certainly not something to cap off. Any of those devices can be replaced, but only by somebody qualified to work on unvented systems.

Unvented work is notifiable and needs a G3 qualification on top of Gas Safe registration. Our engineers hold both, so the boiler side and the cylinder side are dealt with on one visit rather than two appointments and two charges. That is why our unvented cylinder repair sits alongside the boiler work instead of being farmed out.

Swapping stored hot water for a combi to free up an airing cupboard is the change people most often ask us to reverse. A combi cannot run two showers at once, whatever the output figure on the box says, because it heats water as it passes rather than storing it. In a house with three bathrooms the honest answer is usually to keep the cylinder, and to spend the money replacing it if it is old rather than removing it.

Every part is current and at this age almost every failure is a single component rather than a boiler wearing out. An electrode, siphon, turbine or pump head on a six year old Greenstar is a repair, without qualification.

Find the serial number and check the guarantee before anybody quotes you, including us. Worcester terms run for a fixed number of years from installation and a boiler that feels too old to be covered often is not.

If somebody offers you a new boiler at this age, ask which single part has failed and what that part costs on its own. The gap between the two answers is usually the whole conversation.

Still a clear repair: electrode, fan, pump head, condensate siphon, expansion vessel, hot water turbine. These are ordinary wear items and they say nothing about the condition of the rest of the boiler.

The conversation changes with a main heat exchanger, or a second significant failure inside a year. At that point the arithmetic favours replacing the boiler, and the question is not only what has gone today but what goes in the next two winters.

A Greenstar that has had one electrode and is otherwise dry, quiet and holding pressure is a boiler worth keeping. They were built to be kept.

Past this point availability starts to take over from cost. A CDi Classic at fifteen is often still a straightforward repair, because the wear parts on that range are still made, and that is worth establishing before anybody writes the boiler off.

The exception is the older Highflow, where the internal store is the expensive part and the one most likely to have gone. A store is not an economic repair on a boiler of that age, and anybody telling you otherwise is not doing you a favour.

We will tell you whether the part still exists before you spend anything on a diagnosis. Our guide to repair and replacement costs sets out what actually drives the figure.