This guide highlights some common boiler problems and estimated costs for repair for Worcester Bosch, Vaillant, Baxi, Glow-Worm

Work should always be carried out by an expert. Please check Gas Safe registration numbers before agreeing to have work completed.

Prices are estimates. They will be vary depending on brand, model, sizes (i.e. radiators, pumps and expansion vessels) and so on.

After troubleshooting, sometimes there is more to a problem than meets the eye. It’s important not to jump to conclusions.

Hit the quick link to the relevant sections to find faults and estimated costs of repair for your boiler manufacturer.

Worcester Bosch (models after 2004) – Common Boiler Problems

If your boiler was installed after 2004. It’s likely a Greenstar.

This includes the I, SI Compact, I System, CDI Megaflow, CDI, GB162, CDI FS Regular and RI range.

Older boilers will be covered in the next section.

Problem 1: Boiler pressure is too low

Most boilers operate between 1-3 bar. If they drop below (or rise) above this, they won’t function correctly. More importantly, it shows there is a problem.

Potential cause of boiler error

• Leaking radiators, towel rails, or radiator valves

• Automatic air vent needs replacing

• Condense pipe is blocked (in cold weather)

Fix – Leaks

If a radiator or towel rail is split. It will need to be replaced.

If radiator valves are leaking, sometimes they can be fixed by tightening up the nut slightly. In most cases, the olive has been displaced and the easiest option is to change the valve.

You can isolate the leaking radiator by turning the lock shield completely off. The water flow will then bypass this unit until the problem is fixed.

Fix – Automatic air vent

When air collects in a system, it creates noise. An automatic air vent can remove this problem. However, there’s a good chance it’s leaked, seized and now needs to be replaced.

The air vent can be removed (after draining down the system), usually by a quick release pin. The system will then need to be refilled and tested.

Fix – Condense blockage

If soaking warm water on the condense blockage doesn’t fix the issue, the problem is further up the line (potentially the trap or sump) and will require disassembly.

Estimated cost of repair

Fit new TRV (thermostatic radiator valve): £25-40 each

Replace automatic air vent: £50-200

Remove condense blockage: £50-250

Troubleshoot and a boiler losing pressure: £120-350

Problem 2: Flame dies during operation (fault code EA 229)

• Excessive wind is entering the flue

• The sump or condense trap is blocked.

Fix – Wind

Install a branded Worcester Bosch plume management kit. This allows you to direct waste content from the flue any way you want.

Fix – Condense pipe is blocked

Water has likely frozen. You can try to melt the ice using a bucket of warm water. Don’t use hot water is it might crack the condense pipe.

Estimated cost of repair

Install a Worcester Bosch plume management kit: £180-250

Remove condense blockage: £50-250

Problem 3: Boiler won’t ignite (fault codes EA 227 and EA 261)

In the above two cases, the boiler will go through it’s start up sequence followed by a ticking noise (that’s the sound of it trying to ignite).

Potential causes

• No gas supply

• Faulty ignition leads

Fix – No gas supply

If you are on a meter, check you have credit. If you have ran out of credit there should be an emergency credit option.

If you have just moved into the property, it’s likely that the gas was turned off (especially if the property was purchased on vacant possession). My advice would be to get a gas engineer to come and turn on the gas, but also to run a test for leaks.

Fix – Ignition leads

If a gas engineer can isolate the problem to ignition leads, they can be replaced with new ones.

Estimated cost of repair

Turn on gas and run a leak test (including minor repair if there is a leak): £60-150

Replacement of ignition leads: £160-200f7

Replace probes and ignition leads: £200-400

Problem 4: Knocking noises coming from the boiler

This problem will usually be most persistent when you first switch on the boiler/heating.

Potential cause of boiler fault

• Gas valve and/or fan

Fix – Gas valve and fan

Unfortunately, there is no simple fix. The most likely cause could be the gas valve and fan are running out of sync. This could be an issue with the gas valve, the fan, or both.

Estimated cost of repair

Install new gas valve: £150-500

Install new fan: £200-600

Worcester Bosch (models before 2004) – Common Boiler Problems

If your Worcester boiler is not a Greenstar (this includes the I, SI Compact, I System, CDI Megaflow, CDI, GB162, CDI FS Regular and RI range). It was likely made before 2004.

Problem 1: Boiler will not ignite

This assumes the boiler has gone through all the usual start up procedures (no additional knocks or unusual noises compared to usual). The fan will begin to spin in anticipation of the boiler igniting, but it fails to do so.

Potential causes

• Air pressure switch is not functioning correctly

• Fan has stopped working

Fix – Air pressure switch

This is a simple swap out for a new part. The air pressure switch sends a signal to the boiler to say the fan is running (so if your fan is running, there’s no problem there).

In some cases the piping going to the air pressure switch will have simply degraded over time. Replace these alongside the switch.

Fix – Fan

If your boiler doesn’t go through the normal start up sequence (i.e. you can’t hear the fan running) there’s a good chance the fan is your problem.

It might be seized, but as most boiler fans have a sealed bearing unit, trying to unseize the bearings using external lubricant won’t work.

Depending on the age of the boiler and cost, the best option is usually the replace the boiler all together, especially if there are other intermittent problems.

Estimated cost of repair

Install new air pressure switch: £100-250

Install new fan: £200-600

Problem 2: Boiler keeps locking out

There are two scenarios here

Fix – Boiler is locking out before it tries to start up

This is usually electrical. The PCB (printed circuit board) is the heart of your boiler. It’s likely this is the culprit and needs to be replaced.

Fix – Boiler tries to ignite, but then immediately locks out

Each boiler will incorporate probes and ignition leads. It’s likely they have degraded over time and need to be replaced.

Estimated cost of repair

Install a new PCB: £250-500

Replace probes and ignition leads: £200-400

Problem 3: Boiler doesn’t operate, but there is power to it

Many old Worcester boilers were fitted with thermostats which constantly monitor to see if the boiler is overheating.

Potential cause

• Overheat thermostat

Fix – Overheat thermostat

The obvious answer is going to be a direct replacement.

To check this isn’t a onetime issue, it’s worth running a system reset.

Estimated cost of repair

Installation of a new overheat thermostat: £150-320

Problem 4: Water isn’t getting up to temperature

If the lights on the boiler are showing that it’s getting up to temperature, but the water itself isn’t, there is a common culprit.

Potential cause

• Heat exchanger not working correctly

Fix – Heat exchanger

Replace the heat exchanger

Estimated cost of repair

Installation of a new heat exchanger: £400-650

Baxi – Common Boiler Problems

Problem 1: Boiler lockout (common on the Solo 1PF and 2PF)

The most common reason a lockout happens is due to an overheat.

Potential cause

• Low pressure

• Overheat thermostat working incorrectly

• Blockages (especially during colder times of the year)

Fix – Low pressure

Pressure on a boiler can be adjusted using the filling loop. Please be warned, the higher the running temperature of your Baxi, the more pressure will increase. So the pressure may look low when it’s off (or not on full power), but this will increase as it heats up.

Fix – Overheat thermostat

To check this isn’t a onetime occurrence, reset the thermostat.

The 1PF and 2PF both have a reset procedure.

On the 1PF the reset procedure is limited to pressing in the red button for the thermostat.

The 2PF will have a button for the thermostat. Turn that to the off (or reset) position and then back again.

Fix – Blockages

The most common blockage is in the condense pipe. In the winter months, moisture will freeze and block the flow. If the pipe is accessible, pouring warm water over the pipe is usually enough to remove the blockage. If this doesn’t clear the blockage, it’s likely there is a blockage further up (condense trap or sump).

Estimated cost of repair

Installation of a new overheat thermostat: £150-320

Remove condense blockage: £50-250

Problem 2: Boiler is leaking

This is particularly common for the Baxi 30HE, 105HE, 133HE and Duo Tech combis.

Potential cause:

• Housing for diverter valve and pump

Fix – Replace the housing

The housing will degrade over time, and eventually cause boilers to leak water. Replacing the housing is not a particularly easy job.

It’s worth spending an extra hour assessing potential water damage to other parts while the boiler is easy to access.

Estimated cost of repair

Replacement of diverter valve housing: £100-150

Replacement of diverter valve (including housing): £250-380

Problem 3: Banging and knocking noises

This problem occurs regularly on the Duo Tech and HE range.

Potential cause:

• Restricted flow

Fix – Restricted flow

If this problem has just started, the pump could be failing. A replacement would be in order to solve heating pump problems.

However, if restricted flow (and banging) has always been a problem, the pipework may be incorrectly installed or blocked. As an example, we went to a property where 10mm pipework (which actually should have been 15mm) was incorrectly routed.

Newer pumps tend to have a flow rate setting (from 1-3 on the basic Grundfos range). You can turn down the flow range on the head of the pump.

Estimated cost of repair

Replace pump (Grundfos): £240-380

Problem 4: Boiler won’t ignite correctly, or at all

Models most likely to suffer are the Baxi Barcelona and the 100E.

Potential cause

• Leaking seals

Fix – Leaking seals

The combustion chamber seals on the 100E and Barcelona are not particularly durable. They can degrade and then start leaking. The boiler will sense a leak and lock out.

The seals will need to be changed. In some cases it makes sense to change the whole combustion chamber.

Estimated cost of repair

Fix leaking seals or replace combustion chamber: £200-550

Problem 5: No hot water from a combination boiler (Unless heating is on)

Particularly a problem from the 80E and 105E.

Potential cause

• Diaphragm

Fix – Diaphragm

If the diaphragm is faulty, it will need to be replaced. As this is a time consuming job it makes sense to change the entire unit (diverter valve).

Estimated cost of repair

Replace diaphragm (and diverter valve): £250-380

Ferroli – Common Boiler Problems

Problem 1: Boiler is leaking

A common problem on the 30C

Potential causes

• Faulty or loose pump

• Faulty shutoff valve

Fix – Pump

Visually check the pump to see if it’s leaking. If it is:

Pumps shut be rock solid in their housing. If they are not, locate the screws/allen key bolts that are securing them to the boiler and tighten them.

If the pump is still leaking it’s likely it’s blown a seal. It’s usually wise to replace the whole pump.

A temporary fix that sometimes works is to reduce the flow rate of the pump. This will put less pressure on the seal and should reduce the severity of the leak.

Fix – Faulty shut-off valve

It’s rare these can be fixed, and a replacement is likely needed.

Estimated cost of repair

Replace pump (Grundfos): £240-380

Install new shut-off valve: £90-150

Problem 2: Low pressure

Potential cause

• Expansion vessel needs pressurising

Fix – Expansion vessel

It’s likely a new expansion vessel is needed. However, the pressure can be adjusted using a pump.

Estimated cost of repair

Fitting of a new expansion vessel: £175-250

Problem 3: Boiler works sporadically

The boiler doesn’t come on when you set it to, and comes on at times where you’ve not set it to.

Potential cause:

• Mechanical timer is faulty

Fix – New mechanical timer

Replace the existing mechanical timer. They are easy to pick up from heating spares suppliers.

Estimated cost of repair

New mechanical timer (depending on brand): £90-150

Problem 4: Boiler lockout and doesn’t ignite

This is most common on the Ferroli Modena and Falcon range.

Potential cause:

• Faulty PCB

Fix – PCB

The obvious answer is a PCB replacement.

Before jumping to conclusions, test the PCB with a digital multi-meter.

PCB replacements are not cheap. If a boiler is over 10 years old, replacement is usually the most cost effective option.

Estimated cost of repair

Replace PCB: £250-500

Vaillant – Common Boiler Problems

Problem 1: Lack of pressure (fault code F22)

Boilers generally sit in between 1-3 bar. It’s common on the Vaillant Eco-Tec 24, 28 and 624.

Potential cause

• Boiler has lost pressure over time and needs to be topped up.

Fix – Low pressure

Most combination boilers have an external filling loop (a braided hose). On this house is a handle. Turning the handle will top up the boiler pressure.

Don’t overfill the boiler. Boiler pressures will rise as it heats up and too much pressure can put unwanted pressure on internal components.

Problem 2: F75 fault code is shown on the display

Vaillant’s Eco-Tec are prone to a F75 fault code appearing.

Potential cause of boiler fault

• Pump

• Pressure sensor

Fix – Pump

Vaillant used to use Wilo pumps in their Eco-Tec range. They are low quality and they’ve since changed to Grundfos.

If you’ve noticed leaks, the pump is the culprit and the easier option is to get a replacement (from a better known brand like Grundfos).

Fix – Pressure sensor

The F75 fault relates to both the pressure sensor and the pump. If your pump is running but the code is still flashing, it’s likely the pressure sensor.

Replace the sensor. If it’s dirty or blocked, think about running a hot flush through the system and adding a magnetic system filter (this will help this problem happening again).

Estimated cost of repair

Replace pump (Grundfos): £240-380

Install new pressure sensor: £130-£180

Hot flush: £150-350

Click for further details on the Vaillant F75 fault code.

Ariston – Common Boiler Problems

Problem 1: Won’t ignite (fault code A1)

This fault is most common on the Ariston Microgenus 24HE and 28HE.

Potential cause

• Ignition leads

• Gas supply

Fix – Ignition leads

Check for current using a multi-meter. If there is not current. Replace the ignition leads.

Problem 2: Low pressure (fault code E2)

This fault is common on the Microgenus 30HE.

Potential cause:

• Boiler losing pressure (air in system, expansion vessel leakage)

Fix – Low pressure

The quick fix is to top up the boiler using the external filling loop. Top up to roughly 1.5 bar.

A Gas Safe engineer will need to be able to diagnose and fix the fault.

Estimated cost of repair

Replace probes and ignition leads: £200-400

Problem 2: Fault code E34 on display

Many old Ariston Microgenus boilers are affected by this fault code.

Potential cause:

• Air pressure switch

Fix – Air Pressure switch

A simple replacement.

Problem 4: Fault code A41 on display

This problem occurs mainly on the newer Microgenus 2 range. Namely the 24 RFFI.

Potential cause

• Faulty fan

Fix – Fan

Using a multi-meter. Check the fan has a current. If no current shows do the same check on the PCB.

If the reading from the PCB is fine, the fan needs replacing. If the PCB isn’t showing a reading, it’s likely the PCB needs replacing. Generally speaking, replacing the PCB is extremely costly, and usually boiler replacement is recommended.

Estimated cost of repair

Install new air pressure switch: £100-250

Install new fan: £200-600

Vokera – Common Boiler Problems

Problem 1: Fault codes 92/93 and 95

This occurs in extremely cold weather (ice).

Potential cause

• Ice blocking the condense pipe

Fix – Condense blockage

Using warm-hot water, try and melt the ice from the condense pipe. Usually this will travel back to the condense unit thawing that at the same time.

Vokera boilers have a reset button. Reset the boiler before switching the heating back on and the system should clear the fault.

If this doesn’t work. The blockage is likely more severe.

Estimated cost of repair

Troubleshoot and fix condense blockage: £100-250

Problem 2: Ignition problems

This has been particularly common with the Vokera Compact 25HE (fault code A01) and the Vokera Linea24 HE.

Potential problem:

• Gas valve

• PCB

Fix – Gas Valve

You can use a multi-meter to see if there is a current to the gas valve. If there is not a current, the gas valve and solenoid will need replacing.

Fix – PCB

If the PCB is dead and needs replacing, especially if other parts are worn, it’s likely a replacement boiler will be suggested.

Estimated cost of repair

New gas valve fitted: £180-300

Replace PCB: £250-500

Problem 3: Boiler is operating at low pressure

Extremely common in all boilers, not just Vokera’s.

Potential cause

• Air in the system

Fix – Air in the system

First, you want to boost the pressure. Use the external filling loop that’s located under your boiler. Turn the valve until the boiler hits 1.5 bar.

Once you’ve done that, use a radiator key to bleed all the radiators in your house. If the problem persists, an engineer will have to troubleshoot the problem.

Estimated cost of repair

Troubleshoot and fix pressure loss: £120-350

Problem 4: Hot water doesn’t work, or only works when the heating is on

This is common in the Vokera Vision and Flowmatic ranges.

Potential cause

• Diverter valve not working correctly.

Fix – Diverter valve

On some occasions, it’s just the diaphragm washer that needs replacing. If swapping out the washer on the valve doesn’t solve the problem, the whole diverter valve will need to be replaced.

Estimated cost of repair

Replace diaphragm (and diverter valve): £250-380

Glow Worm – Common Boiler Problems

Problem 1: Flame not detected

You’ll get this on Ci models when a F16 fault code appears.

Potential problem

• Probe

• Pilot assembly

Fix – Probe

It’s likely the build-up of sludge in the system has clogged up the probe. It will need to be removed and cleaned. If the problem persists the probe and lead will need to be replaced.

Fix – Pilot assembly

If the probe is clean, the pilot assembly may be jamming. This will be to be disassembled, cleaned and then reassembled.

Estimated cost of repair

Replace probes and ignition leads: £200-400

Replace pilot assembly: £100-180

Problem 2: F9 Fault code appearing on display

The F9 fault code is common on the Glow-Worm Flexicom CX and the Ultracom HXI range.

Potential cause:

• Build up of sludge in system

Fix – Sludged up system

First, the system needs to be cleaned out. This can be done with a powerflush, hot flush or manually draining down and hosing out the radiators.

Once the system is drained and cleaned, inhibitor should be added to the system and a magnetic filter installed (this catches debris). The magnetic filter will be cleaned out during your boiler service.

Estimated cost of repair

Hot flush: £150-350

Install magnetic system filter: £150-250

Problem 3: Flexicom CX showing a F3 fault code

Particularly common with the 30CX.

Potential cause:

• Fan

Fix – Fan

First, you’ll need to confirm the fault is with the fan. Use a multi-meter to check the current. You’ll have to trace back the connections from the PCB. The fan can rarely be fixed, so replacing the fan is usually the only option.

Estimated cost of repair

Install new fan: £200-600

Problem 4: Glow-Worm Betacom showing F61, F62, F63, F64, F65 and F67 fault codes.

This is one of the most common fault codes on all the of Betacom boilers.

Potential cause

• PCB (printed circuit board).

Fix – PCB

A faulty PCB is not good news. The part itself is incredibly expensive. With labour on top, old boilers usually warrant being replaced.

That’s a last resort though. Using a multi-meter you can check to see if the fault is the whole PCB unit failing, or a piece of the PCB. It could be a loose connection for instance.

Replace PCB: £250-500

Potterton – Common Boiler Problems

Problem 1: Boiler is leaking

This is a common fault on the Potterton Promax.

Potential cause:

• Pump seal faulty

• Flow/pump switch faulty

Fix – Pump seal

Remove the pump and replace the seal. If the boiler is particularly old, it might be worth replacing the whole pump unit.

Fix – Flow/Pump switch

The leak is likely coming from the o-ring on the pump/flow switch. A quick fix would be to replace this o-ring. It makes a lot more sense to replace the whole unit.

Estimated cost of repair

Replace pump (Grundfos): £240-380

Problem 2: Boiler makes lots of banging noises when it fires up

This is most common on the Promax HE models.

Potential problem:

• Burner is not working correctly.

Fix – Burner

Newer burners are now ceramic and they rectify noise related issues. Swap out the old burner for a new ceramic one.

Estimated cost of repair

Fit new burner: £280-400

Problem 3: Smell of gas near the boiler

Switch your gas and boiler off straight away.

This is a common problem with the Potterton HE models.

Potential problem

• Leak via the heat exchanger

Fix – Ring a Gas Safe engineer

This is immediately dangerous, and needs to be looked at ASAP.

Estimated cost of repair

Run leak test, identify leak/s and fix (including parts): £150-500