• water leaking from the boiler;

• falling or rising boiler pressure;

• unusual banging, whistling or gurgling;

• radiators not heating correctly;

• the boiler repeatedly switching off;

• ignition failure;

• a frozen condensate pipe;

• a pilot light that will not remain lit;

• an unfamiliar fault code.

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Gas Safe Register identifies intermittent heating or hot water, kettling, water leaks, pressure loss and a boiler switching itself off among the common signs of boiler trouble.

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The exact cause cannot always be identified from the symptom alone. A leak, pressure problem or ignition fault may have several possible causes, so the boiler should be diagnosed rather than repaired by guesswork.

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Boiler Leaking Water

 

A boiler leaking water should be investigated promptly.

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Even a small drip can damage cupboards, flooring, plasterwork or electrical components. A leak may also cause the heating system to lose pressure and eventually stop working.

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Possible causes include:

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• a loose internal connection;

• a worn seal;

• a faulty valve;

• excessive system pressure;

• corrosion;

• a damaged heat exchanger;

• a leaking pump;

• a problem with the condensate pipe;

• water escaping from nearby heating pipework rather than the boiler itself.

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Older boilers can develop internal corrosion, while damaged or blocked condensate pipework can also result in water appearing around a modern condensing boiler.

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Do not remove the boiler casing or attempt to identify an internal leak yourself. Internal work on a gas boiler should be left to an appropriately qualified Gas Safe registered engineer.

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What Should You Do if the Boiler Is Leaking Water?

 

Where it is safe to do so:

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1. Turn off the boiler.

2. Switch off the heating.

3. Contain the water with a tray, bowl or towels.

4. Move possessions away from the leak.

5. Check whether water is coming from the boiler, a pipe joint or a radiator connection.

6. Arrange an inspection by a qualified heating engineer.

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The water supply may also need to be isolated where the leak is substantial, but only do this if you know which valve to use and can reach it safely.

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Vaillant advises turning off the heating and water supply where appropriate, containing the leak and contacting a Gas Safe registered engineer rather than attempting to dismantle the boiler.

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Do not continue repeatedly topping up a leaking system. Adding more water without repairing the fault can increase water damage and introduce additional oxygen into the heating system, which may contribute to corrosion.

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Water Dripping From the Pipe Outside

 

A small pipe passing through the external wall near the boiler may be connected to the pressure-relief valve.

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If water is continually dripping from this pipe, the heating system may be reaching excessive pressure or the valve may not be closing correctly.

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Possible causes include:

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• a faulty pressure-relief valve;

• an expansion-vessel problem;

• the filling loop being left open;

• excessive system pressure;

• contamination preventing the valve from sealing.

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A continuously wet external discharge pipe should be inspected. Worcester Bosch advises checking the outside pressure-relief pipe when a heating system repeatedly loses pressure, as dripping may indicate a faulty valve.

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Do not block or cap this pipe. It forms part of the boiler’s safety arrangements.

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Boiler Not Working

 

When a boiler is not working, begin with a few basic checks that do not require removing the casing or touching gas components.

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Check whether:

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• the property has electricity;

• the boiler display is illuminated;

• the gas supply is available;

• the water supply is on;

• the thermostat is calling for heat;

• the programmer is set correctly;

• the boiler pressure is within the manufacturer’s normal range;

• a fault code is displayed;

• the boiler requires a permitted reset;

• other gas appliances are operating normally.

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A loss of gas, electricity or water can prevent the boiler from producing heating or hot water.

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Refer to the manufacturer’s instructions before resetting the appliance. A reset may clear a temporary interruption, but repeated lockouts indicate that the cause should be investigated.

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Do not continue resetting a boiler that immediately fails again.

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Boiler Has Power but Will Not Fire Up

 

A boiler may have an illuminated display but still fail to ignite.

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Possible causes include:

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• low system pressure;

• no gas supply;

• an ignition fault;

• a faulty gas valve;

• a flame-detection problem;

• a blocked or frozen condensate pipe;

• a thermostat or control fault;

• an internal sensor failure;

• the boiler entering safety lockout.

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The display may show a fault code that helps the engineer identify the relevant part of the system.

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Fault codes vary between manufacturers and models. The code should be checked against the instructions for the exact boiler rather than interpreted using a generic list.

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An ignition fault should be investigated by a Gas Safe registered engineer. Do not attempt to repair or adjust internal boiler components yourself.

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No Heating but Hot Water Still Works

 

Where a combi boiler provides hot water but the central heating does not work, the problem may relate to the heating controls or circulation rather than the boiler’s ability to generate heat.

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Possible causes include:

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• the thermostat being set too low;

• incorrect programmer settings;

• a thermostat communication fault;

• a faulty motorised or diverter valve;

• a circulation-pump problem;

• closed radiator valves;

• air or sludge within the system;

• low pressure;

• a heating sensor fault.

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Check the thermostat and programmer first. Where wireless controls are used, check the batteries and confirm that the receiver appears to be communicating with the thermostat.

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If the controls appear correct but the boiler does not respond, professional diagnosis may be required.

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Heating Works but There Is No Hot Water

 

Where the radiators heat normally but there is no hot water, possible causes include:

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• a diverter-valve fault;

• a flow sensor problem;

• a failed temperature sensor;

• an issue with a hot water cylinder;

• incorrect cylinder controls;

• a motorised-valve fault;

• an immersion heater or electrical issue;

• a scale-restricted heat exchanger.

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The cause will depend on whether the property has a combi boiler or a separate hot water cylinder.

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With a combi boiler, hot water is produced directly by the appliance. A system or regular boiler normally heats water stored within a cylinder, so the cylinder thermostat, valves and controls may also need investigation.

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Boiler Losing Pressure

 

Most sealed heating systems have a pressure gauge or digital pressure display.

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Many boilers normally operate at around 1 to 1.5 bar while the system is cold, although the correct range should always be checked in the manufacturer’s instructions.

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Pressure can fall slightly after radiators have been bled. However, regularly needing to repressurise the boiler may indicate:

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• a leaking radiator valve;

• a leak beneath the floor;

• a boiler leak;

• a faulty pressure-relief valve;

• an expansion-vessel problem;

• air being repeatedly released from the system;

• a connection that leaks only when the heating is hot.

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Check visible radiator valves and accessible pipe joints for water, staining or signs of corrosion. Small leaks can evaporate before forming a visible puddle, leaving discolouration around a joint.

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If the pressure continues to fall after topping up, arrange an inspection rather than repeatedly refilling the system.

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Boiler Pressure Too High

 

Boiler pressure normally rises as the heating water becomes hot, but it should remain within the operating range specified by the manufacturer.

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Excessively high pressure may be caused by:

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• the filling loop not being fully closed;

• too much water being added;

• a faulty expansion vessel;

• a pressure-relief problem;

• another internal fault.

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Where the pressure rises significantly whenever the heating operates and then falls again when it cools, the expansion vessel may require attention.

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Do not release water through unfamiliar boiler valves. Incorrect use of the pressure-relief valve can damage it and cause a continuing leak.

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Boiler Making Banging or Kettling Noises

 

A boiler making a rumbling, whistling or kettle-like noise may be experiencing restricted water flow or overheating within part of the system.

Possible causes include:

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• limescale;

• central-heating sludge;

• a restricted heat exchanger;

• low water flow;

• pump problems;

• trapped air;

• excessive boiler temperature;

• poor system circulation.

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This noise is often described as kettling because it can sound similar to water boiling inside a kettle.

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Kettling does not necessarily mean that the entire boiler needs replacing. The engineer may need to inspect the heat exchanger, pump, system water and circulation before recommending the correct repair.

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Gurgling Noises From the Boiler or Radiators

 

Gurgling commonly indicates air or poor circulation within the heating system.

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Possible causes include:

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• air trapped in radiators;

• low system pressure;

• sludge;

• restricted pipework;

• pump or circulation problems;

• water moving through a condensate pipe.

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Radiators that are cold at the top may contain air and may need bleeding. Radiators that are cold at the bottom are more likely to be affected by sludge or restricted circulation.

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After bleeding radiators, the boiler pressure should be checked because releasing air can cause the pressure to fall.

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Radiators Not Heating Properly

 

Cold or uneven radiators do not automatically mean that the boiler is faulty.

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Possible causes include:

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• air trapped in the radiator;

• sludge;

• a stuck thermostatic radiator valve;

• closed valves;

• poor system balancing;

• an inadequate pump;

• undersized heating pipework;

• a radiator that is too small for the room;

• the heating system being extended without redesign.

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A single cold radiator is more likely to have a local valve, air or balancing problem. Several cold radiators may indicate a wider circulation or control issue.

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A new boiler cannot compensate fully for badly restricted pipework or radiators that are incorrectly sized.

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Boiler Keeps Switching Off

 

A boiler that repeatedly switches itself off may be entering a safety lockout.

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Possible causes include:

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• low pressure;

• overheating;

• poor circulation;

• a pump fault;

• an ignition problem;

• a flame-detection fault;

• a frozen condensate pipe;

• thermostat or sensor problems;

• restricted water flow.

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Record any fault code before resetting the boiler. This can help the engineer understand the sequence of events.

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A boiler that continues to lock out after a reset should be inspected rather than repeatedly restarted.

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Frozen Condensate Pipe

 

Modern condensing boilers produce water that is carried to a drain through a condensate pipe.

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Where part of this pipe runs outside, the water can freeze during prolonged cold weather and create a blockage. The boiler may then display a fault code or stop working.

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A frozen condensate pipe may be suspected where:

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• the boiler stops during freezing weather;

• the external plastic pipe is visibly iced;

• there is a gurgling noise;

• the boiler displays a condensate-related fault.

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Manufacturer instructions may explain how to thaw an accessible external pipe safely using warm—not boiling—water. Do not climb, dismantle the boiler or attempt to reach an unsafe pipe position.

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Where the pipe freezes repeatedly, its route, size and insulation should be reviewed.

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Pilot Light Keeps Going Out

 

Older boilers may use a permanent pilot light.

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A pilot light that repeatedly goes out could be caused by:

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• a faulty thermocouple;

• a gas-supply issue;

• a draught;

• dirt or damage affecting the pilot;

• an internal safety fault.

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Do not repeatedly relight a pilot where there is a gas smell, visible damage or concern about safe operation.

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Modern boilers generally use electronic ignition, so a boiler that fails to ignite may display a fault code rather than have a visible pilot light.

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Boiler Displaying a Fault Code

 

A fault code is intended to identify the area in which the boiler has detected a problem.

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It does not always identify the exact failed component.

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For example, an ignition-related code could result from:

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• no gas supply;

• a faulty ignition component;

• a gas-valve issue;

• a blocked condensate pipe;

• a flame-detection problem.

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Write down or photograph the code and check the boiler’s own instructions.

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Avoid ordering components based only on an online description. The appliance should be tested by someone qualified to confirm the actual cause.

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Boiler Smells Unusual

 

A boiler should not produce a persistent gas, burning or electrical smell.

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Possible warning signs include:

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• a smell of gas;

• scorching or burning smells;

• melted wiring;

• soot or staining;

• unusual fumes;

• a carbon monoxide alarm sounding.

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A gas or carbon monoxide concern must be treated differently from an ordinary boiler fault.

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Leave the property, avoid operating electrical switches or creating flames, and contact the National Gas Emergency Service on 0800 111 999. A suspected carbon monoxide incident also requires fresh air and immediate professional assistance.

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Do not return to using the boiler until it has been declared safe.

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When Is a Boiler Problem an Emergency?

 

Urgent assistance may be needed where:

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• gas can be smelled;

• a carbon monoxide alarm activates;

• there is smoke, scorching or an electrical burning smell;

• water is leaking heavily;

• water is reaching electrical components;

• the boiler appears damaged;

• the property has no heating during severe cold;

• vulnerable occupants depend on the heating;

• the boiler cannot be safely isolated.

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A routine loss of pressure or isolated cold radiator may not be an emergency, but it should still be investigated where the problem persists.

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What Can You Safely Check Yourself?

 

Without removing the boiler casing, a homeowner can usually check:

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• whether the electricity supply is on;

• whether other gas appliances are operating;

• whether the water supply is available;

• the thermostat temperature;

• programmer settings;

• the boiler pressure;

• the displayed fault code;

• visible radiator valves and pipe joints;

• whether an external condensate pipe is frozen;

• whether the boiler instructions permit a reset.

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Only repressurise or reset the boiler where you understand the manufacturer’s instructions.

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Stop if the boiler does not respond normally, pressure rises too high or the same fault returns.

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What Should You Not Attempt?

 

Do not:

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• remove the boiler casing;

• alter gas components;

• repair internal electrical parts;

• block a discharge pipe;

• dismantle the flue;

• bypass safety controls;

• repeatedly top up a leaking system;

• keep resetting a recurring fault;

• continue using a boiler that smells of gas or burning;

• attempt work that requires Gas Safe registration.

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Gas Safe Register states that gas boilers should be installed, maintained and repaired by appropriately registered professionals.

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Does the Boiler Need Repairing or Replacing?

 

Many common boiler problems can be repaired.

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Repair may be appropriate where:

• the boiler is reasonably modern;

• parts remain available;

• the fault is isolated;

• the appliance has otherwise been reliable;

• the repair cost is proportionate;

• the heating system remains suitable.

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Replacement may deserve consideration where:

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• breakdowns are frequent;

• the boiler is badly corroded;

• water is leaking from a major component;

• several expensive parts are failing;

• components are obsolete;

• heating and hot water performance are poor;

• repair costs are accumulating;

• the appliance no longer suits the property.

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The engineer should explain the fault, likely repair cost and condition of the wider boiler before recommending replacement.

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Can Boiler Servicing Prevent Problems?

 

Annual servicing cannot prevent every breakdown, but it can help identify leaks, worn seals, combustion issues, corrosion and developing component faults.

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A service also provides an opportunity to check:

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• safe combustion;

• the flue;

• ventilation;

• operating pressure;

• internal components;

• seals and connections;

• controls;

• signs of leakage;

• stored fault information.

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Regular servicing can reduce the risk of faults remaining unnoticed and may be required to maintain the boiler warranty. Gas Safe Register also advises that regularly serviced appliances can reduce the risks associated with poorly maintained gas equipment.

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Is Your Boiler Not Working or Leaking Water?

 

APN Boilers helps homeowners, landlords and property managers find experienced heating professionals for boiler problems, repairs, servicing and replacement.

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Tell us:

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• the boiler manufacturer and model;

• what the boiler is doing;

• any fault code displayed;

• whether there is heating or hot water;

• whether pressure is rising or falling;

• where any water appears to be leaking;

• whether the fault is intermittent or constant.

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Photographs of the boiler display, pressure gauge and visible leak can also help explain the problem.

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Choose the contact option that suits you best.

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