Boiler, Heating & Plumbing Fault Guides
Symptom-by-symptom fault guides written by a Gas Safe registered engineer: what causes it, what we test on site, and what is safe to check yourself.
Pick the fault that matches what your system is doing. Each page explains the realistic causes in the order an engineer works through them, the on-site tests involved, and the two or three things you can safely check before you call.
Boiler Won't Fire Up
The controls call for heat, the pump may hum, but the burner never lights and nothing gets warm.
combiBoiler Fan Not Running
The boiler tries to start, you hear a click, then it stops without the usual whoosh of the fan spinning up.
combiBoiler PCB Failure
Random lockouts, a display that behaves oddly, or a boiler that ignores the controls entirely.
combiBoiler Overheating and Locking Out
The boiler runs hot for a minute or two, then shuts down on a fault before the room reaches temperature.
combiBoiler Leaking Water
Water pooling under the boiler, staining on the wall below it, or pressure that drops every day.
combiBoiler Pressure Too High
The gauge sits well above 2.5 bar when hot, and water occasionally discharges from a pipe outside.
combiPressure Relief Valve Discharging Outside
A copper pipe on the outside wall keeps dripping, sometimes only when the heating has been on.
systemExpansion Vessel Failure
Pressure that swings dramatically between cold and hot, often with a relief valve dripping outside.
combiBoiler Short Cycling On and Off
The burner lights, runs for a minute or two, drops out and relights, over and over.
systemBoiler Losing Pressure Overnight
You top the system up, everything is fine, and by morning the gauge has sunk again.
combiStuck Diverter Valve
Hot water that is lukewarm, briefly hot then cold, or only works when the heating is off.
combiScaled Plate Heat Exchanger
Hot water that has weakened gradually over a year or two, and runs hotter at low flow than at full flow.
cylinderHot Water Running Too Hot
Taps deliver water hot enough to be genuinely unsafe, particularly at the bath.
cylinderHot Water Runs Cold After a Few Minutes
The shower starts hot and turns cold long before it should, or the bath never fills hot.
cylinderImmersion Heater Not Working
The immersion switch is on but the cylinder stays cold, or it trips the consumer unit.
cylinderUnvented Cylinder Discharging Water
Water running from the tundish under the cylinder, either continuously or after each heating cycle.
waterNo Water Upstairs but Fine Downstairs
Kitchen tap runs normally, bathroom taps upstairs trickle or give nothing at all.
pipeworkBanging Pipes and Water Hammer
A sharp bang or shudder through the pipes when a tap or appliance shuts off.
bathroomDripping Tap Repair
A steady drip from a basin, bath or kitchen tap that carries on after the tap is fully closed.
bathroomToilet Constantly Running Into the Pan
A trickle of water running into the pan long after the flush, often with a faint hiss.
bathroomBlocked Toilet That Won't Clear
The pan fills and drains slowly, or won't clear at all after flushing.
bathroomPoor Shower Pressure
A weak or spluttering shower, especially noticeable when another tap is running.
bathroomElectric Shower Not Heating
Water flows but stays cold, or heats only briefly before going cold again.
radiatorsRadiators Cold at the Top
The bottom of the radiator is hot and the top stays cool, so the room never quite warms up.
radiatorsOne Radiator Not Heating When Others Are
Every other radiator is hot, but one stays cold all the way through.
radiatorsSludge in the Central Heating System
Cool patches at the bottom of radiators, black water when bled, and a boiler that struggles in cold weather.
controlsThermostatic Radiator Valve Not Working
A room that overheats regardless of the number on the valve, or a radiator that never comes on.
controlsRoom Thermostat Not Calling for Heat
The thermostat display is on and the setpoint is high, but the boiler never starts.
controlsSmart Thermostat Not Connecting
The app shows the thermostat offline, or schedules don't reach the boiler.
controlsHeating Comes On When It Shouldn't
Radiators heating in the middle of the night or during a hot afternoon.
systemZone Valve Failure
One zone or the hot water won't heat, or a valve buzzes continuously in the airing cupboard.
systemNoisy Central Heating Pump
A humming, grinding or vibrating noise from the airing cupboard whenever the heating runs.
combiBoiler Making a Kettling Noise
A rumbling or boiling-kettle sound from the boiler, usually worse as it reaches temperature.
radiatorsGurgling Radiators
A trickling or gurgling sound from radiators when the heating starts up.
pipeworkFrozen or Burst Outside Tap
No flow from the garden tap after a cold snap, or water appearing inside the wall when it's opened.
pipeworkBurst Pipe Emergency
Water escaping from a pipe, often after freezing weather, with damage building by the minute.
pipeworkLeak Under the Floor
Warm or damp patches on the floor, unexplained pressure loss, or the sound of running water with all taps off.
pipeworkSeized Main Stop Tap
The internal stop tap won't turn, so you have no way to isolate the water in an emergency.
pipeworkAirlock in Hot or Cold Pipework
A tap that spits, runs briefly then stops, or gives nothing at all after work on the system.
airconAir Conditioning Blowing Warm Air
The indoor unit runs and the fan blows, but the air isn't cold.
airconAir Conditioning Unit Leaking Water Indoors
Water dripping from the indoor wall unit onto the floor or furniture below.
airconNoisy Outdoor Air Conditioning Unit
Rattling, humming or vibration from the outdoor condenser, sometimes worse at certain speeds.
airconBad Smell From Air Conditioning
A musty or sour smell whenever the indoor unit starts.
airconAir Conditioning Not Heating in Winter
The system cools fine in summer but delivers little or no heat when set to heating mode.
airconAir Conditioning Remote Not Working
The indoor unit ignores the handset, or only responds from very close range.
systemHouse Won't Warm Up Even With the Heating On
The boiler runs for hours, radiators are warm, but rooms never reach a comfortable temperature.
combiBoiler Showing a Fault Code
The boiler has stopped and a letter-and-number code is showing on the display.
combiBlocked Condensate Pipe
The boiler stops with a fault, sometimes with a gurgling sound, most often during a cold spell.
combiBoiler Flue Problem or Blockage
Staining around the flue terminal, condensation streaks on the wall, or a boiler locking out on a flue-related code.
combiCarbon Monoxide Alarm Going Off
The CO alarm has sounded, whether or not anyone feels unwell.
combiWhat Happens When a Gas Appliance Is Condemned
An engineer has classified your appliance as At Risk or Immediately Dangerous and it has been turned off.
combiFailed Landlord Gas Safety Check
A CP12 inspection has recorded a defect and you need to know what happens next.
controlsBoiler Not Responding to Controls
Turning the thermostat or programmer up makes no difference at all.
combiOld Boiler With Obsolete Parts
A repairable fault, but the part is no longer manufactured or supported.
combiBoiler Cuts Out in Cold Weather Only
The boiler is fine most of the year, then fails during the first hard frost.
bathroomShower Temperature Fluctuating
The shower runs hot then cold when another tap or the toilet is used.
pipeworkWashing Machine or Dishwasher Waste Leaking
Water under the kitchen units whenever an appliance drains.
bathroomSmelly Drain in the Bathroom
A drain or sewer smell around the basin, bath or shower, sometimes worse in warm weather.
bathroomBathroom Extractor Not Clearing Steam
Persistent condensation, black mould at ceiling corners and mirrors that stay fogged.
radiatorsHeated Towel Rail Not Heating
The bathroom rail stays cold while the rest of the house heats normally.
systemUnderfloor Heating Not Warming Up
The floor stays cool despite the system running and the manifold being warm.
combiBoiler Cuts Out When a Hot Tap Is Opened
Heating is fine, but opening a hot tap makes the boiler stop rather than fire.
combiBoiler Service Overdue — What Happens Next
The annual service has slipped, and you're wondering whether it matters.
radiatorsNew Radiator Not Getting Hot After Installation
A recently fitted radiator stays cold or heats far less than the others.
systemHot Water Works but Heating Doesn't
The cylinder heats normally, but the radiators stay cold whatever the controls say.
combiCombi Boiler Poor Hot Water Flow Rate
Hot water is hot enough but the volume is disappointing, especially in the shower.
systemHeating Slow to Warm Up
Rooms eventually get warm, but it takes far longer than it used to.
combiBoiler Display Is Blank
No lights, no display and no response to any control.
cylinderHot Water Smells or Discolours
A metallic, eggy or musty smell from hot taps, sometimes with discoloured water.
systemTenant Left Without Heating or Hot Water
A rented property has lost heating or hot water and you need it resolved quickly and properly documented.
combiBoiler Bangs or Booms on Start-Up
A distinct bang or whump when the burner lights, rather than a smooth ignition.
radiatorsRadiator Leaking at a Joint
A damp patch or drip at the point where the valve meets the radiator.
pipeworkKitchen Sink Draining Slowly
Water sits in the bowl and drains away far more slowly than it should.
pipeworkOutside Drain Overflowing
Water rising in a gully or inspection chamber, sometimes with foul water backing up indoors.
controlsNo Heating After a Power Cut
Everything else in the house works, but the heating hasn't come back on.
combiGas Hob Not Lighting
Burners click but won't light, or light and immediately go out.
combiGas Fire Not Working Safely
A gas fire that won't light, goes out, or has started to soot up the surround.
controlsBoiler Flow Temperature Set Incorrectly
High gas bills, radiators that are scalding to touch, or a condensing boiler that never actually condenses.
systemMagnetic Filter Full of Black Sludge
The filter bowl is packed with black magnetite at every service.
combiCondensate Pump Failure
The boiler locks out and there's water around the pump unit near the appliance.
radiatorsRadiators Hot Upstairs but Cold Downstairs
The upstairs rooms are warm while the ground floor never quite gets there.
combiGas Smell Near the Meter or Pipework
A gas odour around the meter box, a pipe run or an appliance connection.
combiBoiler Vibrating or Humming Loudly
A hum or buzz you can feel through the wall, often worse at night when the house is quiet.
airconAir Conditioning Tripping the Electrics
The RCD or breaker trips when the system starts, or after it has been running for a while.
airconPoor Airflow From Air Conditioning
The unit runs and cools slightly, but very little air comes out of the vents.
bathroomLeak Through the Ceiling Below a Bathroom
A stain, bulge or drip in the ceiling under a bathroom or en-suite.
cylinderFaulty Cylinder Thermostat
Hot water that either never satisfies or never reaches temperature.
systemHeating Working but Gas Bills Too High
Nothing is broken, but the running cost has climbed noticeably.
pipeworkMicrobore Pipework Problems
Radiators on 8mm or 10mm pipework that block, restrict or heat poorly.
combiCombi Boiler Preheat Not Working
A noticeable delay before hot water arrives, where there didn't used to be one.
pipeworkOutside Tap With No Flow
Nothing comes out of the garden tap even though inside taps are fine.
bathroomWet Room Draining Slowly
Water pooling on the floor of a wet room instead of clearing to the drain.
systemBoiler Undersized for the Property
Heating and hot water compete, showers weaken when radiators run, and the house never quite keeps up in cold weather.
radiatorsCold Loft Room or En-Suite
A converted room at the top of the house that never gets properly warm.
systemBoiler Beyond Economic Repair — What Now
An engineer has said the repair cost is close to the value of the appliance and you need to decide.