FAQs
Boiler repair: questions answered
When the boiler stops, leaks, loses pressure or shows a fault code.
Boiler repair
Why does my boiler keep losing pressure and need topping up every week?
A boiler that needs refilling every week almost always has water escaping somewhere. Common causes are a small leak on a radiator valve or pipe, a failed expansion vessel, or a pressure relief valve letting by onto the outside pipe. Topping up only hides the fault. A Gas Safe engineer can pressure test and find it.
What pressure should my combi boiler be at, and how do I top it up?
Most sealed combi systems sit between about 1 and 1.5 bar when cold, but check the figure in your boiler manual. To top up, open the filling loop slowly while watching the gauge, close it fully at the right pressure, then reset the boiler. If it keeps dropping, book a repair.
My radiators get hot but there is no hot water from the taps. What is wrong?
On a combi boiler, heating without hot water usually points to the diverter valve, a blocked or failed flow sensor, or a scaled plate heat exchanger. Check the hot water temperature setting and that the boiler shows no fault code first. These parts sit inside the sealed boiler case, so the repair needs a Gas Safe engineer.
My boiler stopped in freezing weather and shows a fault. Is it the condensate pipe?
Very likely. Condensing boilers drain water through a plastic condensate pipe, and if the outside section freezes the boiler locks out, often with a gurgling sound. Pour warm, not boiling, water along the outside pipe, then reset the boiler. If it keeps freezing, the pipe may need rerouting, insulating or a larger diameter.
My boiler is showing a fault code. What does it mean and can I reset it?
Each brand uses its own codes, so the same number can mean different things on a Worcester Bosch, Baxi, Ideal or Vaillant. One reset is reasonable for a lockout after checking pressure and the condensate pipe. If the code returns, stop resetting it and get the boiler checked, because repeated lockouts can point to an ignition or gas supply fault.
Water is dripping from underneath my boiler. Is it safe to keep using it?
Switch the boiler off at the fused spur, put a container underneath and keep water away from electrics. Drips can come from a perished seal, a failing pressure relief valve, a leaking pump or a blocked condensate trap backing up. Do not remove the boiler case yourself. A Gas Safe engineer needs to find the source.
Why is my boiler making a loud banging or kettling noise?
Banging or kettling usually means water is overheating inside the heat exchanger because flow is poor. Typical causes are sludge or limescale build up, a failing pump, low system pressure or air in the system. Leaving it can damage the heat exchanger, so have an engineer check flow, the pump and whether the system needs cleaning.
Why are my radiators hot at the top and cold at the bottom?
Hot at the top and cold at the bottom normally means sludge has settled in the radiator, so bleeding it will not help. Cold at the top means trapped air, which bleeding does fix. If several radiators are cold at the bottom, the system may need cleaning and a magnetic filter, which a heating engineer can advise on.
Why does my boiler keep switching on and off every few minutes?
Frequent on and off cycling, known as short cycling, can be caused by a boiler that is oversized for the house, poor water flow, a faulty thermostat or a sensor fault. It wastes gas and wears parts faster. An engineer can check the boiler's output settings, controls and system flow and adjust or repair as needed.
Is it worth repairing a 10 to 15 year old boiler, or should I replace it?
It depends on the fault, the parts price and the boiler's history. A single fault on a well serviced boiler is often worth fixing. Repeated breakdowns, a failed heat exchanger or parts that are hard to get usually make replacement better value. A Gas Safe engineer can give you a repair price to weigh against a new boiler.
An engineer capped off my boiler as unsafe. What happens now?
If a Gas Safe engineer classes an appliance as Immediately Dangerous or At Risk, they can turn it off and label it, with your permission or where required by the gas industry unsafe situations procedure. Do not reconnect it. Ask for the written warning notice, then have the fault repaired by a Gas Safe engineer before using it again.
I can smell gas near my boiler. What should I do?
Call the National Gas Emergency Service on 0800 111 999 straight away. Turn off the gas at the meter if you can, open doors and windows, do not use light switches or flames, and leave the property if the smell is strong. Once the emergency service has made it safe, a Gas Safe engineer can repair the fault.
I rent my home. Who is responsible for fixing the boiler?
In most tenancies in England the landlord must keep the heating and hot water installation in repair and working order, and must arrange gas safety checks. Report the fault to your landlord or letting agent in writing. Tenants should not arrange gas repairs themselves unless the landlord agrees, but can always call 0800 111 999 if they smell gas.
My boiler has stopped working. What should I check first?
Start with three things: power, gas and water pressure. Check the boiler display or light is on and the fused spur switch is on, that other gas appliances work and a prepay meter has credit, and that the gauge reads about 1 to 1.5 bar. If those are fine, call a Gas Safe engineer.
Why is my boiler display completely blank?
A blank display usually means the boiler has no power. Check the fused spur switch next to the boiler is on, look for a tripped circuit breaker or RCD at the consumer unit, and check other sockets work. If power is present and the boiler is still dead, a Gas Safe engineer needs to investigate.
My boiler keeps tripping the electrics. What should I do?
Leave the circuit off and do not keep resetting it, because repeated tripping points to an electrical fault in the boiler, its pump or fan, or the heating controls wiring. Switch the boiler off at the fused spur and book a Gas Safe engineer, who may also involve an electrician for the house wiring.
My prepayment meter has credit but the boiler still will not light. Why?
After a prepay meter runs out it may need the button pressing or the valve resetting before gas flows again, as the meter instructions describe. Light the hob to confirm gas is reaching the house. If the hob works but the boiler still locks out, reset it once, then call a Gas Safe engineer.
Why does my boiler make a gurgling noise when it tries to light and then lock out?
A gurgling sound as the boiler tries to fire often points to a blocked or frozen condensate pipe, because the water cannot drain away. In cold weather, find the outside plastic pipe and thaw it with warm (not boiling) water, then reset once. If it keeps happening, ask a Gas Safe engineer to check the trap and pipe.
How do I thaw a frozen condensate pipe?
Find the plastic pipe running from the boiler to a drain outside; it usually freezes at the open end or a bend. Pour warm, not boiling, water over it or hold a hot water bottle on it until it clears, then reset the boiler once. Never cut the pipe or open the boiler.
How can I stop my condensate pipe freezing every winter?
Ask a Gas Safe engineer to check that the external pipe is the correct diameter, has a good fall, is as short as possible outside and is insulated. Rerouting it internally to a drain is often the lasting fix. Manufacturer guidance sets the sizes, so this is work for an engineer, not a DIY job.
Why does my boiler pressure go up when the heating is on?
Water expands as it heats, so a small rise is normal. A big jump, close to 3 bar, usually means the expansion vessel has lost its air charge. Other causes include a filling loop that is letting water by. Do not drain water yourself from the boiler. Book a Gas Safe engineer to check the vessel.
My boiler pressure keeps rising even when the boiler is off. What causes that?
If pressure climbs while the boiler is cold, mains water is usually getting into the heating system, often through a filling loop that has not fully closed or, on combis, an internal heat exchanger fault. Check the filling loop valves are fully shut. If it still rises, a Gas Safe engineer needs to diagnose it.
Water is dripping from a small copper pipe outside. Is that my boiler?
It is probably the pressure relief valve discharge pipe. It drips or runs when system pressure gets too high, often because of a flat expansion vessel or overfilling. Check the gauge. If it is high, do not top up further and book a Gas Safe engineer, as the valve may also need replacing.
What is an expansion vessel on a boiler?
It is a tank with an air cushion on one side that absorbs the extra volume of heating water as it warms up. When the air charge is lost, pressure swings high when hot and the relief valve can discharge. Checking and recharging it means working inside the boiler, so it is an engineer job.
Can I recharge my boiler's expansion vessel myself?
We recommend you do not. Recharging properly involves isolating and draining the boiler, opening it and setting the charge to the manufacturer's figure, then refilling. It is part of a standard service check. A Gas Safe engineer can test whether the vessel holds its charge or needs replacing.
Why are my radiators cold at the top and warm at the bottom?
That usually means air has collected at the top. With the heating off and cool, use a radiator key on the bleed valve at the top corner, open it slowly with a cloth ready and close it once water appears. Then check the boiler gauge and top up if it has dropped.
Only one or two radiators are not heating. What could be wrong?
Check the valves at both ends are open and that a thermostatic valve is not set low or stuck. Bleed the radiator if the top is cold. If it is still cold, the cause may be a stuck valve pin, a blockage or a balancing issue, which a heating engineer can sort.
How do I bleed a radiator safely?
Turn the heating off and let the radiators cool. Hold a cloth under the bleed valve, turn the key a quarter turn until air hisses, and close it as soon as water comes out. Afterwards check the boiler pressure gauge, because bleeding lowers pressure on sealed systems, and top up if needed.
Do I need to top up the boiler after bleeding radiators?
On a combi or sealed system, often yes, because releasing air lets pressure fall. Check the gauge with the boiler cold and top up to the figure in your user guide using the filling loop. Tank-fed open-vented systems refill themselves from the feed and expansion tank in the loft.
Why does my boiler need a condensate pump?
Condensing boilers produce acidic water that must drain away. Where there is no suitable drain nearby, a small pump lifts it to one. If the pump fails, the boiler can lock out. Check the pump has power and is not overflowing, then call an engineer if it is not running.
Why does my combi hot water go hot then cold?
Fluctuating hot water can come from low flow at the tap, a blocked plate heat exchanger from limescale, or a failing sensor or diverter valve. Check whether it happens at every tap. If it does, book a Gas Safe engineer. In hard water areas, scale protection on the mains feed can help.
Why does my combi only give hot water when I fully open the tap?
Combis need a minimum flow before they detect a demand and fire. If you only get heat with the tap fully open, the flow sensor or filter may be partly blocked, or mains pressure may be low. Check other taps, then ask a Gas Safe engineer to look at the boiler.
My combi boiler is not doing heating or hot water. What is the likely cause?
If both have stopped, look for a common cause: no power, no gas, low pressure or a lockout code. Note any code or flashing light, do the safe checks and reset once. If it will not run, book a Gas Safe engineer and give them the code.
Why is my boiler making a high-pitched whistling noise?
Whistling can come from restricted water flow, air in the system, or a combustion or gas valve issue inside the boiler. Bleed radiators and check pressure first. If the noise comes from the boiler itself while firing, stop using it and book a Gas Safe engineer, as combustion faults need proper testing.
Why are my pipes banging when the heating comes on?
Banging pipes are often caused by loose pipe clips, air in the system, or expansion as pipes heat and rub against joists. Bleeding radiators can help. If the noise comes with low pressure or a fault code, or sounds like it is in the boiler, ask a heating engineer to look.
I see green or white crust around a radiator valve. Is that a problem?
Green or white crust usually marks a slow leak where heating water has evaporated and left deposits. It can explain pressure loss on a sealed system. Note which valves are affected. A heating engineer can repack or replace the valve and recheck the system pressure.
Do I need to keep my heating on when I go away in winter?
Leave the boiler powered with frost protection or the heating on a low setting so pipes and the condensate pipe do not freeze. Check the pressure before leaving. If you are away for a long time, ask a neighbour to check the property, and know where your stopcock is.
Why does my boiler fire up then cut out after a few seconds?
This can be a flame detection problem, a gas pressure issue, a blocked condensate or flue, or a fan or sensor fault. Note any code. Check other gas appliances work and the condensate pipe is not frozen, reset once, then book a Gas Safe engineer if it repeats.
My heating is on but the house never gets warm. Why?
Check the room thermostat setting and position, radiator valves and the boiler flow temperature. Cold spots on radiators suggest air or sludge. If the boiler runs constantly without heating the house, it may be undersized, poorly balanced or faulty. A heating engineer can check.
What flow temperature should I set on my combi boiler?
Lower flow temperatures, often around 50 to 60°C for radiators, help condensing boilers run more efficiently, but your home may need more in very cold weather. Your boiler's user guide gives the range. If rooms will not heat at lower settings, ask an engineer to review the system.
How do I find the model and serial number of my boiler?
Look for the data badge, usually on the underside or behind the front panel flap, and in the user guide. It lists the model, gas council (GC) number and serial number. Give these with any fault code when you book, so the engineer brings the right parts.
How can I tell how old my Worcester Bosch boiler is?
Worcester serial numbers include a three-digit fabrication date (FD) code showing the month and year the boiler was made. You will find it on the data badge. Worcester's support team or your engineer can decode it. It is useful for warranty and parts queries.
What is a GC number on a boiler?
The GC (gas council) number identifies the exact boiler model and gas type. Engineers use it to find the correct manual, settings and spare parts. It is printed on the data badge and in the user guide, so note it down when reporting a fault.
What is a filling loop?
A filling loop is the flexible hose or built-in key and valve under a combi or system boiler that connects mains water to the heating, letting you top up pressure. It should be fully closed afterwards. Some are detachable and must be stored away after use.
What is a diverter valve in a combi boiler?
The diverter valve switches the boiler's heat between the radiators and the hot water. If it sticks, you may get hot water but no heating, or warm radiators while running a tap. It is an internal part that a Gas Safe engineer would test and replace.
What does the circulating pump do in a heating system?
The pump moves hot water from the boiler around the radiators and back. If it fails or seizes, radiators stay cold and the boiler can overheat or show a pump code. Pump replacement involves isolating and draining parts of the system, so leave it to an engineer.
What does balancing radiators mean?
Balancing adjusts the lockshield valves so water is shared evenly, letting radiators furthest from the boiler heat as well as those nearest. If some rooms warm quickly while others lag, balancing can help. An engineer can do it with temperature readings.
Should I use radiator leak sealer?
Leak sealers can stop tiny hidden leaks, but some manufacturers restrict their use and the wrong product can harm a boiler. Ask your engineer to check the manufacturer's guidance first. A visible leak should always be fixed properly rather than sealed.
What information should I give when booking a boiler repair?
Give the boiler make, model and GC number, the fault code or light pattern, when it started, and what you have already checked such as pressure and gas supply. Mention any leaks, noises or smells. This helps the engineer bring the right parts. CombiCert can be reached on 07393 220694.
Can an engineer tell if my gas supply is low at the boiler?
Yes. A Gas Safe engineer measures gas pressure at the boiler and the meter with a gauge, while the boiler runs. Low pressure can cause ignition lockouts. If the meter supply is low, the gas network operator may need to attend.
Can I drain my central heating system myself?
On a tank-fed system it is possible but often messy and can introduce air. On sealed systems it affects pressure and inhibitor. Draining is usually done by an engineer during repairs or upgrades, who refills, vents and re-doses inhibitor properly.
What causes an airlock in a heating system?
Airlocks form when air gets trapped in pipes or radiators, often after draining, a repair or very low pressure. Symptoms include cold radiators and gurgling. Bleeding radiators helps. Persistent air may point to a pump, vent or pipework issue for an engineer.
What does the red zone on my boiler pressure gauge mean?
Many gauges mark a safe band, often green, around 1 to 2 bar, with red areas for too low and too high. Below the band, top up per your guide. Near 3 bar, do not top up and call a Gas Safe engineer, as the relief valve may soon open.
My boiler pressure gauge reads zero. Is the gauge broken?
It may be genuinely empty after a leak or relief valve discharge, or the gauge may have failed. Try topping up slowly and watch the needle. If it does not move while water flows in, stop and call an engineer, as the gauge or a pressure sensor may need replacing.
Can I paint my radiators?
Yes, with a heat-resistant radiator paint, when the heating is off and cool. Avoid painting over the bleed valve and TRV heads. Metallic paints can reduce heat output slightly. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
Why is one radiator always hotter than the rest?
Usually the system is unbalanced, with the nearest radiators taking more flow. Adjusting lockshield valves spreads the heat. A heating engineer can balance the system properly with temperature readings. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
What does frost protection on my boiler do?
Frost protection fires the boiler or runs the pump if the temperature inside the boiler drops very low, to stop it freezing. It only works if the boiler has power and gas. It does not protect external condensate pipes from freezing.
Can I fit a new radiator myself?
Changing a radiator is plumbing work rather than gas work, but it involves draining or isolating, refilling and venting, and on sealed systems re-pressurising and inhibitor. If unsure, ask a heating engineer. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
Can CombiCert repair a boiler brand it does not install?
Yes, engineers repair most common gas boiler brands, subject to parts availability. Give the make, model and fault code when you call 07393 220694. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
Why has my boiler lost pressure after an engineer visit?
Pressure can drop slightly after work as air vents out. Top up to the recommended level once cold. If it keeps dropping, contact the engineer. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
What is the hot water flow sensor in a combi?
It detects water flowing when you open a hot tap so the boiler fires. If it fails, the boiler may not respond to taps. It is an internal part for a Gas Safe engineer. Give the engineer your boiler make and model, as flow sensors differ between models.
What is a plate heat exchanger in a combi?
It transfers heat from the heating water to the mains water for hot taps. Limescale can block it, causing fluctuating or poor hot water. It is an internal part replaced by a Gas Safe engineer. Never remove the boiler casing yourself, as anything inside it is a job for a Gas Safe registered engineer.
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CombiCert Ltd, Gas Safe registered 621146, The Old School House, Park Lane, Telford, Shropshire TF3 4TG. Open Monday to Saturday 08:00 to 19:45, closed Sunday. Email hello@combicert.co.uk.
Smell gas? Call the National Gas Emergency Service on 0800 111 999.