Why Is My Boiler Not Working? All Your Boiler Questions Answered – Burgess Hill & West Sussex
- BCooke Plumbing & Heating
- Mar 26
- 13 min read
Your boiler was fine yesterday. Now it's making strange noises, leaking water, or refusing to heat your home. You're Googling symptoms at 7am before work. We get it.
This guide covers the most common boiler problems homeowners face. For each one, we explain what's likely causing it, what you can safely check yourself, and when you need to call a Gas Safe registered engineer.
Some of these fixes are simple. Others need a professional. Knowing the difference saves you time, money, and a cold shower.
Why Is My Boiler Losing Pressure?
This is the single most common boiler problem we see. You check the gauge, it reads below 1 bar, and your boiler has locked out. No heating. No hot water.
Your boiler's pressure gauge should sit between 1 and 1.5 bar when the system is cold. When the heating runs, it rises slightly towards 2 bar. That's normal. The water expands as it heats up.
A drop below 1 bar triggers most modern boilers to shut down as a safety measure.
What causes it?
A leak in the system. This is the most common cause. The leak might be at a radiator valve, a
pipe joint, or somewhere under the floorboards where you can't see it. Even a tiny drip, barely enough to notice, can drain enough water over days or weeks to drop the pressure.
Walk around your house. Check every radiator valve. Look at pipe joints, especially where copper meets plastic. Check under the boiler itself. Damp patches on ceilings, walls, or skirting boards near pipes are a giveaway.
You recently bled your radiators. Bleeding releases trapped air, but it also lets out a small amount of water. The pressure drops. This is normal and easy to fix by topping up via the filling loop.
A faulty expansion vessel. Inside your boiler (or sometimes mounted externally) there's a vessel that absorbs the pressure change as water heats and expands. It contains a rubber diaphragm and a charge of air. Over time, the diaphragm can fail or the air charge leaks out. When this happens, pressure spikes when the heating runs, water gets released through the pressure relief valve, and the pressure drops when it cools down. You might notice water dripping from the overflow pipe outside.
A faulty pressure relief valve (PRV). The PRV is a safety device that opens if pressure gets too high. If it's stuck slightly open or has debris in it, it leaks water constantly. Check the small copper overflow pipe that exits through your outside wall near the boiler. If it's dripping or wet, the PRV is likely the problem.
What you can do
You can repressurise the boiler yourself using the filling loop. This is usually a silver braided hose underneath the boiler with one or two valves.
Turn off the boiler and let it cool
Locate the filling loop
Open the valve (or valves) slowly
Watch the pressure gauge rise to between 1 and 1.5 bar
Close the valves fully
Switch the boiler back on
If you have to do this more than once or twice a year, something is wrong. A boiler that needs topping up every few weeks has a leak or a faulty component. Don't keep topping it up and ignoring it. The underlying problem will get worse.
When to call an engineer
If you can't find a visible leak but pressure keeps dropping, call a Gas Safe engineer. Hidden leaks under floors or inside walls need professional detection. If the overflow pipe outside is dripping, the expansion vessel or PRV needs attention. Neither is a DIY job.
Why Is My Boiler Leaking Water?
Water around or under your boiler is never normal. Don't ignore it, even if the boiler still seems to work.
Where is the water coming from?
From underneath the boiler. Internal components have failed. The heat exchanger, pump seals, or internal valves can corrode and leak over time. You'll see water pooling on the floor beneath the unit. Turn the boiler off. Don't try to remove the casing yourself.
From the pressure relief valve overflow pipe. This is the copper pipe that goes through the wall to the outside. A steady drip or flow of water here means the PRV is releasing pressure. Usually caused by a failing expansion vessel, an over-pressurised system, or a faulty PRV itself.
From pipe connections. Joints where pipes connect to the boiler can work loose. Temperature changes cause pipes to expand and contract repeatedly. Over years, this loosens fittings.
From the condensate pipe. Modern condensing boilers produce acidic water as a byproduct. This drains away through a plastic condensate pipe, usually into an external drain. If this pipe gets blocked or cracked, you'll see water where it shouldn't be.
Is a leaking boiler dangerous?
A water leak from a boiler is not as immediately dangerous as a gas leak. But it causes damage. Water near electrical components creates a short circuit risk. Persistent leaks damage walls, floors, and ceilings. Damp encourages mould growth.
If the leak is significant, turn off the boiler and the water supply to it. Place towels and a container to catch the water. Call an engineer.
Don't try to tighten internal boiler components yourself. The casing should only be removed by a Gas Safe registered engineer.
Why Is My Boiler Making a Loud Noise?
Boilers aren't silent, but they shouldn't be loud either. A low hum during operation is normal. Anything beyond that tells you something needs attention.
Kettling (rumbling, banging, popping)
If your boiler sounds like a kettle coming to the boil, that's called kettling. It happens when limescale or sludge builds up on the heat exchanger. The buildup restricts water flow. Water gets trapped in the heat exchanger, overheats, and turns to steam. The steam bubbles collapse as they hit cooler water. That's the banging and rumbling you hear.
This is common in hard water areas. West Sussex has moderately hard to hard water depending on your exact location. Burgess Hill, Haywards Heath, and Hassocks all have relatively hard water supplies.
Left untreated, kettling reduces efficiency and shortens the life of your heat exchanger. A power flush clears the sludge from your system. Adding a magnetic filter (like a MagnaClean) and inhibitor fluid prevents it from coming back.
If limescale is the problem rather than sludge, the heat exchanger may need descaling or replacing. An engineer can tell the difference.
Buzzing or humming louder than usual
A failing pump often buzzes or hums more loudly as it struggles. The bearings wear out. Sometimes the pump speed is set too high for the system. An engineer can test pump operation and adjust or replace it.
Electrical components like the fan, PCB, or gas valve can also produce unusual buzzing sounds when they're failing.
Vibrating
Loose brackets, panels, or internal components vibrate during operation. Sometimes the boiler casing itself is loose. Check the external screws on the casing. If tightening them doesn't help, the vibration is internal and needs professional attention.
Why Is My Boiler Whistling?
A high-pitched whistling sound from your boiler is an early sign of the same problems that cause kettling. Restricted water flow through a narrowed pipe or heat exchanger makes the water whistle as it's forced through.
Think of it like partially covering the end of a garden hose. The water speeds up through the smaller opening and makes noise.
Common causes:
Limescale buildup narrowing the heat exchanger passages
Sludge restricting flow through pipes or the heat exchanger
A closed or partially closed valve somewhere in the system
Air trapped in the system
The pump running too fast for the system
If the whistling only started recently, check that all radiator valves and the valves around the boiler are fully open. Someone might have knocked one partially closed.
If all valves are open and the whistling persists, you likely need a power flush or heat exchanger inspection. Call a Gas Safe engineer.
Why Is My Boiler Making a Gurgling or Banging Noise?
Gurgling and banging are different problems, though they sometimes occur together.
Gurgling
Air trapped in your heating system causes gurgling. You'll often hear it in the radiators and pipes as well as the boiler itself. Air gets into sealed systems through micro-leaks, during maintenance, or through the automatic air vent on the boiler.
Bleeding your radiators is the first thing to try. Use a radiator key to open the bleed valve at the top of each radiator. Start with the radiator furthest from the boiler. Hold a cloth underneath. Open the valve until water flows steadily with no air bubbles, then close it. Check your boiler pressure after bleeding all radiators and top up if needed.
If you bleed the radiators and the gurgling comes back within days, air is getting into the system somewhere. A Gas Safe engineer needs to find out where.
Banging (water hammer)
Sudden banging in pipes when the heating fires up or shuts down is called water hammer. It happens when water flow stops or changes direction abruptly. Loose pipework amplifies the noise.
A faulty pump, a sticking valve, or pipework that isn't properly clipped to joists can cause this. The fix depends on the cause. Securing loose pipes sometimes solves it. Other times the pump or a valve needs replacing.
Banging from the boiler itself
Banging from inside the boiler (as opposed to the pipes) points back to kettling, a failing pump, or ignition problems. If the boiler bangs when it fires up, the gas valve or ignition system may need attention. This needs an engineer.
Why Is My Boiler Not Heating Water?
You turn on the hot tap and get cold water. The radiators might be working fine, or they might be cold too. The cause depends on which scenario you're dealing with.
Hot water not working but heating is fine
On a combi boiler, a faulty diverter valve is the most likely cause. The diverter valve directs hot water either to your radiators or to the hot water outlet. When it fails, it gets stuck in the heating position and can't switch to provide hot water when you open a tap.
A diverter valve replacement is a routine job for a Gas Safe engineer.
Heating not working but hot water is fine
This can be a faulty motorised valve (on system boilers), a thermostat problem, or a programmer/timer issue.
Check the obvious things first. Is the thermostat set high enough? Is the heating timer or programmer set to the right schedule? Has someone accidentally turned it off? Are the batteries dead in a wireless thermostat?
If the controls seem fine, the motorised valve that directs water to the heating circuit may have failed. Or there's an airlock preventing circulation through the radiators.
Neither heating nor hot water
Check the boiler display for an error code. Check the pressure gauge. Check that the gas supply is on (other gas appliances in the house should still work). Check the electricity supply to the boiler.
If everything looks normal but the boiler won't fire up, it needs professional diagnosis. The fault could be the PCB, gas valve, fan, ignition leads, or several other components.
Why Does My Boiler Keep Switching Off?
A boiler that fires up then shuts itself down after a few minutes is called "cycling" or "locking out." It's frustrating because the boiler appears to work but won't stay on long enough to heat your home.
Low water pressure
If the pressure has dropped below the minimum threshold (usually around 0.5 to 0.8 bar depending on the model), the boiler shuts down to protect itself. Check the pressure gauge and repressurise if needed. See the pressure loss section above.
Overheating
The boiler has safety devices that shut it down if it gets too hot. Restricted water flow (from sludge, a failing pump, or closed valves), a faulty thermostat, or a blocked heat exchanger can all cause overheating.
Blocked condensate pipe
In cold weather, the condensate pipe can freeze. The boiler detects the blockage and shuts down. More on this below.
Faulty thermostat
If your room thermostat is misreading the temperature, it tells the boiler to shut off too early.
Try turning the thermostat up a few degrees. If the boiler stays on longer, the thermostat may need replacing or repositioning.
Ignition failure
If the boiler tries to light but fails, it locks out after a set number of attempts. Faulty ignition leads, a dirty or worn electrode, or a gas valve problem can cause this.
Pump failure
The pump circulates water through the system. If it's failing, water doesn't move fast enough, the boiler overheats, and it shuts down. You might hear the pump struggling or notice that radiators near the boiler get hot while distant ones stay cold.
If your boiler locks out repeatedly, note the error code on the display before resetting it. The code tells the engineer exactly where to start looking.
Why Is My Boiler Showing a Fault or Error Code?
Modern boilers display error codes when something goes wrong. These codes vary by manufacturer, but they all serve the same purpose: telling you (or your engineer) what the boiler has detected.
Some common codes across popular brands:
Vaillant: F22 (low water pressure), F28 (ignition failure), F75 (pressure sensor fault)
Worcester Bosch: EA (ignition/flame fault), E9 (overheating), A1 (pump or circulation fault)
Baxi: E119 (low water pressure), E133 (gas supply fault), E168 (circuit board fault)
Ideal: F1 (low water pressure), F2 (flame loss), L2 (ignition lockout)
Before calling an engineer, write down the exact code showing on your display. Some codes have a simple fix. Low pressure codes (like F22 or E119) often just need repressurising.
Others need professional diagnosis.
Don't repeatedly reset the boiler and hope the code goes away. If it comes back after one reset, there's a real fault. Repeated resetting can cause further damage.
Your boiler's manual lists all error codes and their meanings. If you've lost the manual, search for your boiler model online. Most manufacturers have the manuals available as free PDF downloads. We've also put together a separate guide covering common boiler error codes explained in more detail.
Why Is My Condensate Pipe Frozen?
This is a winter problem. If your boiler stops working during a cold snap and shows an error code, a frozen condensate pipe is a strong possibility.
What is a condensate pipe?
Modern condensing boilers extract extra heat from exhaust gases. This process creates acidic water (condensate) that drains away through a plastic pipe, usually into an outside drain or soil pipe. Part of this pipe often runs along an external wall or through an unheated space.
Why does it freeze?
When temperatures drop below zero, water in the exposed section of the condensate pipe freezes. Ice blocks the pipe. The boiler detects the blockage and shuts down, typically showing a specific error code.
How to thaw it
You can safely thaw a frozen condensate pipe yourself.
Find the condensate pipe. It's a white or grey plastic pipe (usually 22mm) coming out of the bottom of your boiler and running to an outside drain. Identify the frozen section. It's usually the lowest point of an external run, an elbow, or where the pipe is most exposed.
Pour warm water over the frozen section. Use water from the kettle after it has cooled for a minute or two. Not boiling water. Boiling water can crack the plastic pipe. A hot water bottle held against the pipe also works. Keep pouring until the ice clears and you hear water flowing again.
Once thawed, reset the boiler. It should fire up normally.
Preventing it from happening again
Lag the external section of the condensate pipe with foam pipe insulation. This is cheap, available from any DIY store, and takes minutes to fit. Your engineer can also reroute the pipe to reduce exposed sections, or increase the pipe diameter to make freezing less likely.
During your annual boiler service, ask your engineer to check the condensate pipe route and insulation. If you're not sure what a service involves, we've covered that in detail in our guide: What Is a Boiler Service?
Why Does My Boiler Pilot Light Keep Going Out?
If you have an older boiler with a permanent pilot light (a small flame that stays lit all the time), this section is for you. Most modern boilers use electronic ignition and don't have a permanent pilot light.
Common causes
A faulty thermocouple. The thermocouple is a safety sensor that sits in the pilot flame. It detects whether the pilot is lit. If the flame goes out, the thermocouple tells the gas valve to shut off. When the thermocouple itself fails, it can't sense the flame even when it's lit, so it shuts the gas off. Replacing a thermocouple is a straightforward job for a Gas Safe engineer.
Draughts. A strong draught near the boiler can blow the pilot light out. Check for draughts from windows, doors, or vents near the boiler. This is more common in boilers installed in garages, utility rooms, or near external doors.
A dirty pilot assembly. Dust, debris, or carbon buildup on the pilot jet reduces the flame size until it can't stay lit. Cleaning the pilot assembly during a service prevents this.
Gas supply problems. If the pilot won't light at all, check that other gas appliances in your home work. If nothing works, there may be a problem with your gas supply. Check that your gas meter valve is open. If the supply is fine but the pilot won't light, the gas valve on the boiler may be faulty.
If your boiler has a permanent pilot light, it's old. Pilot lights waste gas because they burn continuously whether you need heating or not. If your pilot light keeps failing, it might be time to consider whether a new boiler would be more economical in the long run.
Should I Repair or Replace My Boiler?
This is the question that comes up eventually with every boiler. There's no single answer, but there are clear signals that point one way or the other.
When repair makes sense
Your boiler is under 10 years old. The repair is affordable relative to a replacement. The fault is a one-off, not a recurring problem. The boiler is still under warranty. Replacement parts are readily available for your model.
When replacement makes sense
Your boiler is over 15 years old. You're paying for repairs more than once a year. The repair quote is close to (or more than) the cost of a new boiler. Your boiler has an efficiency rating of D or below (older boilers can be as low as 60-70% efficient). The manufacturer has stopped making parts for your model. Your energy bills keep climbing despite regular servicing.
The numbers
A modern condensing boiler runs at 90%+ efficiency. If your old boiler runs at 70% efficiency, you're wasting 20p of every pound you spend on gas. Over a year, that adds up to hundreds of pounds on your energy bills alone.
Factor in the cost of repeated repairs on an ageing boiler, and a new installation often pays for itself within a few years. And you get a warranty (typically 5 to 10 years) that covers parts and labour.
If you're unsure, ask your engineer for an honest opinion during your next boiler service. A good engineer will tell you whether a repair is worthwhile or whether you're throwing money at a boiler that's on its way out.
Quick Reference: Boiler Problems at a Glance
Problem | Likely Cause | DIY or Engineer? |
Pressure dropping | Leak, expansion vessel, PRV | DIY to top up. Engineer if it keeps dropping. |
Leaking water | Internal component failure, loose joints | Engineer |
Loud banging/rumbling | Kettling, sludge, limescale | Engineer (power flush) |
Whistling | Restricted water flow, limescale | Engineer |
Gurgling | Air in system | DIY (bleed radiators). Engineer if it returns. |
No hot water | Diverter valve, thermostat, controls | Check thermostat first. Then engineer. |
Keeps switching off | Low pressure, frozen condensate, thermostat | Check pressure and condensate pipe. Then engineer. |
Error code showing | Various faults | Note the code. Engineer. |
Frozen condensate pipe | Sub-zero temperatures | DIY (warm water). Lag the pipe to prevent it. |
Pilot light going out | Thermocouple, draught, dirty pilot | Engineer |
Need a Boiler Repair in Burgess Hill or West Sussex?
B.Cooke Plumbing & Heating is based in Burgess Hill and covers Haywards Heath, Hassocks, Hurstpierpoint, and surrounding towns across West Sussex.
All our engineers are Gas Safe registered. We carry common parts on our vans so many repairs are completed in a single visit.
Whether your boiler is losing pressure, leaking, making odd noises, or won't fire up, we can diagnose and fix the problem.
Comments