What the symptom usually means
Boilers rarely fail without warning. They give you a sound, a reading or a code first, and knowing which tells us what to bring.
No hot water, but the heating works
On a combi this usually points at the diverter valve or the hot water sensor. With a cylinder, the motorised valve, cylinder thermostat or immersion. Normally a part, not a new boiler.
Pressure keeps dropping
A sealed system holds its pressure. If the gauge falls week after week, water is going somewhere: a weeping radiator valve, a pinhole under the floor, or a failed expansion vessel.
Banging, gurgling or kettling
Kettling is scale or sludge on the heat exchanger boiling water in the wrong place. Gurgling is usually air. Neither fixes itself, and both make the boiler work harder.
Water under or around the boiler
Switch a leaking boiler off. It may be a seal, a connection or a corroded heat exchanger — one is a cheap repair, one is usually the end of the boiler.
An error code on the display
Codes vary by manufacturer, so the same number means different things on different boilers. Write it down before you reset. One reset is reasonable; if the fault returns, stop.
Radiators cold in the wrong places
Cold at the top, warm at the bottom means air, and bleeding sorts it. Cold at the bottom means sludge settled in the radiator, which needs flushing.
Four things you can safely check first
A fair number of call-outs turn out to be something the homeowner could have sorted in five minutes. All four are outside the casing — nothing here asks you to open a boiler.
The pressure gauge
Most sealed systems sit between roughly 1 and 1.5 bar when cold. Below the marked range, the boiler may lock out. Topping up through the filling loop is a normal homeowner job and the manual shows where it is. Doing it often means a leak.
The thermostat and the programmer
Flat batteries in a wireless thermostat, a schedule lost to a power cut and the clocks changing all cause call-outs that are not the boiler. Set the thermostat well above room temperature and see whether it fires.
Power and the gas supply
Check the fused spur next to the boiler is on and the consumer unit for a tripped breaker. On a prepayment meter, check the credit, and check other gas appliances still work.
The condensate pipe in cold weather
A condensing boiler drains through a plastic pipe, often through an external wall. In a freeze it blocks with ice and the boiler shuts down. A warm — not boiling — cloth or jug of water along the outside pipe usually clears it.
When to stop and call
Some symptoms are not a fault to investigate. They are a reason to switch the boiler off.
- You can smell gas — leave and call 0800 111 999 first
- Soot or scorch marks around the boiler casing
- A yellow or orange flame rather than crisp blue
- The boiler is leaking, or water is near its electrics
- The same error code comes back after one reset
- Headaches or drowsiness that ease when people leave the property
- The casing is off, or someone has already removed it
If you can smell gas, open the windows, leave the electrical switches alone, get out and call 0800 111 999. For a leak that is water rather than gas, see our emergency plumber page.
Repair or replace
There is no neat rule, and anyone who gives you one is selling a boiler.
- Age. Under about eight years old, a repair usually makes sense.
- Parts. Discontinued parts can cost more than the boiler is worth.
- What failed. A fan, pump, diverter valve or PCB is a repair; a corroded heat exchanger is not.
- History. Three call-outs in two winters is a pattern, not bad luck.
- The system around it. Sludge will kill a new boiler as happily as the old one.
We will tell you which side of the line your boiler falls on.
The annual service is the cheap bit
A boiler serviced once a year has its faults found in September rather than mid-January. Most manufacturers also require it to keep the warranty valid.
A service is not the same as a landlord gas safety check. The HSE is clear you should not assume one covers the other: the check confirms the appliance is safe on the day; maintenance is a separate duty.
If you let the property, that check is due on every landlord-provided gas appliance every 12 months.
Gas Safe registered engineers
Hem is not Gas Safe registered. Gas work booked through us is carried out by Gas Safe registered engineers we work with. Hem is NAPIT registered, 64777, and fully insured.
Inside the M25 and just outside
One team, house, flat or portfolio.
From £75 an hour for members
Against our standard rate of £120, with no call-out fee on any plan.
Boiler questions we get asked
My boiler has stopped. What should I check before calling?
The pressure gauge, the thermostat and programmer, and the power — including the consumer unit and the fused spur beside the boiler. In freezing weather, check the condensate pipe outside. Then note any error code and call 020 4634 2681.
Why does my boiler keep losing pressure?
Because water is escaping somewhere: a weeping radiator valve, a leak on buried pipework, or a failed expansion vessel venting through the pressure relief pipe outside. Topping it up masks the problem rather than fixing it.
What does kettling mean?
The banging or rumbling of limescale or sludge on the heat exchanger, making water boil against it instead of flowing past. London water is hard, so it is common here. The fix ranges from a chemical clean to replacing the exchanger.
Should I repair or replace my boiler?
It depends on age, parts availability, what failed and how often. A fan or diverter valve on an eight-year-old boiler is worth repairing. A corroded heat exchanger, or a third repair in two winters on an older unit, usually is not.
How often should a boiler be serviced?
Once a year is the standard advice, and most manufacturers require it to keep the warranty valid. It is when small faults get caught before they become a January breakdown.
Is a service the same as a gas safety certificate?
No. The landlord gas safety check, the CP12, confirms the appliance is safe on the day; a service is maintenance. The HSE is explicit that one does not cover the other.
Are your engineers Gas Safe registered?
Hem is not Gas Safe registered. Gas work booked through us is carried out by Gas Safe registered engineers we work with, and you can ask to see the card on the day. Hem is NAPIT registered for electrical work, 64777, and fully insured.
Also see gas servicing, gas safety certificates, plumbing and membership.
The landlord gas duties on this page come from the HSE guidance Gas safety for landlords and letting agents and were last checked on 12 September 2026. It is a summary, not legal advice. Landlords remain responsible for confirming their own obligations.