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Most heating problems in a home with a combi boiler come back to four things: air, sludge, pressure and settings. Understand those and you can sort out a fair amount yourself, describe the rest clearly to an engineer, and recognise when someone is selling you a job you don't need.
How a wet central heating system works
Picture a loop. The boiler heats water, and a pump inside the casing pushes it out along the flow pipe. The water passes through each radiator, gives up some of its heat to the room, then travels back along the return pipe to be heated again. Nothing is used up; the same water goes round for years.
In a sealed system, which is what almost every combi runs, that loop is closed off from the mains and kept at a modest pressure. An expansion vessel, a small steel tank with a rubber diaphragm, usually tucked inside the boiler, absorbs the swelling of the water as it warms. If pressure ever climbs too high, a relief valve lets water out through a small copper pipe that ends outside the wall.
The "combi" part means the same boiler also heats tap water straight from the mains, using a separate heat exchanger, so there is no hot water cylinder. While a hot tap runs, most combis pause the heating, which is why radiators can cool a little during a long shower.
Sludge, and why radiators go cold at the bottom
Steel radiators and iron fittings corrode slowly from the inside. The result is magnetite, a fine black iron oxide that looks like wet soot when you drain a radiator. It is heavy, so it settles along the bottom of each panel, blocking the waterways and stopping hot water reaching the lower half. It also collects in the boiler's heat exchanger and pump, where it causes noise, overheating and breakdowns.
Where a radiator stays cold tells you a lot:
| Where it's cold | Likely cause | First thing to try |
|---|---|---|
| Top cold, bottom warm | Air trapped in the radiator | Bleed it |
| Bottom and middle cold, top warm | Sludge settled in the panel | Flush the radiator or the system |
| Whole radiator cold, others fine | Valve shut or TRV pin stuck | Check both valves and free the pin |
| Radiators far from the boiler lukewarm | System out of balance | Balance with the lockshield valves |
| Every radiator cold, hot taps fine | Boiler, pump, timer or thermostat | Check settings, then call an engineer |
Bleeding, balancing and flushing
People use these words loosely, but they are three separate jobs.
Bleeding
Bleeding lets trapped air out. With the heating off and the radiators cool, fit a radiator key to the small square valve at the top corner, hold a cloth underneath and turn it a quarter turn anticlockwise. You'll hear a hiss; close it as soon as water spits out. Then look at the boiler gauge, because every radiator you bleed lowers the pressure slightly. A radiator that needs bleeding every week is a sign that air or gas is getting in somewhere, and that needs investigating.
Balancing
Balancing shares the hot water fairly. Water takes the easiest route, so radiators near the boiler take most of the flow and the ones at the end of the run stay lukewarm. Each radiator has a lockshield valve, usually under a plain plastic cap, that sets how much water it receives. Balancing means closing the nearest ones down a little and opening the furthest until they all warm at a similar rate. It takes an afternoon, some patience and ideally a clip-on pipe thermometer.
Flushing
Flushing removes sludge. A single radiator can be taken off the wall and hosed out in the garden. A whole system can be cleaned by circulating a chemical cleaner and then draining and refilling, or by power flushing, where a pump machine drives water and cleaner through each radiator in turn at high flow. Which approach makes sense depends on how bad the sludge is and how old the pipework is; we compare them in what is radiator flushing.
Combi pressure basics
The gauge on the front of the boiler shows the pressure in the heating loop, not your mains pressure. With the system cold, most combis want to sit at around 1–1.5 bar, but your manual gives the exact figure. As the heating runs and the water expands, the needle rises, often towards 2 bar, and that is normal.
Below about 1 bar, many boilers stop with a low-pressure fault. You top up through the filling loop, a braided hose or built-in key linking the mains to the heating circuit, opening it slowly and closing it once the gauge reaches the right level. If you are topping up every few weeks, water is escaping: a weeping valve, a hidden leak, a flat expansion vessel or a relief valve that lets by. Our guide to a combi boiler losing pressure works through each cause in a sensible order.
Pressure heading towards 3 bar is a different problem, often a filling loop left slightly open or an expansion vessel that has lost its air charge. At that point the relief valve may start dripping from the pipe outside.
The annual boiler service
A gas boiler should be serviced once a year by an engineer who is qualified and registered for gas work under the rules where you live. Registration schemes differ from country to country, so ask to see the engineer's card or certificate before work starts. Oil and electric boilers have their own specialists. Never open the sealed casing of a gas boiler yourself.
The exact checklist follows the manufacturer's instructions, but in general terms a service covers:
- the appliance, flue and flue terminal, checked for damage, corrosion and blockage;
- gas pressure and combustion, tested with a flue gas analyser;
- the burner, heat exchanger and condensate trap, inspected and cleaned as needed;
- seals, electrical connections and safety devices;
- the expansion vessel charge and system pressure;
- the magnetic filter, if one is fitted;
- a written record of what was done and any faults found.
Keep those records; manufacturers often require them for the warranty.
Inhibitor and magnetic filters
Corrosion inhibitor is a liquid added to the heating water to slow the reaction that produces sludge. It is diluted every time water is drained or topped up, so a system that has had work done, or that loses pressure often, may have little protection left. An engineer can test the level during a service, and more can be added through a radiator or the filter.
A magnetic filter is a canister fitted on the return pipe just before the boiler. Water flows past a strong magnet inside, which catches magnetite before it reaches the heat exchanger. Cleaning it takes a few minutes: isolate, unscrew, wipe the magnet, refill. Filters cost little compared with a new heat exchanger, and fitting one after a flush keeps the system clean for longer.
Habits that get more heat from less fuel
- Turn down the heating flow temperature. Condensing boilers recover extra heat from their exhaust when the water coming back to them is below roughly 55°C. Many are left at a high factory setting and rarely condense. A lower setting with slightly longer heating periods often keeps rooms just as warm, though draughty older homes may need more in the coldest weeks.
- Use TRVs properly. The numbers on a thermostatic radiator valve correspond to a room temperature, not to how fast the radiator heats, so turning it to maximum won't warm the room any quicker. Leave the radiator in the room with the main thermostat fully open, so the two controls don't fight.
- Keep radiators clear. Sofas and long curtains trap the heat behind them.
- Bleed and check pressure in early autumn, before the first cold week. Our home plumbing maintenance calendar sets out this and the other seasonal jobs month by month.
- Think about emitters when you renovate. Larger radiators or underfloor heating let the boiler run cooler; we weigh the two up in underfloor heating vs radiators.
When to call a professional
- You smell gas: leave the property, don't touch switches, and call your country's gas emergency number from outside.
- A carbon monoxide alarm sounds, or you see sooty marks around an appliance.
- Water is dripping from inside the boiler casing.
- Pressure keeps falling and you can't find a visible leak.
- The same fault code returns again and again, or the boiler bangs and rumbles like a kettle.
- The flue is damaged, loose or boxed in without an inspection hatch.
- Anything involves the gas supply, the inside of the boiler or the flue.
Questions readers ask
Should I bleed radiators with the heating on?
No. Switch it off and let the radiators cool first. Hot water can scald, and with the pump running you may pull air in rather than let it out.
How often does a system need flushing?
There is no fixed interval. A system with a magnetic filter and a healthy inhibitor level can stay clean for many years. Flushing makes sense when the symptoms appear: cold bottoms on several radiators, black water when you bleed, or a new boiler going onto old pipework.
My combi works fine. Does it still need a service?
Yes. Combustion and safety devices can drift without any obvious symptom, and a service catches small faults, such as a blocked condensate trap or a flat expansion vessel, before they cause a breakdown in midwinter.
