How to maintain your heating system before winter

Book an annual boiler service in early autumn, run the heating for 20 minutes before the cold sets in to catch faults while it is still easy to get an engineer, bleed any radiators that are cold at the top, check the system pressure sits between 1.0 and 1.5 bar when cold, test the thermostat and replace its batteries, lag the external condensate pipe to prevent freezing, and clear the boiler flue of leaves and debris. Ten minutes of checks in September prevents the most common mid-winter breakdowns.

Book the annual service early — not in December

An annual boiler service by a Gas Safe registered engineer is the single most effective piece of winter preparation. It is also a condition of almost every manufacturer's warranty: miss a service and a heat exchanger failure that would have been covered becomes a bill you pay yourself.

Timing matters. Every heating engineer in London is at capacity from the first genuinely cold week onwards, and a breakdown in January can mean days without heat. Booking in September or early October means you get a slot easily, and any parts identified as failing can be ordered and fitted before you actually need the boiler.

A proper service is more than a visual glance: it includes combustion analysis with a flue gas analyser, checks on gas pressure and rate, seal and casing integrity, condensate flow, and inspection of the heat exchanger, burner and safety devices.

Run the heating before you need it

Systems that have sat idle all summer often will not start cleanly. Pumps seize, diverter valves stick, and airlocks develop. Turn the heating on for twenty minutes on a mild day in early autumn and stand and listen.

You are checking that the boiler fires without repeated lockouts, the pump runs quietly, hot water comes through at temperature, and every radiator gets hot. If only some radiators heat, or hot water works but heating does not, you probably have a stuck diverter valve or a circulation problem — both far easier to resolve in October than in a cold snap.

Bleed the radiators properly

If a radiator is hot at the bottom and cold at the top, air is trapped inside it. Turn the heating off and let the system cool, then use a radiator key on the small valve at the top corner, holding a cloth beneath. Turn a quarter turn anticlockwise, let the hiss of air escape, and close it as soon as water appears.

Work from the radiator nearest the boiler on the ground floor outwards, then upstairs. Once you have finished, check the boiler pressure gauge — bleeding releases pressure and you will usually need to top the system back up.

A radiator that is cold at the bottom instead is a different problem: that is sludge — magnetite build-up settling in the base. Bleeding will not fix it. That needs a power flush or a chemical clean, and it is worth dealing with before winter because sludge also wrecks pumps and blocks heat exchangers.

Check and top up the system pressure

On a sealed combi or system boiler, the pressure gauge should read roughly 1.0 to 1.5 bar when the system is cold. It will rise as the water heats — that is normal.

If it reads below 0.8 bar the boiler may lock out. Top up using the filling loop: the braided hose or built-in valves beneath the boiler. Open slowly, watch the gauge, and close as soon as you reach 1.2 bar. Do not overfill; above about 2.5 bar the pressure relief valve will discharge outside.

The important point is frequency. Needing a top-up once or twice a year is normal. Needing it every week means you have a leak — from a radiator valve, a pipe joint, a failed expansion vessel or the PRV itself — and that needs investigating rather than repeatedly refilling.

Lag the condensate pipe before the first freeze

This is the most common cause of winter boiler breakdowns in the UK, and it is almost entirely preventable. Condensing boilers produce acidic condensate that drains away through a plastic pipe. Where that pipe runs outside, it can freeze solid in sub-zero weather, the condensate backs up, and the boiler shuts down on a fault code.

Fit waterproof external pipe lagging along the whole exposed run before winter. If the pipe is 21.5mm, upgrading the external section to 32mm also helps significantly.

If it does freeze, you can often revive it yourself: pour warm — not boiling — water along the exposed pipe, or hold a hot water bottle against it, then reset the boiler. If it refreezes repeatedly, the pipe run needs rerouting internally.

Test the thermostat and controls

Turn the thermostat well above room temperature and confirm the boiler fires within a minute or so. If nothing happens, replace the batteries in a wireless stat before assuming a boiler fault — flat batteries in a wireless thermostat are a genuinely common false alarm.

Check the timer or programmer is set to the schedule you actually want and that the clock is correct after the October clock change. Set the thermostat around 18–21°C: each degree higher adds roughly 10% to your heating bill.

Thermostatic radiator valves also seize over summer. Turn each one from fully closed to fully open a few times; if the pin under the head is stuck down, free it gently with pliers or a little lubricant.

Balance the system and improve efficiency

If some rooms are always cold while others roast, the system is unbalanced — too much flow is going to the nearest radiators. Balancing involves adjusting the lockshield valves so each radiator reaches a similar temperature difference between flow and return, typically around 11–12°C.

For a combi boiler in condensing mode, lowering the flow temperature to around 60°C (rather than the factory 75–80°C) typically improves efficiency by 6–8% with no loss of comfort in a well-insulated home. A magnetic filter fitted on the return, and cleaned annually, keeps sludge out of the heat exchanger and protects the whole system.

Check the flue, ventilation and outside

Look at the flue terminal outside. Clear leaves, cobwebs, nests and any garden growth that has crept across it during summer, and make sure nothing obstructs the terminal within the manufacturer's clearances. A blocked flue is a carbon monoxide risk, not just an efficiency issue.

Check that any air vents serving the boiler or a gas fire are unobstructed. Confirm your carbon monoxide alarm works and is within its expiry date — most units last seven to ten years.

Finally, locate your stopcock and know where the gas emergency control valve is. Knowing these before something goes wrong saves a great deal of stress at two in the morning.

Frost protection and unoccupied properties

If you are away over Christmas, do not switch the heating off entirely. Set the thermostat to a background 12–15°C, or use the frost protection setting, so pipework in unheated voids and lofts does not freeze and burst.

In older London terraces and top-floor flats, lag any accessible pipework in the loft and check the loft hatch closes properly. Burst pipes caused by freezing are one of the most expensive and disruptive claims a homeowner can face, and they overwhelmingly happen in empty properties during cold spells.

When to call an engineer rather than DIY

Bleeding radiators, topping up pressure, lagging a condensate pipe and changing thermostat batteries are all safe homeowner tasks. Anything involving the gas supply, the boiler casing, the flue or internal components is not — by law, gas work must be carried out by a Gas Safe registered engineer.

Call an engineer if you see a yellow rather than blue flame, black or sooty marks around the boiler, persistent lockouts or fault codes, water leaking from the boiler itself, banging or kettling noises, or if your CO alarm sounds. In the case of a CO alarm or a gas smell, turn off the gas at the meter, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.

We service and repair boilers and central heating across Bethnal Green, Hackney, Shoreditch, Stratford and the wider East and Central London area, with Gas Safe registered engineers and same-week appointments through autumn.

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