Winter Plumbing Checklist for London: Prevent Frozen Pipes

A winter plumbing checklist for London homes comes down to five jobs, done in autumn: lag every pipe and tank in the loft and other unheated spaces, isolate and drain the outside tap, book the boiler service, protect the condensate pipe, and find and test your stop tap. Then keep some heat on through freezing spells.

London's winters are mild on average, and that leaves many homes unprepared for short, sharp cold snaps that arrive with little warning. Pipework that has caused no trouble for years can freeze in a single night if it runs through a cold loft, under a ventilated floor or along an outside wall. Preparation belongs in autumn. Once the forecast turns, engineers' diaries are filling and there is little time left to lag a loft or book a service.

The cost of getting it wrong is high. A split pipe in a loft can soak through a ceiling before anyone notices, and a frozen condensate pipe can leave a household without heating or hot water in the coldest week of the year. Landlords also carry a legal duty to keep the water, heating and hot water installations in repair, so winter preparation protects tenants as well as the property.

How Do You Prevent Frozen Pipes in a London Home?

Do four things before the first frost: insulate every pipe and tank in unheated spaces, keep a low level of heat in the house, fix drips, and find your stop tap. Most freezes happen in lofts, under suspended floors, along outside walls and in outbuildings, where pipework sits outside the heated part of the home.

The reason is simple physics. Water expands by roughly 9% in volume as it freezes. Inside a pipe, that expansion raises the pressure on the pipe wall and joints until one of them splits or pulls apart. While the pipe stays frozen, the ice plugs the gap. The damage shows when it thaws and water escapes, which is why many bursts are discovered as temperatures rise.

Some London homes carry more risk than others:

• Victorian and Edwardian terraces with suspended timber floors. The void beneath the floorboards is ventilated through airbricks (the perforated bricks set low in the outside wall), so pipes running under the floor sit in outside air.
• Loft conversions, where pipework often runs through the eaves voids behind the knee walls (the short walls at the sides of the room). These voids are frequently as cold as the outside.
• Flats with pipes on external walls, or running through unheated communal stairwells and risers.

How to Insulate Pipes and Tanks in the Loft

Fit foam pipe lagging, the tubular insulation that slips over a pipe, on every exposed pipe in the loft. Match it to the pipe size: 15mm and 22mm are the common sizes. Tape the joins and cut the lagging to cover bends, valves and elbows, because a short bare section is where a pipe freezes first.

Many older London homes are tank-fed. The cold water storage cistern is the large tank in the loft that feeds the cold taps and the hot water cylinder. Homes with an open-vented heating system also have a smaller feed and expansion tank, which tops up the radiator circuit. Fit an insulation jacket on both, or wrap the top and sides with suitable insulation, and keep the lids on. Lag the overflow (warning) pipe where it crosses the cold space so that it can still carry water away if the tank's float valve fails.

Do not lay loft insulation underneath the tank. Heat rising from the rooms below is what stops the water in it from freezing. After any loft insulation top-up, check that insulation has not been packed under the tank and that any pipes left lying above the new layer are lagged. A better-insulated loft is a colder loft.

In a loft conversion, open the access hatches to the eaves voids and check the pipes there. They are easy to forget.

Outside Taps, Under-Floor Pipes and Outbuildings

Close the outside tap's isolation valve, usually on the pipe just inside the wall, then open the outside tap to drain the water left in it. If there is no isolation valve, fit an insulated tap cover. Lag pipes under suspended timber floors where you can reach them through a hatch or lifted board, and never block the airbricks: they ventilate the void and protect the joists from damp and rot.

In garages, garden offices and outbuildings, insulate toilet cisterns, tanks and pipes. If the building will not be used over winter, drain them instead. During a cold snap, open the cupboard doors under sinks on outside walls so that warm air from the room can reach the pipes.

Heating, Drips and Your Stop Tap

Keep the heating running at a low setting during freezing spells, including overnight. Many programmers have a frost setting, and some systems have a separate frost thermostat. Both switch the boiler on only when the temperature nears freezing, which protects the pipework without heating the whole house to a comfortable level.

Repair dripping taps and leaking valves promptly. A constant drip into a waste or overflow pipe can freeze where the pipe leaves the building and block it.

Then find the stop tap, the valve that shuts off the mains water to the house. Most are under the kitchen sink, and some are in a cupboard, garage or cellar. In older terraces it is often under the floorboards near the front door. Turn it off and on again now to check that it moves, and label it so that everyone in the house knows where it is. The external stop tap under the pavement belongs to the water company, which is Thames Water for most of London. Treat it as a last resort, because it can be hard to locate or open in an emergency.

Is Your Boiler Ready for a London Winter?

Book the annual service in September or October, run a few simple checks yourself, and protect the condensate pipe. These steps remove the most common reasons a boiler stops in cold weather. Engineers' diaries fill quickly from November, and a fault found at an autumn service is usually cheaper and faster to fix than a breakdown in January.

If your annual boiler service is due, book it now and run these checks in the meantime:

• Run the heating for an hour. Check that every radiator heats fully. A radiator that is cold at the top has trapped air. Bleed it with a radiator key, holding a cloth under the valve, and close the valve when water appears.
• Check the pressure gauge. Boiler manufacturers typically give a cold pressure of around 1 to 1.5 bar for a sealed system, but your manual gives the correct figure. If it reads low, top it up with the filling loop, the flexible connector under the boiler that lets mains water into the heating circuit. Bleeding radiators lowers the pressure, so recheck it afterwards. If the pressure keeps dropping, there is a leak or faulty component, and an engineer needs to find it.
• Fit or test a carbon monoxide alarm in every room with a boiler or other fuel-burning appliance. Since October 2022, the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 have required landlords in England to fit one in any room with a fixed combustion appliance, excluding gas cookers.

Any work on the boiler itself must be done by a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998. The same regulations require landlords to arrange an annual gas safety check, give tenants a copy of the gas safety record within 28 days and keep records for two years.

Service prices depend on the boiler, its age and access to it, so ask for a fixed price before you book. You can ask InstaGasWorks for a quote at any time.

What a Boiler Service Before Winter Should Cover

Before any work starts, check the engineer's Gas Safe ID card and confirm that the back of it lists the right appliance categories for your boiler. A proper service includes a flue gas analysis (a reading taken with an analyser to confirm the boiler is burning gas safely and cleanly), an inspection of the flue and seals, a gas pressure check, testing of the safety devices and cleaning of the condensate trap. Our guide to what a boiler service includes explains each step.

Annual servicing is usually a condition of the manufacturer's warranty, so a missed service can leave a winter breakdown uncovered. For landlords, a service and a gas safety check are different things: the check is the legal inspection that produces the gas safety record, and the service is maintenance. Both can be booked in a single visit.

Why a Frozen Condensate Pipe Stops Your Boiler

Under the Building Regulations, condensing boilers have been required for most new gas boiler installations in England and Wales since April 2005, so most boilers fitted since then are condensing models. A condensing boiler recovers extra heat from its flue gases and produces mildly acidic water as a result. This drains through a plastic condensate pipe, which often runs outside to a drain. When the outside section freezes, water backs up and the boiler locks out to protect itself.

The signs are a gurgling noise, a fault or lockout code on the display (codes vary by manufacturer, so check the manual) and a boiler that will not restart in freezing weather. To thaw the pipe, pour warm water along the outside section from the outlet end upwards, or hold a hot water bottle against the bends and the open end. Then reset the boiler as the manual describes.

Use warm water, never boiling. Boiling water can crack the plastic pipe and its fittings and can scald you if it splashes. Do not climb a ladder on icy ground to reach a high pipe.

Industry guidance recommends that external condensate runs are at least 32mm in diameter to reduce the risk of freezing. Keep the outside section short, lag it with weatherproof insulation and lay it with a good fall towards the drain. The most reliable fix is to have an engineer reroute the pipe internally, which removes the cold section altogether.

What to Do if a Pipe Freezes or Bursts

Turn off the water at the stop tap first, every time. Then either thaw the frozen pipe slowly or, if it has burst, drain the system and keep clear of electrics before calling a plumber.

Tank-fed homes: closing the stop tap cuts off the mains supply, but the cold water storage tank in the loft still holds water and will keep feeding the pipes below it. Drain it by running the bathroom cold taps and flushing the toilets, and close the gate valve (a valve with a round wheel handle) on the tank outlet if there is one.

When you call an emergency plumber, ask three questions before they set off: the expected arrival time, the callout charge and the hourly rate.

Thawing a Frozen Pipe Safely

The signs of a frozen pipe are no water or only a trickle from one tap, frost on an exposed pipe, or a toilet cistern that will not refill. Close the stop tap and open the affected tap so that meltwater has somewhere to go. Work from the tap end back towards the frozen section with a hot water bottle, a towel soaked in warm water or a hairdryer on a low setting, keeping the hairdryer and its cable away from any water.

As the pipe thaws, check it for splits or drips before turning the stop tap back on. Call a plumber if you cannot find or reach the frozen section.

Never use a naked flame or blowtorch to thaw a pipe. The fire risk is serious, and sudden heat can damage the pipe and its joints.

Dealing With a Burst Pipe

1. Close the stop tap. Switch off the boiler and any immersion heater (the electric element inside a hot water cylinder).
2. Open all the cold taps and flush the toilets to drain the system. In a tank-fed home, close the gate valve on the tank outlet in the loft as well.
3. If water is near sockets, light fittings or the consumer unit (the fuse box), switch off at the consumer unit only if you can reach it without standing in water. Otherwise keep clear.
4. Move belongings out of the way, catch the water, photograph the damage and contact your insurer early.
5. In a flat, warn the neighbours below and tell the managing agent or freeholder. Tenants should report the burst to their landlord straight away.

Our guide to water leak repair in London sets out what to do while you wait for help.

Your Winter Plumbing Checklist for London Homeowners and Landlords

Work through the checklist in three stages: autumn preparation, extra steps for a property that will stand empty, and a clear split between jobs you can do yourself and jobs for a Gas Safe engineer or plumber. Save or print it for winter 2026.

In Autumn, Before the Cold Sets In

• Book the boiler service, or for landlords the annual gas safety check. Both can be done in one visit.
• Lag the loft pipes, including the overflow, and fit jackets on the cold water tank and the feed and expansion tank.
• Close the outside tap's isolation valve and drain the tap, or fit an insulated cover.
• Find, test and label the stop tap.
• Inspect the condensate pipe run: its length, diameter, insulation and fall.
• Test the carbon monoxide alarm.
• Save a trusted plumber's number in your phone.

If the Property Will Stand Empty

An empty property carries extra risk because nobody is there to notice a leak.

• Keep the heating on low or on its frost setting, or have a plumber drain the system down.
• Check your insurance policy's unoccupancy clause. Policies often set conditions on heating or draining down once a home has been empty for a certain period, and that period varies, so read your own wording.
• Leave a key with a neighbour, along with contact details for someone who can help in an emergency, and arrange regular visits.
• Landlords should plan for void periods between tenancies, show each new tenant where the stop tap is and give them an emergency contact.

Jobs to Leave to a Gas Safe Engineer or Plumber

Gas work in the right-hand column must go to a Gas Safe registered engineer. For the water-side jobs, such as a seized stop tap or a repaired burst, our plumbing services page explains what we cover.

Start Your London Winter Plumbing Checklist Before the First Frost

Three jobs this week deal with the most common winter plumbing problems: find and test the stop tap, book the boiler service and lag the loft. If something does go wrong, closing the stop tap first limits the damage more than any other step. When our Gas Safe engineers service your boiler, you can ask them to check the condensate pipe and any accessible loft pipework in the same visit, so get in touch to book before the winter diaries fill.