How to Stop a Condensate Pipe Freezing

If your boiler has stopped working in a cold snap, the chances are the condensate pipe has frozen¹. Modern condensing boilers, including combis, drain acidic water through a plastic pipe, and when temperatures drop below freezing the water inside an outside pipe can freeze and block it. The boiler then shuts down to protect itself, leaving you without heating or hot water².

The fix is usually simple. If the pipe is at ground level and you feel confident, thaw it with warm water from a jug or a hot water bottle held against the pipe, starting at the end nearest the ground³. Never use boiling water, and never use a naked flame². Once it has thawed, reset the boiler and check the pressure.

To stop it happening again, lag the outside pipe with waterproof and weatherproof lagging⁶. That is the single most useful thing an ordinary household can do before the cold arrives.

Why a frozen condensate pipe stops your boiler

A condensing boiler produces water as it burns gas, and that water has to go somewhere. It leaves through the condensate pipe, a plastic pipe that usually runs to an outside wall or drain¹⁰. In cold weather the water inside an external pipe can freeze and cause a blockage, and if the condensate cannot drain away the boiler may automatically switch off to protect itself². The pipe on condensing boilers may freeze when temperatures drop below freezing, and that will cause the boiler to shut down³.

The signs are fairly clear. The boiler stops working, leaving you without heating or hot water, and you might hear a gurgling noise from the boiler or from the condensate pipe outside². If your boiler has stopped working in the cold, check the condensate pipe before assuming anything more serious⁶.

Condensate pipes are commonly found in properties with a combi boiler, as well as other modern boilers¹¹. In winter the condensation in the pipe can freeze and burst the pipe, and cause pipes inside your home to do the same¹¹. That is why prevention matters as much as the thaw.

Lag the outside pipe with waterproof, weatherproof lagging

The best defence is to stop the pipe freezing in the first place. To help prevent your condensate pipe freezing, lag the pipe with waterproof and weatherproof lagging⁶. Insulating the pipe can prevent this from happening¹², and a Gas Safe engineer can insulate the condensate pipe for you if you would rather not do it yourself¹³.

Water companies give the same advice for pipes generally. Wrap exposed pipes in cold locations such as the garage, loft or outdoors with lagging or pipe insulation to prevent freezing and bursting¹⁴. Use waterproof lagging for outdoor pipes¹¹. Good quality lagging protects from freezing, helps insulate your hot water system and saves you money¹⁵. Foam tubing is cheap and simple to fit¹⁶, and lagging exposed pipes helps prevent bursts and reduces the risk of water loss during freezing temperatures¹⁷.

A few other habits help through a cold spell. Set your heating to come on for short periods every day¹⁹, and if you are going away, turn off the water at the stop tap and drain pipes and tanks, and ask a neighbour to turn the heating on and off regularly¹⁹. Know where your stop taps are and that they work¹⁹. If a pipe does freeze or burst, locate the stop valve and turn it off immediately²⁰.

How do you unfreeze a condensate pipe?

First, find the blockage. Pipes normally freeze at their most exposed points, so check the open end or any bends first, and run your hand down the pipe to find an area colder than the rest⁹. Check the end of the pipe: if water is not coming out, you can see ice, or it feels very cold, it is probably frozen⁸.

Then thaw it, working from the end nearest the ground. Hold a hot water bottle or warm heat pad against the pipe², or pour warm, not boiling, water over it using a jug or watering can². Pour hot, but not boiling, water on the pipe; you might need a few kettles or jugs to defrost it⁸. If safe to do so, pour warm water along the pipe²¹. Several applications may be needed²².

Only attempt defrosting condensate pipes that are at ground level and if you feel confident to do so. If the pipe is higher up or you are unsure, call a professional heating specialist³.

Using a heat wrap, heat pad or warm water

There are two safe ways to warm the pipe, and both work on the same principle: gentle heat applied to the outside.

A hot water bottle or warm heat pad held against the pipe is the simplest². A microwaveable heat pack or a warm damp cloth works too, and several applications may be needed²². You can also ask your plumber to install a heat wrap, so the pipe can be warmed without removing the lagging⁸.

Pouring warm water is the other route. Pour hot, but not boiling, water on the pipe, and repeat until it starts to thaw⁸. Carefully remove any pipe lagging first and pour warm water down the side of the pipe, repeating the process until you start to see it thaw⁹.

Whichever you use, start at the end nearest the ground⁴. If you are dealing with a frozen water pipe rather than a condensate pipe, the advice differs: turn off your water supply at the stop tap, switch off the immersion heater and central heating, leave the cold tap on so thawed water can escape, and thaw the pipe slowly with a hairdryer on warm, a hot water bottle or a heated cloth²³. If you spot that one of your pipes might be frozen, turn off your inside stop tap immediately²⁴.

Restarting the boiler after thawing

Once the pipe is clear, the boiler usually needs a reset. Follow the maker's instructions for your model; if you have an Ideal Logic Combi2, its manual sets out the reset procedure. Before you do, check the pressure gauge, and if it reads low, follow the maker's guide to repressurising a combi boiler.

If the boiler locks out again straight away, or the pipe refreezes within hours, stop resetting it and call a Gas Safe registered engineer. Repeated lockouts can point to a different fault, and a boiler breakdown in cold weather is worth diagnosing properly rather than guessing. If you rent your home, your landlord is responsible for repairs, and our guide to broken boilers in rented homes explains what they must do.

If you have no heating at all and need help, our gas and heating failures section covers the wider picture, including finding a Gas Safe registered engineer and extra support for vulnerable households.