Frozen Condensate Pipe: How to Fix It Safely

Woke up to a cold house and a boiler showing an error code? If it’s below freezing outside, there’s a good chance your condensate pipe has frozen. The good news: this is often fixable yourself in under 15 minutes. Here’s exactly how to do it safely.

What Is a Condensate Pipe?

Every modern condensing boiler produces condensate, a slightly acidic wastewater created when the boiler extracts heat from exhaust gases. This water needs to go somewhere, so it drains through a plastic pipe (usually white or grey, 20-22mm diameter) that runs from your boiler to an external drain, soil stack, or internal waste pipe.

The problem? This pipe often runs outside, exposed to the elements. When temperatures drop below freezing, the water inside can freeze solid, creating a blockage. Your boiler detects that it can’t drain properly and locks out for safety.

This is actually your boiler doing its job correctly. It’s protecting itself from damage. The fix is usually straightforward.

How to Tell If Your Condensate Pipe Is Frozen

A frozen condensate pipe is one of the most common causes of boiler lockouts in winter. Here’s how to identify it:

Signs to look for:

• It’s below freezing outside. This is the obvious one. If temperatures haven’t dropped below zero, it’s probably not a frozen condensate.
• Your boiler was working fine yesterday. Sudden failure after a cold night is the classic pattern.
• Error code on display. Most boilers will show an ignition failure or lockout code (see fault codes section below).
• Gurgling sound when boiler tries to start. This is the boiler trying to push condensate through a blocked pipe.
• Visible ice on the external pipe. Go outside and look. If you can see frost or ice on the condensate pipe, that’s your answer.

Where to find the condensate pipe:

Look for a plastic pipe (white, grey, or black) coming out of your boiler or through an external wall. It typically runs down the wall and terminates at:

• A drain or gully at ground level
• A soil stack (the main waste pipe from your bathroom)
• An internal waste pipe under a sink (less common, and less likely to freeze)

The pipe is usually 20-22mm in diameter, much smaller than standard waste pipes. If it runs externally for any distance, it’s vulnerable to freezing.

Tip: The condensate pipe is usually smaller than standard waste pipework and is plastic all the way. It is not the copper pressure relief discharge pipe (which is a safety valve outlet and should not be touched).

Safety First: Before You Do Anything

A frozen condensate pipe is not dangerous in itself. It’s just an inconvenience. Your boiler has locked out as a precaution, which is exactly what it’s designed to do. A frozen condensate is usually not a gas or carbon monoxide issue. However, if you have a CO alarm sounding or feel unwell (headaches, nausea, dizziness), treat it as an emergency and follow the safety steps above.

That said, don’t ignore it. A boiler that keeps trying to fire against a blocked condensate can eventually cause other problems. Fix it, reset the boiler, and if you’re not confident, call an engineer.

How to Thaw a Frozen Condensate Pipe (Step-by-Step)

This is a job most people can do themselves. Here’s how:

What you’ll need:

• Kettle or jug
• Warm water (NOT boiling)
• Towel (optional, to wrap around pipe)
• Hot water bottle or heat pack (optional, for stubborn freezes)

Step 1: Locate the frozen section

Go outside and find where the condensate pipe exits your property. The blockage is usually at one of these points:

• The end of the pipe where it terminates at the drain
• Any bends or elbows where water can collect
• The most exposed section (north-facing walls freeze first)

Look for visible ice, frost, or a section that looks different from the rest. If you can’t tell, start at the lowest point (the drain end) and work upward.

Step 2: Prepare warm water

Important: Use warm water, NOT boiling.

Boiling water can crack the plastic pipe due to thermal shock. You want water that’s hot to the touch but not scalding, around 40-50°C. Freshly boiled water that’s been left to cool for 5-10 minutes is about right.

Step 3: Pour warm water over the frozen section

Slowly pour the warm water over the outside of the pipe, starting from the end nearest the drain and working back toward the boiler. You may need several kettles’ worth.

As the ice melts, you might hear water starting to flow through the pipe. That’s a good sign.

Step 4: Check for flow

Once you’ve thawed the pipe, go back inside and check if the boiler is ready to restart. Some boilers will automatically attempt to fire once the blockage clears; others will need a manual reset.

Step 5: Reset the boiler

Press the reset button on your boiler (usually marked with a flame symbol or “Reset”). Wait 60 seconds. The boiler should attempt to ignite.

Only reset once. If it locks out again, stop and move to What if the boiler won’t restart?

If it fires up and runs normally, you’re done. Make a note of what happened so you can take steps to prevent it freezing again (see prevention section).

What NOT to Do

Some DIY “fixes” can make things worse or create safety hazards:

🚫 Do NOT use boiling water

Pouring boiling water directly onto cold plastic can crack or split the pipe. Use warm water only.

🚫 Do NOT use a naked flame

Blow torches, lighters, or heat guns should never be used on plastic pipework. You will melt or deform the pipe, creating a much bigger (and more expensive) problem.

🚫 Do NOT hit the pipe to break the ice

You’ll crack the pipe. Plastic becomes brittle in cold weather.

🚫 Do NOT repeatedly reset the boiler

If the pipe is still frozen, repeatedly resetting the boiler won’t help and can stress the ignition system. Thaw the pipe first, then reset once.

🚫 Do NOT ignore it

A frozen condensate won’t fix itself. The ice will only get worse, and your home will stay cold.

Fault Codes That Indicate a Frozen Condensate Pipe

Most boilers don’t have a specific “frozen condensate” code. Instead, they display an ignition failure or lockout code because the boiler can’t complete its startup sequence. Here are the common codes by brand:

Important: These codes can also indicate other problems (gas supply issues, faulty electrodes, etc.). If you’ve thawed the condensate pipe and the code persists, the cause is something else. Call an engineer.

Note: Codes vary by model and display type. In freezing weather, many boilers show an ignition or lockout code even though the underlying cause is a frozen condensate route. If unsure, see our manufacturer Help Hub pages for model-specific guidance.

What If the Boiler Won’t Restart?

You’ve thawed the pipe, pressed reset, waited 60 seconds… and the boiler still won’t fire. Now what?

Check these first:

1. Is there still ice in the pipe? The blockage might be further along than you thought. Try pouring warm water over a longer section of pipe.
2. Is there ice inside the boiler? In extreme cold, the condensate trap inside the boiler itself can freeze. This requires an engineer. Do not open the boiler casing yourself.
3. Has the pipe split? Check for cracks or drips. If the pipe is damaged, you’ll need a repair before the boiler will run safely.
4. Is there another problem? The cold snap may have coincided with an unrelated fault. Check boiler pressure, check the gas supply (do other gas appliances work?), and check the electrics.

Still not working?

Time to call an engineer. The fault may be unrelated to the condensate, or there may be internal damage that needs professional diagnosis.

How to Prevent Your Condensate Pipe Freezing Again

Once you’ve fixed the immediate problem, take steps to stop it happening next time. Here are the quick wins:

1. Insulate the pipe

Foam pipe lagging is cheap (a few pounds from any DIY store) and easy to fit. Cover the entire external run of the condensate pipe, paying special attention to bends and the termination point.

Important: Standard pipe lagging will slow freezing, but won’t prevent it entirely in a prolonged cold snap. It buys you time, not immunity.

London note: If your boiler is also noisy (kettling) or radiators have cold spots, winter lockouts can be compounded by poor circulation from magnetite sludge. A power flush clears the system and can improve overall reliability, not just in winter.

2. Keep heating on low overnight

When temperatures are forecast to drop below -3°C, keep your heating on a low setting overnight (even 15°C is enough). This keeps warm water flowing through the condensate pipe regularly, preventing ice from forming.

3. Pour warm water down the pipe before bed

On very cold nights, pouring a kettle of warm water down the external drain where the condensate terminates can help prevent ice forming at the outlet.

4. Check the pipe route

Is the pipe longer than it needs to be? Does it run along a north-facing wall? Does it have unnecessary bends where water can collect? A poorly routed pipe is more likely to freeze.

Permanent Solutions (Get It Fixed Properly)

If your condensate pipe freezes every winter, the quick fixes above are just sticking plasters. Here are the proper solutions:

Option 1: Reroute the pipe internally

The best solution is to have the condensate pipe run entirely inside the property, connecting to an internal waste pipe (under a sink or to the soil stack). No external exposure = no freezing.

This isn’t always possible depending on your property layout, but it’s worth investigating.

Option 2: Increase the pipe diameter

A larger bore pipe (32mm instead of 22mm) is less likely to freeze solid because the water can flow around partial ice buildup. Some installations use 22mm for the entire run when 32mm would be more appropriate.

Option 3: Fit a condensate pipe heater

Trace heating cables can be wrapped around the pipe to keep it above freezing. These are powered by mains electricity and switch on automatically when temperatures drop. Effective but adds to running costs.

Option 4: Install a condensate pump

A condensate pump pushes the wastewater out more forcefully and can route it to a more protected drain point. Useful when gravity drainage to a suitable internal waste pipe isn’t possible.

Option 5: Relocate the boiler

In some cases (usually during a boiler replacement), it makes sense to relocate the boiler to a position where the condensate route is shorter, more protected, or entirely internal.

For any of these permanent solutions, you’ll need a Gas Safe registered engineer. We can assess your setup and recommend the most cost-effective fix.

If your boiler is older and repeatedly suffering winter lockouts, it may be worth reviewing overall reliability and efficiency. See new boiler installation in London or book a boiler service before next winter to get ahead of the problem.

When to Call an Engineer

You can fix a frozen condensate pipe yourself in most cases. Call an engineer if:

• The boiler won’t restart after thawing. There may be another fault.
• The pipe is cracked or damaged. Needs replacing before the boiler can run safely.
• You can’t access the pipe safely. Some installations require ladders or are in awkward locations.
• Ice has formed inside the boiler. Do not open the casing yourself.
• This keeps happening every winter. You need a permanent solution, not repeated quick fixes.
• You’re not comfortable doing it yourself. There’s no shame in calling a professional. We’d rather you call us than make a problem worse.

Frequently Asked Questions

London-Specific: Why Frozen Condensate Is So Common Here

London’s housing stock creates particular challenges for condensate pipes:

• Victorian and Edwardian terraces: Boilers often installed in kitchens at the back of the property, with condensate running to rear drains. North-facing rear walls are colder.
• Flats and conversions: Limited options for condensate routing means longer external runs. Basement flats are especially vulnerable because the pipes run at ground level where cold air pools.
• Listed buildings and conservation areas: Restrictions on external pipework can force awkward routes.
• Older installations: Many London boilers were installed 10-15 years ago when condensate freezing wasn’t as well understood. Minimum-spec installations are now causing problems.

If you’re in London and your condensate keeps freezing, we can assess your installation and recommend the most practical permanent fix for your property type.

Summary: Quick Reference