Boiler condensate pipe blocked

Boiler Condensate Pipe Blocked? How to Find and Fix the Cause

Last updated on October 6th, 2023

Condensing boilers produce condensate wastewater from the flue pipe (chimney) that needs to run freely down through the boiler, into a condensate trap, then down a condensate pipe and discharged into a drain.

Having your boiler condensate pipe blocked means your boiler will not work. Boiler condensate pipes become blocked regularly, often from freezing in the cold winter.

I have been fitting and repairing boilers for over a decade. Here I’ll share what I’ve learned over the years from my on-the-job experiences.

How to Check if Boiler Condensate Pipe is Blocked

If a boiler is not working and I suspect it could be because of the condensate pipe being blocked, the first thing I do is remove the condensate pipe from the boiler.

Disconnect condensate pipe from boiler
Disconnected condensate pipe

Here’s how to check if a boiler condensate pipe is blocked:

  1. Place a Plumb Tub or bucket under the boiler
  2. Pull the condensate pipe out of the bottom of the boiler
  3. Reset the boiler
  4. Turn the heating on to see if it works

This should only be done on boilers with a rubber connector connecting the condensate pipe to the boiler.

If the heating comes on when the pipe is removed then your condensate pipe is blocked.

You should refit the pipe back into the boiler and work on unblocking it.

Do not leave the condensate pipe out of the boiler and put the heating on. If the condensate trap fails you can get carbon monoxide spilling into the room.

If your boiler is trying to fire up and gurgling, this can be a sign of a condensate pipe being blocked.

How to Fix a Blocked Condensate Pipe

To fix a blocked condensate pipe you need to unblock it by finding and clearing out the blockage.

How to fix a blocked boiler condensate pipe:

  • Thaw out a frozen pipe
  • Clean out waste pipe
  • Clean out the end of the pipe outside
  • Clean the condensate trap

Frozen Condensate Pipe

The most common cause of a boiler condensate pipe becoming blocked is when it freezes.

You will need to defrost the ice by pouring hot water over the plastic pipe outside. This should melt the ice and clear the blockage. Then you will need to reset the boiler.

A frozen condensate pipe will usually freeze because the pipe diameter is not big enough or there’s not enough of a fall on the pipe.

This means the condensate wastewater is always sitting in the pipe and therefore freezes easily.

Clean Out Waste Pipe

If the condensate pipe runs under a kitchen sink to a waste pipe, then the waste might be full of gungy slop that has built up over the years.

Even if the water from the sink is running away, it could still be partially blocked enough for the boiler condensate wastewater to back up to the boiler.

This will need to be cleaned out to fix it. Or you can run a new condensate pipe from the boiler directly to a drain or soil pipe.

Blocked waste pipe
Blocked waste pipe

Clean Out the End of Pipe

Another way condensate pipes can get blocked is at the end outside where it discharges into a drain.

To fix it you should check the end of the pipe for any leaves and dirt that might be restricting or blocking it and clear them out.

Clean the Condensate Trap

The condensate trap in the boiler could be full of dirt and other debris on certain boilers. This will need to be removed and cleaned out properly and then refitted.

Dirty condensate trap
Dirty condensate trap

If the condensate trap is blocked then removing the pipe to check (as above) will not help. You will only be able to tell by looking at the trap.

Condensate Pipe Dripping Outside

If your boiler condensate pipe is dripping outside into a drain then this is normal.

When your heating is on, the condensate pipe will have condensate wastewater running through the pipe regularly and discharging into the drain.

Condensate pipe outside
White condensate pipe

If the pipe is dripping from a fitting or damage to the pipe, then you should get this repaired as soon as possible.


A blocked boiler condensate pipe is a very common repair job for engineers in the winter.

The first thing you should do if you suspect a condensate pipe has frozen is to pour hot water over it and reset the boiler.

Cleaning out a blocked condensate pipe because of other blockages can be very tricky so running a new pipe in might be the best solution.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.

Boiler Condensate Pipe Blocked FAQs

Is a leaking condensate pipe dangerous?

Yes, a leaking condensate pipe can potentially be dangerous. If your condensate pipe is leaking outside then it is not dangerous, but you don’t want young children or animals to access and ingest it.

However, if it is leaking inside the house onto a gas pipe for example, then this is very dangerous and needs to be fixed ASAP!

A condensate pipe must be in plastic because the condensate wastewater inside is acidic and it eats away at metals like copper and brass.

If it leaks onto any copper or brass pipes or fittings then this will eventually eat right through the metal.

Condensate pipe under boiler
White condensate pipe under boiler

Where is the condensate pipe on my boiler?

The condensate pipe will be under your boiler. It should be the only white (rarely black) plastic pipe.

It runs from the bottom of your boiler to a drain or condensate pump.

Can you use copper pipe for condensate?

No, you can not use copper pipe or brass for condensate pipe. Condensate wastewater is acidic and it will corrode the copper pipe and any other metals. Condensate pipe and fittings must be plastic.

Brass fitting on condensate pipe
Brass fitting used on condensate


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

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11 thoughts on “Boiler Condensate Pipe Blocked? How to Find and Fix the Cause”

  1. Hello. My gas boiler is Wolf FGB-K-28. The condensate goes over washing machine hose about 7m. It worked well until recently when condensate began to overflow between boiler condensate output and upper end of hose. I tested the hose with water and it seems it is not blocked – water leaked at the other end. I noticed a small concavity on boiler output and it seems its purpose is for air supply inside pipe/hose below. Give me some emaill address if possible to send photos.

  2. Our condensate pipe goes outside and finishes underground but when we get heavy rain it backs up and boiler won’t work is there any solutions for this? Thanks

    1. Hi Charlotte,
      It sounds like you have a condensate soakaway fitted if it’s going underground. You could fit an air break in the pipe outside so that when the water backs up it pours out of it. It’s basically a gap in the pipe so water will go down the pipe but not up.

  3. Hi Ian
    My boiler is coming up with error E133. The burn light keeps going on and off does that mean the pilot light has gone out? It makes a thudding noise as though its trying to light up.

  4. Hi Ian, our condensation pipe goes under the sink and is blocked (I assume from sludge) would it work to carefully send some drain cleaner down the out pipe in the first instance to start clearing the blockage before calling in a plumber?

  5. Hi, is it possible for the pipe to be blocked and for water to back flow up into the boiler and spray inside? Ours appears to have done that. When the pipe exits the house it is level for about a metre before going into the drain pipe. It happened whilst someone was taking a shower.



    1. Hi Ian,
      It is possible for the pipe to block and water to backflow into the trap and maybe even spray inside the boiler, but it could also be something else that’s spraying, probably on the hot water side of the boiler.

      1. Thanks Steven, it’s happened twice in 7 months, it’s only 2 years old and been serviced. The only thing that’s changed is the pipe was moved to accommodate a part garage conversion so more of it was moved outside.



  6. Hi, I am having a problem with my condensate pipe from my Worcester combi boiler freezing and the boiler going off. This has only happened twice in eight years and both times after prolonged freezing periods like this week and the beast from the east a few years ago.
    It appears to be fitted correctly with a 22mm pipe and elbow out of the wall then into about one foot of larger pipe probably 40mm to drain.
    I am guessing the problem may be round the 22mm elbow,would lagging help ? The pipe is on the second floor so a ladder would have to be used to put hot water on it.

    1. Hi Paul,
      The condensate pipe should be at least 32mm all the way through the wall from outside to in, then it can drop down to 22mm inside the property. If there is any 22mm pipe or fittings outside then this should be upgraded to 32mm or 40mm and lagged.

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