By Steven Reid · Last updated on August 23rd, 2026 · 17 Comments
Condensing boilers produce wastewater (from the flue gases) that needs to run freely down through the boiler, into a trap, then down a plastic pipe and into a drain. Having a blocked condensate pipe means your boiler will not work. I have been repairing boilers for over 15 years and fixed many condensate pipe problems.
How to check if condensate pipe is blocked
A boiler trying to fire up and gurgling is a sign of a condensate pipe being blocked. If a boiler is not working, and I suspect it could be because of a blocked condensate, the first thing I do is remove the plastic pipe from the boiler.
How to check if it is blocked:
- Place a bucket under the boiler
- Pull the condensate pipe out of the bottom of the boiler
- Reset the boiler
- Turn the heating on to see if it works
This should only be done on boilers with a rubber connector connecting the pipe to the boiler.
If the heating comes on when the pipe is removed the condensate pipe is blocked. Refit the pipe back into the boiler and work on unblocking it. Do not leave the condensate pipe out of the boiler and leave the heating on. If the condensate trap fails you can get carbon monoxide spilling into the room.
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 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
A frozen condensate pipe is one of the most common causes of a boiler condensate blockage during winter. Ice prevents the wastewater from draining, causing it to back up towards the boiler and make it lock out.
Pour warm water that is not freshly boiled over the affected section, starting at the open end and working back towards the wall. Once water begins draining freely, reset the boiler according to its instructions. Follow my complete guide on how to clear a frozen condensate blockage if you need the full process.
Condensate pipes usually freeze because the external section is undersized, too long, poorly insulated or does not have enough fall. Dips and horizontal sections allow wastewater to collect and freeze more easily.
Thawing clears the immediate blockage, but the underlying installation problem should also be corrected. See how to prevent a condensate pipe from freezing again for advice on pipe sizing, insulation and trace heating.
Clean out waste pipe
If the condensate pipe runs under a kitchen sink to a waste pipe, 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 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.
Clean out the end of the 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. The trap needs to be removed and cleaned out properly and then refitted. If the 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.
Conclusion
The biggest problem with condensing boilers is the pipe becoming blocked or frozen. The first thing you should do if you suspect a pipe has frozen is to pour hot water over it and reset the boiler.
Cleaning out a blocked pipe because of other blockages can be very tricky so running a new pipe in might be the best solution.
Feel free to ask any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.
FAQs
Can you use copper pipe for condensate pipe?
No, you can not use copper or brass for pipe or fittings on a condensate pipe. The condensate wastewater is acidic and it will corrode the copper and brass or any other metals. Condensate pipe and fittings must be plastic.
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 be able to access and ingest it. However, if it’s leaking inside the property onto a gas pipe, for example, this is very dangerous and needs to be fixed ASAP!
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.
Our ten year old Baxi combi boiler is reverberating intermittently when heating and hot water is on. The sound is like distant thunder This started happening when the shower tap was turned up to maximum heat. It now wont seem to shut down even when turned off, after working ok for several hours. Does this sound like a condensate pipe blockage? The problem appeared just as the weather turned cold, the boiler having performed faultlessly for ten years.
no it doesn’t sound like a condensate pipe blockage.
hi my names denise been told by my boiler cover engineer that i have a frozen condense pipe on my boiler and that when boiler was fitted by another company the pipe is wrong size its 22ml and should be 44ml im without heating now for 2 days and my boiler company that im insured with says my policy dont cover them changing the pipe
Yeah because it wasn’t fitted to the manufacturers instructions, it’s the installers fault. Try pouring some hot water on the plastic pipe outside and reset the boiler.