Blocked condensate pipe: signs, causes and fixes
A blocked condensate pipe stops a condensing boiler working: the acidic water it produces has nowhere to go, it backs up into the boiler, and a sensor locks the boiler out to protect it. The usual signs are a gurgling noise from the boiler, a lockout or fault code, and sometimes water dripping from underneath. In cold weather the cause is usually ice, but a pipe can block at any time of year with sludge in the boiler's internal trap, leaves or soil at the outside end, a sagging run that holds water, or a failed condensate pump. You can safely check the outside end of the pipe. Anything inside the boiler casing is work for a Gas Safe registered engineer.
Loyal Plumbers · Updated
Discuss your plumbing workWhat the condensate pipe does
Every modern gas boiler is a condensing boiler. It cools its flue gases enough to recover extra heat, and in doing so produces water, often several litres on a cold day. That water is mildly acidic. It collects in a trap inside the boiler and drains away through a plastic pipe, usually white or grey, to a sink waste, a soil stack, a gully or a soakaway outside.
If the pipe cannot drain, the condensate fills the trap and backs up into the boiler. Most boilers detect this and shut down, which is why a blocked pipe looks like a boiler breakdown.
Signs the condensate pipe is blocked
One of these on its own can have other causes. Two or three together point strongly at the condensate. If the weather has been freezing, start with our guide to a frozen condensate pipe instead.
- A gurgling or bubbling noise from the boiler, especially as it tries to fire
- The boiler locks out and shows a fault code, then does the same again after a reset
- Water dripping from the underside of the boiler or around the condensate connection
- No water coming out of the end of the pipe outside while the boiler is running
- Water pooling where the pipe ends, or the end sitting in mud, leaves or a blocked gully
Fault codes for condensate problems differ between makes and models, and the same code can have other causes. Check the manual or the manufacturer's website before assuming it is the pipe.
Why condensate pipes block when it isn't freezing
Several causes have nothing to do with the weather, and each one points to a different fix.
| Cause | What you see | Who fixes it |
|---|---|---|
| Sludge in the boiler's internal trap | Gurgling and lockouts, outside pipe clear | Gas Safe registered engineer, often at a service |
| Leaves, soil or debris at the outside end | End of pipe buried, blocked or inside a full gully | You can often clear it |
| Pipe sagging or run with too little fall | Pipe dips between clips; water sits in the low point | Plumber or heating engineer |
| Pipe crushed, cracked or disconnected | Water leaking at a joint or inside a cupboard | Plumber or heating engineer |
| Blocked sink waste or stack it connects to | Sink also slow; boiler locks out when the sink is used | Plumber |
| Condensate pump failed | Pump not running, water in its reservoir | Plumber or heating engineer |
| Soakaway silted up | Pipe ends in the ground and water no longer drains away | Plumber or heating engineer |
What you can safely check
Turn the boiler off at its controls first. Then follow the pipe from where it leaves the wall to where it ends. Clear leaves or soil from the open end and make sure it is not sitting in standing water or packed into a gully grid. Look along its length for a section that sags between clips, a joint that has pulled apart or a pipe that has been crushed by a bin or a ladder.
If the pipe drains into the kitchen or bathroom waste indoors, run that sink. If it is slow, the blockage may be in the waste, not the condensate pipe, and our guide on how to unblock a sink can help.
Once you have cleared an obvious blockage, reset the boiler following the manufacturer's instructions. Our guide on how to reset a boiler explains how, and when you should not keep trying. If it locks out again, stop there: repeated resets will not clear a blocked internal trap.
Do not remove the boiler casing or try to take out the internal condensate trap yourself. On most boilers the trap is inside the sealed case, and opening it is work for a Gas Safe registered engineer. Condensate is mildly acidic, so wear gloves and keep it off skin and eyes.
When the boiler has a condensate pump
Where a boiler is fitted somewhere the pipe cannot drain downhill, such as a basement or a kitchen with no nearby waste, a small pump lifts the condensate to a drain. If the pump fails, has lost power or has a blocked reservoir, the boiler will lock out in exactly the same way. Check the pump's power supply and listen for it running when the boiler fires. Our guide on boiler condensate pump maintenance covers what these pumps need.
Stopping it happening again
Most internal blockages are sludge that builds up slowly. The trap is normally checked and cleaned at the annual boiler service, which is one of the reasons a missed service can lead to a winter breakdown.
Outside, the pipe should run with a steady fall, be properly clipped so it cannot sag, and end where it cannot be buried or blocked. Manufacturers also recommend keeping the external length short and using a larger pipe outside, which helps against both blockage and freezing. If your pipe has a long external run or was clearly put in as an afterthought, our guide to boiler condensate pipe routing explains the options for improving it.
What an engineer will do
A Gas Safe registered engineer will check the fault history, remove and clean the internal condensate trap and its connections, check the siphon for sludge, and test the pipe outside for fall and blockages. They will clear or re-lay any section that holds water, fix leaking joints, and check the pump or soakaway if there is one. The boiler is then run and checked to make sure the condensate drains away freely and it no longer locks out.
Get your boiler running again
Loyal Plumbers covers London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Gas work is carried out by Gas Safe registered engineers. Call 020 4652 8585 and tell us the boiler make and model, any fault code on the display, and where the condensate pipe runs.
Our call-out charge is £60, and the engineer explains what they have found and what the work involves before starting. You can read more about our boiler repair and boiler servicing work.
Common questions
- How do I know if my condensate pipe is blocked?
- The boiler usually gurgles, locks out with a fault code and does the same again after a reset. There may be water dripping from under the boiler, and no water coming out of the pipe outside while it runs.
- Can a condensate pipe block when it isn't freezing?
- Yes. Sludge in the boiler's internal trap, leaves or soil at the outside end, a sagging pipe that holds water, a blocked sink waste or a failed condensate pump can all block it at any time of year.
- Can I clear a blocked condensate pipe myself?
- You can clear debris from the outside end, check the pipe for sags or damage and check the waste it connects to. The trap inside the boiler is behind the sealed casing and needs a Gas Safe registered engineer.
- Is a blocked condensate pipe dangerous?
- The boiler shuts down to protect itself, so it is not usually dangerous, but you lose heating and on a combi hot water too. Condensate is mildly acidic, so wear gloves if you handle it. If water is leaking inside the boiler, switch it off and call an engineer.
- Does a boiler service clean the condensate trap?
- Checking and cleaning the condensate trap is normally part of an annual service, following the manufacturer's instructions. Ask the engineer to confirm it was done.
Sources and further reading
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