Boiler condensate not draining

26 Nov 2023
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United Kingdom
Hi all,

Got a bit of an odd one here and I've not been able to find a solution. I have an engineer attending this week but anything that could be suggested to get things working in the meantime would be greatly appreciated given the cold weather at the moment.

I have a Worcester Bosch Greenstar 4000 25 kW that's been installed for several months without issue.

The Issue
Yesterday the boiler locked out due to flame failure. After restarting, it ran for a short while with a gurgling sound before throwing another flame failure fault. I figured this was a blocked condensate drain.

I popped the cover off and the condensate trap was full. So, I disconnected the rubber outlet from the condensate drain and into a bucket. Nothing happened and the plastic condensate pipe didn't appear backed up. Puzzled, I went to have a look online and suddenly, all the water drained out and half filled the bucket in one go. I also popped the cap off the trap and emptied another reasonable amount of water.

Connected back up and working fine for the evening and a couple of hours this morning.

It's gurgling again and the condensate trap is full. Disconnected the drain again and no water came out; the drain outlet is also completely dry. I took the cap off the condensate trap and got some water out, enough to stop it gurgling again. The trap is still reasonably full.

I've left the boiler running with the condensate outlet into a bucket. The trap is full, but no water has drained from the condensate outlet at all.

It's as if the boiler isn't draining any condensate out at all, despite the trap being full, almost as if there's an airlock (if this is possible?).

Running the boiler with the cap off the bottom of the condensate trap into a bucket does result in condensate dripping out.

Condensate Drain
The condensate pipe runs into the wall and not outside the property.

This was poorly installed when the house was built. After this new boiler was installed, I had some water leaking through the ceiling below once the boiler was running for around an hour. My installer came out, cut out a small piece of ceiling and found an elbow joint leaking. Whilst the elbows were standard, the pipe itself was actually electrical conduit of a slightly different diameter and the elbow had been badly sealed with adhesive. The engineer replaced this section of pipe and used adaptors to connect and seal the drain. There have been no more leaks since.

The pipe seems to run through the wall from here and into the downstairs toilet. As this is behind a wall, I haven't been able to inspect how this is connected to a soil stack. Presumably there's a trap/u bend before the soil pipe connection.

Any advise appreciated!
Last edited:
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