Run it inside wherever you can: at least 19 mm bore (typically 22 mm plastic), falling at least 45 mm per metre to an internal soil stack or waste. If it must go outside, keep the run short, use at least 30 mm bore (typically 32 mm), insulate it and end it at a gully, suitable drain or soakaway, following the boiler instructions.
A condensing boiler gets its efficiency by cooling its flue gases until the water vapour in them condenses. That water, the condensate, is mildly acidic and runs whenever the boiler is condensing, so it needs a pipe that drains it away reliably. Many condensate faults are not the boiler at all: a pipe that sags, runs too flat or freezes outside backs up and locks the boiler out, usually on the coldest morning of the year.
What the condensate pipe has to do
Condensate leaves the boiler through its condensate trap, which holds a water seal so that flue products cannot escape down the pipe, and then drains by gravity, or by a pump, to a suitable discharge point. Because it is acidic, the pipework is plastic: condensate corrodes copper and some other metals.
The pipe has to keep falling from the boiler all the way to where it discharges, with no dips or long flat runs where water can sit. Standing water is what freezes first and what blocks with sludge.
Where it can discharge
The industry’s installer guidance for new and replacement boilers ranks the options. Use the first one that works for the property, and check the boiler instructions, which can be stricter.
- An internal soil and vent stack: the preferred connection.
- An internal waste pipe from a sink, basin, bath or shower, connected downstream of that fitting’s trap.
- A condensate pump, where no gravity route to an internal point is possible, pumping to one of those internal points.
- Outside, an external gully, with the pipe ending below the grating but above the water level.
- An external soil stack or rainwater downpipe, but only where it runs to a combined foul and rainwater drain, and with an air break at the connection so that water cannot back up into the pipe.
- A purpose-made condensate soakaway, close to the boiler but clear of the foundations and of other buried services.
Pipe size and fall
Inside the building the usual minimum is 19 mm internal diameter, which in practice means 22 mm plastic pipe, unless the boiler instructions say otherwise. It should fall at least 45 mm per metre, about 2.5 degrees, away from the boiler and take the shortest practical route. Over a 3 metre run that is at least 135 mm of fall; the drain fall calculator turns a gradient into millimetres over any length.
Where the pipe has to run outside or through an unheated space, the exposed section is increased to at least 30 mm internal diameter, typically 32 mm plastic waste pipe, with the increase made before the pipe leaves the building. A wider bore takes longer to freeze solid and is harder to block with ice. Keep that external run as short as possible; general guidance puts it at no more than about 3 metres.
Internal and outside diameters are easy to mix up in an exam question: 22 mm and 32 mm are the pipe sizes you buy, while 19 mm and 30 mm are the bores the guidance sets as minimums.
Stopping it freezing
Good routing reduces the risk; it cannot remove it. A long, severe freeze can still freeze a correctly installed external run, so a freeze on its own does not prove the installation was wrong. It is still a reason to check the route, size, fall and insulation.
- Keep the route inside the heated part of the building wherever possible. Pipe in an unheated loft, garage or void can still freeze, even though it is indoors.
- Where it must go outside, keep it short, upsize it and insulate it, joints included, with waterproof, UV-resistant insulation.
- Keep a continuous fall with no dips, and avoid long horizontal runs where condensate can pool.
- Keep the boiler’s condensate trap and any required air break: removing either is never a frost fix.
- In an exposed spot, consider a route kept inside with a condensate pump rather than a long gravity run outside.
A frozen condensate pipe: symptoms and the safe fix
The usual sign is a boiler that has locked out with a fault code in freezing weather, sometimes with a gurgling noise as the condensate backs up. The blockage is usually where the pipe is most exposed: at a bend, a low spot or the open end outside.
If the frozen section can be reached safely from the ground, thaw it with warm water, never boiling, or with a hot-water bottle or heat pack held against it, then reset the boiler as its instructions describe. Boiling water can crack plastic pipe, and spilt water refreezes underfoot, so take care where you stand, and do not climb a ladder in icy conditions to reach a pipe.
If it keeps freezing, change the installation instead of thawing it again: reroute it inside, upsize and insulate the external section, or fit a condensate pump. That is a job for a heating engineer following the boiler instructions.
What assessments and exams test
Condensate questions turn up in CENWAT boiler work and in Level 2 and Level 3 plumbing, and they usually test the reasoning rather than a table:
- the preferred discharge point, and when an external downpipe is acceptable;
- why the pipe is plastic, and what the trap and its water seal are for;
- the minimum fall and the minimum bore inside and outside, including the internal-against-outside-diameter trap;
- why the exposed section is upsized and insulated, and what to check after a freeze.
Quick answers
What size should a condensate pipe be?
Inside the building, at least 19 mm internal diameter, which usually means 22 mm plastic pipe, unless the boiler instructions say otherwise. Any section that runs outside or through an unheated space is increased to at least 30 mm internal diameter, typically 32 mm plastic waste pipe, before it leaves the building.
What fall does a condensate pipe need?
At least 45 mm for every metre of run, about 2.5 degrees, falling away from the boiler all the way to the discharge point with no dips or flat sections. Over a 3 metre run that is at least 135 mm.
Can a condensate pipe go into a rainwater downpipe?
Only where the downpipe runs to a combined foul and rainwater drain, and with an air break at the connection so that water cannot back up into the pipe. A downpipe that runs to a surface-water drain or a soakaway for rainwater is not a suitable discharge point under the installer guidance.
How do you unfreeze a frozen condensate pipe?
Find the frozen section, usually at a bend or the open end outside. If you can reach it safely from the ground, pour warm (not boiling) water over it or hold a hot-water bottle against it until it clears, then reset the boiler using its instructions. If it freezes repeatedly, have the pipe rerouted, upsized or insulated.
Can condensate pipe be copper?
No. Condensate is acidic, so condensate pipework is plastic; copper and some other metals corrode in it.