What is a condensate pipe?
A condensate pipe is the drain for the water a condensing boiler produces as it works. A condensing boiler cools its flue gases until the water vapour in them turns back into liquid, recovering extra heat in the process. That liquid, called condensate, collects in a trap inside the boiler and runs out through a white or grey plastic pipe, usually from the underside of the boiler.
The amount varies with how hard the boiler is working and how cool the water returning from the radiators is. Guidance from the Heating and Hotwater Industry Council (HHIC) puts it at around 2 litres an hour when a boiler is condensing well. Some boilers use a siphon trap that lets the water out in batches rather than a steady trickle, so it is normal to see nothing for a while and then a short flow.
Why the water is acidic
Burning gas produces carbon dioxide and small amounts of nitrogen oxides. Some of these dissolve in the condensing water and make it mildly acidic. That is why the pipe is plastic rather than copper, and why the condensate must go to a proper drain, never onto a path, lawn, flower bed or into a water butt. Over time it can etch concrete and kill plants.
It is easy to confuse the condensate pipe with the copper pipe from the pressure relief valve, which usually pokes out through the wall and only drips when the system pressure is too high. Our guide to the boiler pressure relief valve explains how to tell them apart.
Where does a boiler condensate pipe go?
A boiler condensate pipe should go to an internal foul drain wherever possible, such as the soil stack or a kitchen, utility or bathroom waste pipe. The HHIC installer guide, which follows BS 6798:2014 and the manufacturers' instructions, lists the options in order of preference.
| Termination point | Inside or outside | Notes |
|---|---|---|
| Soil stack | Inside | Preferred route; the pipe stays warm and cannot freeze |
| Sink, basin, bath or shower waste | Inside | Connection point and any extra trap follow the manual |
| Condensate pump to an internal drain | Inside | Used when there is no fall, such as a basement boiler |
| External drain or gully | Outside | Pipe ends below the grating but above the water level |
| Rainwater downpipe or hopper | Outside | Only where the downpipe goes to a combined foul and rainwater drain, with an air break |
| Purpose-made soakaway | Outside | Last resort where no drain is available |
Many older installations run the pipe straight out through the wall and down to a gully, because it was the quickest route. These are the ones most likely to freeze in a cold snap.
Internal and external routing
An internal route is the safest because the pipe stays in heated space. The guidance calls for a condensate trap with a water seal of at least 75 mm, which most current boilers have built in, to stop drain smells coming back into the house. If the pipe joins a waste pipe, the installer follows the manual on where to connect and whether an extra trap or air break is needed.
An external route is allowed when an internal one is not practical, but it needs more care. The HHIC guidance says the external run should take the most direct and most vertical route and be no longer than 3 metres. Horizontal runs, dips and long exposed lengths give the water somewhere to sit and freeze. Any part of an internal pipe that passes through a cold space, such as a garage or loft, should be insulated too.
What size should a condensate pipe be?
A condensate pipe inside the house should be at least 19 mm internal diameter, which is usually 22 mm plastic pipe. A pipe that runs outside should be at least 30 mm internal diameter, usually 32 mm pipe, and it should be increased to that size before it passes through the wall. These figures appear in the HHIC guide and in manufacturer manuals such as the Ideal Logic Max installation guide.
| Requirement | Typical figure | Source |
|---|---|---|
| Internal pipe | At least 19 mm internal diameter (22 mm pipe) | HHIC guide, manufacturer manuals |
| External pipe | At least 30 mm internal diameter (32 mm pipe) | HHIC guide, manufacturer manuals |
| Fall on horizontal runs | At least 45 mm per metre (BS 6798); many manuals ask for 52 mm per metre | BS 6798, Ideal Logic Max manual |
| Maximum external length | 3 metres | HHIC guide |
| External insulation | At least 19 mm thick, weatherproof and UV resistant | HHIC guide |
The larger external pipe matters because a thin trickle in a narrow pipe freezes much faster than the same water in a wide one, and a wider pipe takes longer to block completely with ice.
How do I stop my condensate pipe freezing?
The best way to stop a condensate pipe freezing is to route it internally. If it has to run outside, it should be 32 mm pipe, as short and vertical as possible, and protected by insulation or trace heating. The HHIC guidance asks for at least one of these measures on an external run:
- Insulation at least 19 mm thick with a weatherproof outer layer.
- Trace heating, an electric heating cable along the pipe controlled by an outdoor thermostat, under insulation.
- A high-volume siphon unit fitted inside, which releases condensate in larger bursts so it clears the pipe before it can freeze.
If your pipe does freeze, thawing it is usually a safe DIY job with warm (not boiling) water. Our guide to a frozen condensate pipe goes through it step by step, and boiler not firing up explains why ice in the pipe stops the boiler lighting.
Condensate pipe insulation
Adding condensate pipe insulation to an exposed external pipe is one of the few boiler-related jobs a homeowner can reasonably do. Use foam pipe lagging sized for 32 mm pipe, ideally a type rated for outdoor use with a UV-resistant or waterproof cover, or wrap standard lagging in weatherproof tape. Split it along its length, fit it round the pipe without pulling on any joints, and seal the seams.
Leave the last few centimetres of the pipe uncovered where it enters the gully or drain, and do not tape over the open end. The water must be able to leave freely, and insulation that sits in wet leaves at the bottom just freezes solid itself.
If the pipe is 22 mm outside, has long horizontal sections or dips, or has frozen before even with lagging, insulation alone may not be enough. That is the point to ask an engineer about rerouting it inside or upsizing it.
Signs of a condensate pipe problem
| What you notice | Likely cause | What to do |
|---|---|---|
| Gurgling from the boiler in freezing weather, then a lockout | Ice in the external pipe | Thaw it, then insulate |
| Ignition or flame code in cold weather, such as Vaillant F28 or Ideal L2 | Frozen or blocked condensate | Check the pipe before calling anyone |
| Drips or a damp patch under the boiler | Leaking joint or trap | Engineer; see boiler leaking water |
| Drain smell near the boiler | Trap dried out or air break problem | Engineer |
| Staining or a puddle where the pipe ends | Poor termination, pipe not reaching the drain | Engineer to correct the route |
When to call an engineer
Call a Gas Safe registered engineer if the boiler keeps locking out when the pipe is clear, if water is leaking from under the boiler or if the pipe needs rerouting, resizing or a condensate pump. The boiler's internal trap should be checked and cleaned at the annual boiler service, which is also a good time to ask whether an external pipe meets current guidance.
Do not open the boiler case to reach the condensate trap. It sits beside gas and electrical parts, and only a Gas Safe registered engineer should work inside the boiler. Trace heating runs on mains electricity, so it should be fitted by a qualified electrician or an engineer who is qualified to do the electrical work.