Which pipe should I use for a boiler condensate drain?
A condensing boiler recovers heat by cooling flue gases below their dew point, and the water that condenses out carries dissolved combustion products, making it mildly acidic — typically around pH 3 to 4. That is why the drain is plastic and why it must not be run in copper or discharged where it will corrode something. The far more common failure is mechanical: on the first hard frost of the year, condensate freezes in an external pipe, the boiler detects the blockage and locks out, and thousands of households lose heating on the same morning. The fixes are all preventive — keep the run internal if possible, otherwise use a larger diameter, insulate it with weatherproof lagging, keep it short and steeply falling, and avoid external elbows and horizontal sections where ice can form.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| Plastic waste pipe (typically 22 mm internal) | Unaffected by the acidity of the condensate and easy to run with fall. | Too narrow a pipe outdoors freezes readily. |
| Larger diameter for external runs (32 mm or more) | The recognised measure against freezing, alongside insulation and keeping the run short. | Bulkier and more visible on an external wall. |
| Internal termination to a waste stack | By far the most reliable arrangement, because nothing is exposed to frost. | Not always possible depending on the boiler position. |
| Copper | Nothing here. | Boiler condensate is acidic, around pH 3-4, and it corrodes copper and steel. |
Technique that changes the outcome
- Terminate internally into a waste stack wherever the boiler position allows
- Use at least the diameter the boiler manufacturer specifies for external runs, and insulate it
- Maintain a continuous fall with no dips that can hold water
- To thaw a frozen pipe, pour warm — not boiling — water over it and reset the boiler
What goes wrong
- Copper or steel for condensate
- Long external runs with elbows and shallow falls
- Discharging onto a path where it will freeze into a sheet of ice
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other pipe questions
| For | Use |
|---|---|
| Hot and cold water | Copper or PEX for most installations — PEX is faster and more freeze-tolerant, copper is more robust and takes heat without question. |
| Waste and drainage | Solvent-weld or push-fit PVC-u or polypropylene, at the right diameter and fall — and ABS and PVC cements are not interchangeable. |
| Central heating | Copper, or a barrier-grade plastic pipe — non-barrier plastic lets oxygen into the system and corrodes it from the inside. |
| Gas | Steel, copper of the right grade, or yellow MDPE underground — and in most jurisdictions only a registered installer may do the work. |
| An underground water supply | Blue MDPE for potable water, in one continuous length at the required depth, with no joints under a building. |
| Compressed air | Copper, steel, stainless or a purpose-rated aluminium or HDPE air system — never PVC, which fails explosively. |
| Irrigation | Low-density polyethylene tubing with barbed fittings for drip lines, and PVC or MDPE for buried mains. |
Common questions
- Which pipe should I use for a boiler condensate drain?
- Plastic, never copper, at a generous diameter with continuous fall — and insulate any external run, because frozen condensate is the classic winter breakdown.
- What should I avoid?
- Copper or steel for condensate Long external runs with elbows and shallow falls Discharging onto a path where it will freeze into a sheet of ice
Related pages
- Which pipe should I use for hot and cold water? Copper or PEX for most installations — PEX is faster and more freeze-tolerant, copper is more robust and takes heat without question.
- Which pipe should I use for waste and drainage? Solvent-weld or push-fit PVC-u or polypropylene, at the right diameter and fall — and ABS and PVC cements are not interchangeable.
- Which pipe should I use for central heating? Copper, or a barrier-grade plastic pipe — non-barrier plastic lets oxygen into the system and corrodes it from the inside.
- Which pipe is used for gas? Steel, copper of the right grade, or yellow MDPE underground — and in most jurisdictions only a registered installer may do the work.
- Which pipe should I use for a buried water supply? Blue MDPE for potable water, in one continuous length at the required depth, with no joints under a building.
- Which pipe should I use for compressed air? Copper, steel, stainless or a purpose-rated aluminium or HDPE air system — never PVC, which fails explosively.
- Which pipe should I use for irrigation? Low-density polyethylene tubing with barbed fittings for drip lines, and PVC or MDPE for buried mains.
Sources
- Typical range compiled from manufacturer specifications — At One Place