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Air conditioning condensate pumps and drains in commercial buildings

Updated 23 October 2026 · SEO Dons Editorial

How does condensate leave a commercial air conditioning unit?

Cooling takes moisture out of the air, and that water has to leave the building. A gravity drain relies on pipework falling continuously from the unit to the discharge point. A pumped drain lifts the water first, using a pump built into the unit, a small pump beside it, or a tank pump collecting from several units. The choice usually comes down to whether the ceiling void allows a continuous fall.

A unit already dripping is covered in the leaking water guide. The retrofit guide shows where drainage sits when cooling is added to an existing office.

What do manufacturers ask for on a gravity drain?

PointWhat the manual says
FallDaikin asks for a slope of at least 1/100 so air is not trapped in the piping. Mitsubishi Electric asks for a pitch of more than 1/100 towards the discharge side.
Air ventsMitsubishi Electric says never to provide an air vent pipe, because the drain may be ejected.
TrapsFor its ducted PEFY units, Mitsubishi Electric says no trap or irregularity on the way, and no odour trap at the discharge port.
Grouped drainsDaikin allows drain pipes to be combined with correctly gauged pipes and T-joints. Mitsubishi Electric says to make the piping large and keep collected pipes 10 cm below the unit's drain port.
InsulationDaikin says to insulate the complete drain piping in the building.
End of the pipeMitsubishi Electric says to end it where no odour is generated, and not in any drain where ionic gases are generated.

A fall of 1 in 100 is 10 mm of drop per metre, so a 10 m run needs 100 mm of height before supports and joints are counted. The traps row shows why one manual's rule should not be applied to every unit. Gravity is not automatically safer: Sauermann, a pump maker, says blockages often occur in gravity-fed systems because the very low fluid pressure lets a plug build up.

Which types of condensate pump are used?

TypePublished example
Built into the indoor unitMitsubishi Electric says its PLFY-P-VLMD-E cassette has a built-in drain pump that can lift condensate up to 583 mm from the ceiling's surface. Daikin's FXFQ cassette manual shows rising drain piping 675 mm perpendicular to the unit.
Mini pump in a ceiling void, behind the unit or in trunkingSystemair describes its MINI pump as suitable for false ceilings, behind air conditioning units or in the cable duct. Sauermann's Si-30 is sold for units up to 20 kW.
Tank pump for larger or multiple unitsSauermann says its tank pumps are made for large heating, refrigeration and central air conditioning systems producing substantial condensate. The Si-82 has a 2 L tank and can be floor mounted, wall mounted or hung from a ceiling.

These are makers' own examples, not recommendations. The data sheet for the exact model governs.

Does a pump's rated lift tell you what it will deliver?

No. A headline figure is a limit, not a duty point. Sauermann's Si-30 data sheet gives a maximum flow of 20 l/h and a maximum discharge head of 10 m, with the flow at that head falling to 4 l/h. The Si-82 data sheet has a table of actual flows. Through 5 m of tube it lists 380 l/h at a 1 m vertical discharge head and 30 l/h at 5 m, and through 20 m of tube it lists no flow at 5 m.

Sauermann's checklist says to confirm that a pump copes with the length of the drain tubes and the amount of condensate produced. Selection therefore starts from the real lift and tube length, reads the flow at that point, and compares it with every unit on the pump. For example, four cassettes sharing one tank pump need the flow at the real head to cover all four together.

What should a pump alarm do?

Pumps carry a safety (alarm) switch. Sauermann lists the Si-30's as a normally closed contact rated 3 A resistive at 250 V, and the Si-82's as 4 A at 250 V. Systemair's MINI pump has a 3-stage float switch with OFF, ON and ALARM positions. The Si-30 kit includes a cable with two power wires and two safety switch wires, so the alarm is a separate pair of conductors that someone has to connect to the unit.

Sauermann says a connected alarm will automatically switch the unit off if the pump malfunctions, and says to check that it is connected. It adds that the pump should run independently of the unit, which can keep producing condensate long after it is switched off. A commissioning test that fills the reservoir until the alarm trips, and confirms the unit stops, proves both. The commissioning and handover guide covers the records.

Where can condensate be discharged?

Approved Document H, the Building Regulations guidance on drainage, does not settle it. Its condensate paragraph says condensate drainage from boilers may be connected to sanitary pipework. It is not a rule for air conditioning, and none of the official pages used here names air conditioning condensate. What they do cover is permission:

  • England. The Environment Agency says to check with the sewerage undertaker, usually the local water company, before making a new public sewer connection or discharging anything other than domestic sewage. It says no permit is needed to discharge uncontaminated water such as clean rainwater from roofs to surface water.
  • England and Wales. Section 106 of the Water Industry Act 1991 entitles an owner or occupier to have drains communicate with the public sewer, but the sewerage undertaker has twenty-one days after notice to refuse on specified grounds.
  • Scotland and Northern Ireland. NetRegs says permission is needed from the water and sewerage company before discharging anything to its drainage system, and a tenant may need the landlord's permission to connect to a private surface water drain.

Whether a drain's condensate counts as clean water is for the undertaker or regulator to confirm, not an assumption for a design.

How do retrofits design out the usual failure points?

  • Prove the fall on the real route first. If 1 in 100 cannot be found, choose a unit with a built-in pump or plan the lift, sizing any pump from its flow-at-head table.
  • Keep the pump reachable and cool. Sauermann says pumps typically switch off above a certain temperature, and one in a poorly insulated attic can trip.
  • Keep the detection float level. Sauermann says it can overflow before alerting the pump if not perfectly horizontal. Daikin warns that a unit tilted against the flow can stop its float switch working.
  • Clamp joints and avoid pinched tubes. Sauermann says joints either side of the pump should be secured with wire clamps.
  • Wire and test the alarm. Daikin's installer test is to pour about 1 l of water through the air discharge outlet and check for leaks.
  • Agree the discharge point in writing with the building owner and, for a new sewer connection, the sewerage undertaker, then record the pump model, alarm wiring and discharge point in the handover pack.

Readers weighing a full installation will find the wider scope on the commercial air conditioning installation page, and can send an enquiry through the site's form.

Sources

Common questions

Can one pump serve several indoor units?

Tank pumps are made for larger loads: Sauermann's Si-82 is rated at 500 l/h with a 5 m maximum discharge head. The check is the flow at the real head and tube length against the combined condensate of every unit on the pump. How one alarm contact will stop every unit it protects is worth settling at design stage.

Are condensate pumps noisy?

Makers publish sound levels under stated conditions. Sauermann lists 20 dBA for the Si-30 at 1 m, measured to EN ISO 3744 with the pump operating with water, and 45 dBA for the Si-82 in application. Those are laboratory figures, so position and mounting matter. Outdoor plant noise is a separate matter, covered in the guide to air conditioning noise.

Can condensate be piped into a rainwater downpipe?

The official pages checked describe clean rainwater and uncontaminated water going to surface water routes, and none names air conditioning condensate. A tenant may also need the landlord's permission to connect to a private surface water drain. The building owner and the sewerage undertaker are the people to ask before a downpipe is used.

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