When to use a pump vs. gravity
Whenever you can run the drain with a fall of at least 1–2% (1–2 cm per metre) to the sewer or outdoors, choose gravity — fewer parts, no power, no noise. A pump comes in where the unit's drain pan sits below the discharge point: wall splits in basements, cassettes and ducted units above suspended ceilings, indoor heat-pump units in plant rooms with no nearby connection.
Remember that in cooling a 3.5 kW split produces roughly 0.5–1.0 l/h of condensate; larger cassettes and heat pumps during defrost produce considerably more. A pump only transports condensate — it does not replace a correctly graded gravity section downstream of it.
Sizing flow rate and lift height
Size on two parameters: flow rate (l/h) and maximum lift (m). Manufacturers quote peak flow at zero lift — at real height it can drop by half, so read from the performance curve, not the headline figure. Rule of thumb: pick a pump with at least 2× the flow margin over the unit's condensate rate, and never run below 70% of the rated lift.
Peristaltic and mini pumps (mounted in the casing or in a trunking) typically give 6–15 l/h and 6–10 m of lift — enough for splits up to ~7 kW. For cassettes, ducted units, higher capacities and heat pumps use float-switch tank pumps rated 100–500 l/h. Lift is measured from the pump level to the highest point of the tube, not to the end of the discharge.
Installation, level sensor and alarm protection
Mount the pump and sensor (detector) perfectly level — a tilted float will misread switching. Run the discharge tube first vertically up to the apex, then on a gravity fall; avoid long horizontal pressurised runs. Fit an anti-siphon loop (swan neck) on the discharge tube above the max water level, so air breaks back-siphoning and the pump does not drip after it stops.
Always wire the pump's alarm contact (NC/NO) to the unit controller so that a high-level trip cuts the compressor or cooling mode. This anti-flood interlock is a mandatory standard — without it a pump failure ends in a flooded ceiling. Discharge tube diameter is usually 6 mm (peristaltic) or at least 1/2" for tank pumps.
Commissioning, service and common mistakes
Before handover, pour a test charge of about 0.5–1 l of water into the pan and verify the full cycle: start, empty, stop, and alarm trip. Check there is no back-siphoning and that the fall after the pump carries condensate away. Use a trap on the gravity drain downstream of a pump with care — combined with a pump it can form air locks.
The most common faults are a float fouled with biofilm and a blocked pan/tube — advise the client to clean and use biocide tablets each season. Other traps: missing anti-siphon loop (dripping), sensor not mounted level, underestimated lift, and an alarm left unconnected. Pump clicking loudly? It usually runs dry or draws air through a leaking connection.