Night Purge, Free Cooling and Mixed-Air: AHU Control Modes to Spec

A unit sized correctly for design load can still cost more to run than it should, because the fan, dampers and heat recovery bypass were never asked to do anything other than run at 100 % constant flow with the heat exchanger active. Night purge, free cooling and mixed-air economizer control all reuse the same casing and coils — but each needs damper authority, leakage class and fan turndown numbers that a mechanical-cooling-only spec never mentions. Leave them out and the controls contractor either can't implement the sequence or implements it against hardware that fights it.

Three modes, three different jobs

Free cooling (sometimes "economizer mode") replaces mechanical cooling with outdoor air whenever outdoor conditions are more favourable than the return air setpoint — a straight temperature comparison in a dry climate, an enthalpy comparison wherever humidity swings independently of temperature. EN 16798-3 sets the ventilation and building-classification framework this logic sits inside for non-residential buildings; the switching logic itself is a controls sequence, not a standard, but the AHU has to physically support it.

Night purge (night cooling) runs the supply fan — often without mechanical cooling and sometimes without occupancy-driven ventilation demand — during unoccupied hours to flush heat out of exposed thermal mass before the building re-opens. It only works where the structure has real mass to charge (exposed concrete soffits, not a lightweight fit-out) and where night outdoor temperature drops meaningfully below the following day's peak — most of temperate Europe, most of the cooling season.

Mixed-air control is the damper choreography that makes both possible: outdoor air, return air and exhaust dampers modulating together, from minimum fresh-air position up to 100 % outdoor air, instead of a fixed minimum-OA damper sized only for occupied ventilation rate.

What the spec has to state

Parameter	Mechanical-cooling-only spec	Free cooling / night purge spec
OA damper range	Fixed minimum position	0–100 % modulating, sized for full design flow at full-open
Damper leakage class	Unspecified	EN 1751 Class 3 minimum, Class 4 where winter free-cooling hours matter
HRS during free cooling	N/A	Bypass or stop — recovering heat from air you're deliberately not conditioning wastes fan energy for nothing
Fan turndown	Not applicable at part load	VFD range covering purge flow (often design flow or above) down to minimum ventilation
Sensors	Supply/return temperature	Outdoor T + RH (or enthalpy), return T, differential logic point
Interlocks	Frost only	Frost, plus purge lockout below a minimum outdoor temperature and above a maximum humidity

The leakage class line is the one that gets skipped most often. A Class 2 damper leaks several percent of design flow even fully closed; during a winter free-cooling event with the heat recovery bypassed, that leakage is untreated sub-zero air bypassing the exchanger entirely — a frost and comfort risk the coil selection never accounted for. Coil frost protection strategy assumes the bypass path is actually sealed when closed; a leaky damper defeats the strategy it's paired with.

Sizing the fan for purge, not just design flow

Night purge is usually run at or above design airflow, because the point is maximum heat extraction in a limited overnight window — but the fan, duct velocities and acoustic path were selected against occupied-hours SFP targets, not a purge condition nobody hears because the building is empty. Two things follow: first, check duct velocity and any silencer pressure drop at the purge flow rate, not just design flow, so the fan curve doesn't stall against a duct system sized for a lower velocity. Second, size the VFD turndown range down to true minimum ventilation, not just the occupied setpoint — mixed-air economizer control needs the fan to hold stable at whatever flow the free-cooling sequence calls for, which won't always be at the fan's best-efficiency point. The fan-power consequence of running off that point is the same physics as part-load SFP droop — the spec should ask for the selection curve at the purge duty, not assume the 100 %-flow SFP figure still applies.

What this means for the selection, not just the sequence

None of this changes the coil or filter selection — it changes what the AHU has to be capable of doing at duties the design-day calculation never runs. A free-cooling and night-purge capable unit needs modulating, low-leakage dampers rated for full-flow OA, a bypass or stop on the heat recovery section, and a fan curve checked at purge flow as well as design flow — all decisions made once, at spec stage, that a controls-only retrofit can't add later without re-ducting the damper section.

In AirSelect3D, damper apertures and heat recovery bypass are part of the same 3D model that drives the fan and coil selection, so a free-cooling or night-purge duty can be checked against the same geometry and curves as the design-day case, not a separate assumption.

Check your AHU's free-cooling and purge duty →