ASHRAE 90.1 Building Controls, Airside Economizers, Mixed-Air Dampers & Free Cooling
HVAC Airside Economizer Mixed-Air & Free-Cooling Formula
Calculate Modulating Outside-Air Damper Fraction (%), Mixed-Air Plenum Temperature T_mix (°C), Minimum-Ventilation Baseline Mixed Temp (°C), and Chiller Sensible Free-Cooling Saved (kW).
Running a chiller in 11 °C shoulder weather wastes 70+ kW when modulating AHU economizer dampers hit 14.5 °C supply air for free!
Commercial internal-load buildings (data rooms, offices, retail) still demand `13–15 °C` cold supply air even when outdoor dry-bulb `T_oa` sits comfortably at `5–16 °C`! Under ASHRAE 90.1 Airside Economizer control, whenever `T_oa < T_lockout` (high-limit dry-bulb/enthalpy changeover, e.g. `19.0 °C`), the AHU modulates opposing Outside-Air (`OA`) and Return-Air (`RA`) dampers above ASHRAE 62.1 Minimum Ventilation `f_min` (`~15%–25%`) to hit Mixed-Air Setpoint `T_set`: solving steady-mass energy balance `f_oa * T_oa + (1 - f_oa) * T_ra = T_mix` yields Modulated Outside-Air Fraction `f_oa = MIN(1, MAX(f_min, (T_ra - T_set) / (T_ra - T_oa)))` (reverting immediately to `f_min` whenever `T_oa >= T_lockout` so hot outdoor air is locked out)! Compared against Minimum-Ventilation Baseline `T_mix,min = f_min * T_oa + (1 - f_min) * T_ra`, Sensible Chilled-Water Load Unloaded is `Q_free = rho_air * c_p * V_dot * MAX(0, T_mix,min - T_mix)` (`1.20 * 1.006 * Q_m3_s * Delta_T`) kW!
Verify on an 8.50 m³/s (`18,000` CFM) Office Air-Handling Unit—Supply_Airflow `V_dot` = 8.50 m³/s (B2), Return_Air_Temp `T_ra` = 24.0 °C (B3), Cool_Autumn_Outside_Air `T_oa` = 11.0 °C (B4), Mixed-Air_Discharge_Setpoint `T_set` = 14.5 °C (B5), Minimum_Code_OA_Fraction `f_min` = 0.20 (`20%`, B6), and Economizer_High-Limit_Lockout `T_lock` = 19.0 °C (B7): Outside-Air Damper modulates to `73.08%` OA (E2), Mixed-Air Temp hits `14.50 °C` with ZERO mechanical cooling (E3, vs `21.40 °C` at minimum `20%` OA in E4), unloading `70.80` kW of sensible chiller load (E5)!
| ASHRAE 90.1 Economizer KPI (8.5 m³/s, Tra=24 °C, Toa=11 °C -> Tset=14.5 °C, fmin=20%) | Formula | Verified Output |
|---|---|---|
| Modulated Outside-Air Damper (%) | IF(Toa>=Tlock, fmin, CLAMP((Tra-Tset)/(Tra-Toa)))*100 | 73.08% OA (20.00% min @ 22 °C lockout) |
| Actual Mixed-Air Temp T_mix (°C) | f_oa * Toa + (1 - f_oa) * Tra | 14.50 °C (23.60 °C @ 22 °C lockout) |
| Min-OA Baseline Temp T_base (°C) | f_min * Toa + (1 - f_min) * Tra | 21.40 °C (23.60 °C @ 22 °C lockout) |
| Sensible Free-Cooling Saved (kW) | 1.20 * 1.006 * Q_m3_s * MAX(0, Tbase - Tmix) | 70.80 kW (0.00 kW @ 22 °C lockout) |
ASHRAE 90.1 Modulating Economizer Damper & High-Limit Lockout Formula
Validate `Airflow > 0` m³/s, `T_set < T_ra`, `f_min` in `[0, 1]`, and `T_lock <= T_ra`!
=LET(qf_s, B2, tr_c, B3, to_c, B4, ts_c, B5, fm_n, B6, tl_k, B7, IF(OR(qf_s <= 0, ts_c >= tr_c, fm_n < 0, fm_n > 1, tl_k > tr_c), "Invalid Economizer Input", LET(fo_a, IF(to_c >= tl_k, fm_n, MIN(1, MAX(fm_n, (tr_c - ts_c) / (tr_c - to_c)))), tm_x, fo_a * to_c + (1 - fo_a) * tr_c, tb_s, fm_n * to_c + (1 - fm_n) * tr_c, ROUND(1.2 * 1.006 * qf_s * MAX(0, tb_s - tm_x), 2))))Warm 22.0 °C Summer Afternoon High-Limit Lockout Check (Set Outside Air Toa B4 = 22.0 °C)
Warm outdoor weather to `B4 = 22.0 °C` (`>= 19.0 °C` lockout): Economizer High-Limit Lockout snaps Outside-Air Damper E2 back to minimum `20.00%` code ventilation, Mixed-Air Temp E3 matches Baseline `23.60 °C` (E4), and Free-Cooling Savings E5 drop cleanly to `0.00` kW as the chilled-water coil takes over!
=IF(ISTEXT(E5), E5, ROUND(IF(B4 >= B7, B6, MIN(1, MAX(B6, (B3 - B5) / (B3 - B4)))) * 100, 2))Try the example
Use the original values or change them to test your understanding.