When to use: Visualize AHU operation with an animated cross-section showing mixed-air dampers, preheat coil, cooling coil, supply fan, and reheat coil. Adjust outdoor air temperature, OA fraction, coil setpoints, and fan speed — watch airflow particles move through the unit and see temperatures update at each section.
This simulator models a commercial AHU with mixed-air dampers, a preheat coil, a cooling coil, a supply fan, and a reheat coil — calculating air temperatures and energy at each section. HVAC engineers use it to visualize sequence-of-operations logic, size coil capacities, and evaluate fan energy for ASHRAE 90.1 compliance.
An air handling unit conditions outdoor and return air through a series of thermal sections. Mixed air temperature (MAT) is calculated as MAT = OA% × T_outdoor + (1 − OA%) × T_return. If MAT falls below the preheat setpoint (typically 45°F), the preheat coil activates to prevent freezing of the cooling coil or ductwork.
The cooling coil reduces the air temperature to the leaving air temperature (LAT), typically 55°F for comfort cooling. If the LAT is below the zone setpoint, a reheat coil (hot water, electric, or steam) raises the supply air temperature to the discharge setpoint. Cooling energy is calculated as Q = 1.08 × CFM × ΔT, where 1.08 is the volumetric heat capacity of air (BTU/hr per CFM per °F).
Fan power follows the cube law: at 80% speed, a fan uses only (0.8)³ = 51% of full-load power. Variable frequency drives (VFDs) exploit this relationship to achieve significant energy savings at part-load conditions as required by ASHRAE 90.1 Section 6.5.3.
ASHRAE Standard 90.1 governs AHU energy performance for commercial buildings and sets fan power limits (W/CFM), economizer requirements, and supply air temperature reset controls. The ASHRAE Handbook of Fundamentals provides psychrometric equations and heat transfer methods used in coil calculations.
ASHRAE Standard 55 defines thermal comfort boundaries that dictate supply air temperatures. SMACNA standards govern duct construction and leakage. For variable air volume (VAV) systems, ASHRAE Guideline 36 defines the High-Performance Sequences of Operation used in modern BAS programming.
Mixed air temperature control is critical: if the OA damper opens too wide during cold weather without adequate preheat, the cooling coil can freeze. A minimum 45°F mixed air temperature setpoint is standard practice before air enters downstream coils. Cooling coil leaving air temperature (LAT) is typically selected at 55°F for sensible heat ratio and dehumidification targets.
Reheat energy is a significant operating cost. ASHRAE 90.1 limits simultaneous heating and cooling to reduce energy waste. VAV systems with terminal reheat must meet minimum supply air temperature and minimum airflow requirements. Fan speed should be reduced to the minimum needed to maintain duct static pressure setpoint — a VFD is required for fans over 5 HP per ASHRAE 90.1.
Set the outdoor air temperature to your design day condition (e.g., 95°F for summer, 0°F for winter). Adjust the outdoor air percentage to meet your ASHRAE 62.1 ventilation requirement. Set the cooling coil leaving air temperature (55°F default) and reheat setpoint to match your zone requirements.
Watch the temperature at each section update in the animated canvas. The energy panel shows cooling tons, heating MBH, and fan BHP. Reduce fan speed to observe the cube-law energy savings. In winter, increase OA% until preheat activates, demonstrating freeze protection logic.
55°F is the standard LAT for comfort cooling in commercial buildings. Lower LATs (50–52°F) are used when higher dehumidification is needed, such as in humid climates or spaces with high latent loads.
When outdoor air is below about 40°F and the OA damper is open, the mixed air temperature can drop low enough to freeze condensation on the cooling coil. Preheat coils maintain a minimum mixed air temperature — typically 45°F — to prevent freeze damage and allow the cooling coil to operate safely.
1.08 combines the volumetric specific heat of air (0.018 BTU/ft³·°F) with a 60-minute-per-hour conversion. The full formula is Q (BTU/hr) = CFM × 60 min/hr × 0.075 lb/ft³ × 0.24 BTU/lb·°F × ΔT, which simplifies to 1.08 × CFM × ΔT.
Fan power follows the affinity law: power scales with the cube of speed. A fan at 80% speed uses 51% of full-load power; at 60% speed it uses only 22%. ASHRAE 90.1 requires VFDs on fans larger than 5 HP in VAV systems to capture these savings at part-load conditions.
Economizer mode uses outdoor air for free cooling when outdoor conditions are suitable — typically when OA enthalpy or dry-bulb temperature is below a setpoint. ASHRAE 90.1 requires air-side economizers in most climate zones for systems above a threshold size. The AHU opens the OA damper to 100% and reduces or eliminates mechanical cooling.
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