ℹ️ About This Calculator
The Air Handling Unit (AHU) is the central element of a ducted HVAC system. It conditions the supply air (cools, heats, humidifies or dehumidifies) and distributes it through the duct network. Correct AHU sizing determines the supply air temperature differential, airflow rate, coil duty, and fan power — all of which must be coordinated to meet the room cooling load without overcooling or undercooling.
ASHRAE 62.1 specifies minimum outdoor air ventilation rates for acceptable indoor air quality. NBC Part 8 provides the Indian equivalent. For offices: minimum 10 L/s per person plus 0.3 L/s per m² of floor area. AHUs must be designed to handle the minimum fresh air requirement in addition to the recirculated air for cooling. The mixing of fresh and return air happens in the AHU mixing box before the filters and coil.
📐 AHU Sizing Formula (ASHRAE)
❓ Frequently Asked Questions
What is a typical AHU supply air temperature for Indian conditions?
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12–14°C is typical for central AHU systems serving office buildings. For precision cooling in data centres: 18–24°C supply. Lower supply temperatures mean less airflow is needed for the same cooling load, but risk condensation on surfaces and discomfort from cold drafts near diffusers. 13°C supply air with a 24°C room setpoint gives an 11°C differential — the standard design point.
What is the difference between AHU and FCU (Fan Coil Unit)?
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AHU: Large central unit with fresh air intake, filters, cooling coil, fan, and sometimes humidifier. Serves entire floors or buildings through ductwork. FCU: Small room-level unit with cooling coil and fan only — no fresh air intake. FCUs recirculate room air over the coil. Central systems with AHUs provide better air quality and filtration; FCUs are simpler and cheaper but require a separate fresh air system (DOAS — Dedicated Outdoor Air System).
How do I select the AHU filter efficiency?
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ASHRAE 52.2 / IS 14823 filter ratings: G4 (coarse, EU4) — protects AHU coil from large dust; F7 (medium, EU7) — standard for commercial offices; H13 (HEPA) — hospitals, clean rooms. Most office AHUs use G4 pre-filter + F7 final filter. Hospitals and pharma require F7 + H13 HEPA. Green buildings (IGBC/LEED) require minimum MERV-8 (approximately F5) for IAQ credits.
What external static pressure should I design the AHU for?
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ESP is the pressure the AHU fan must overcome (excluding the AHU internal resistance). Typical values: small office AHU (short duct run): 200–350 Pa. Medium commercial: 350–600 Pa. Large central plant AHU with long duct runs: 600–900 Pa. AHU manufacturers rate their fan at a specific ESP — ensure the selected AHU can deliver the design airflow at the calculated system ESP.
How often should AHU filters be changed?
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In Indian urban conditions (high dust levels): G4 pre-filters — monthly or when pressure drop doubles from initial clean value. F7 bag filters — quarterly or semi-annually. HEPA filters — annually or when pressure drop exceeds 250 Pa. Always replace filters on schedule — a clogged filter increases fan power consumption by 20–40% and reduces airflow to the space, impairing both comfort and air quality.
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