Air-Side Economizer Free-Cooling Hours
Free-cooling capacity and the annual ton-hours an air-side economizer offsets.
Example
You enter
- Supply airflow (CFM) 4000
- Mix-to-supply delta-T (°F) 20
- Economizer-eligible hours 1500
You get
- Sensible free-cooling capacity 86400
- Ton-hours offset (annual) 10800
Checked against published worked examples from: ASHRAE sensible-heat relation + Standard 90.1 economizer.
Details, formula, and sources
ScopeSensible free-cooling capacity (BTU/hr) and annual ton-hours of mechanical cooling offset from supply airflow and the mix-to-supply delta-T over the economizer-eligible hours.
Formula or tableQ_sens = 1.08 * CFM * dT (sensible heat, standard air); ton-hours = Q_sens * hours / 12,000. dT is the mix-to-supply delta-T.
Source and editionASHRAE sensible-heat relation Q = 1.08 * CFM * dT (public); air-side economizer changeover per ASHRAE Standard 90.1, by name.
Free accessASHRAE 90.1 free read-only at ashrae.org; the 1.08 sensible factor is public.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namescfm, delta_t_f, hours, q_sens_btuh, ton_hours
- Standard air 1.08 BTU/(hr.CFM.F) at sea level, 70 FASHRAE Fundamentals