Typical HVAC Rules of Thumb

Note: Do not assume these values to be LAWS. They are only typical values, which SHOULD BE VERIFIED for each application. Also, if you’re interested in learning even more, here’s a free download for you: Standardized EMCS Energy Savings Calculations (20.3MB PDF)

General Information

Air Movement 6 ACH
1 CFM/ft2
Ventilation Rate 5 CFM/person
Fan Energy 1000-1500 CFM/hp
Electric Motors .8-.85 kW/hp
Lighting 2-4 watts/ft2
Chiller Size 300-400 ft2/ton
Ton 12,000 BTU/ton
Chilled Water 2.4 GPM/ton (10° rise)
Condenser Water 3 GPM/ton (10° rise)
Chiller Input .7-.8 kW/ton
Chillers & Pumps & Towers .9-1.0 kW/ton
Absorbers 18 lbs steam/ton
Boiler Horsepower 33,479 BTU/hr = 9.8 kW
Rebuilt Systems Design 55°F supply air
30°-40° rise reheat coils
Dual Duct & Multizone Design 55° cold deck
70-105° hot deck – with ODA Reset Schedule
VAV 55°F cooling
10% box leakage flow
40-50% minimum fan volume
People Load 450 BTU/person/hr
People Density Office 140 ft2/person
Theater 14 ft2/person
Heat Transmission See ASHRAE Handbook of Fundamentals
Glass 1 BTU/ft2/°F
Good Wall 0.1 BTU/ft2/°F
Masonry Wall 0.15-0.3 BTU/ft2/°F
Thin Poor Wall 0.25-0.5 BTU/ft2/°F
Overall Building 0.15-0.5 BTU/ft2/°F
Infiltration 0.5-1.5 ACH without Building Pressurization

Chiller Efficiencies

EER COP kW/ton
Air-cooled
(including condenser power > 150 tons)
ASHRAE Standard 90.1 1999 9.6 2.8 1.26
Good 9.9 2.9 1.21
Best 10.6 3.1 1.13
Water-cooled
(>300 ton centrifugal compressor)
ASHRAE Standard 90.1 1999 16.7 4.9 0.72
Good 18.5 5.4 0.65
Best 26.7 7.8 0.45