General Rules of Thumb for HVAC in Construction

​​Here are some HVAC rule of thumb guidelines for construction:

1. Sizing: The general guideline for sizing HVAC systems is to calculate the cooling load in British Thermal Units (BTUs) or tonnage based on the square footage of the area to be conditioned. 

Cooling Load: On average, an HVAC system should be sized to provide 1 ton (12,000 BTU) of cooling capacity for every 400 to 600 square feet of conditioned space. 

Heating Load: For heating, a rule of thumb is to provide 25 to 35 BTU per square foot of conditioned space.

2. Ventilation: For commercial buildings, a rule of thumb is to provide 15-20 cubic feet per minute (CFM) of outside air per person in occupied spaces. Residential buildings typically require 0.35-0.4 air changes per hour (ACH) for ventilation. Aim to provide 7.5 to 15 cubic feet per minute (CFM) of outdoor air per person in the conditioned space. Additionally, the ventilation system should be able to provide around 0.35 air changes per hour for good indoor air quality.

3. Ductwork: The velocity of air in ducts should be kept between 600-900 feet per minute (FPM) for supply ducts and between 700-1000 FPM for return ducts. Duct sizing is determined based on the required airflow rate and the friction loss in the ductwork. The size of ducts should be based on airflow requirements and the length of the duct run. A general rule of thumb is to size the main trunk line at 0.1 to 0.15 inches of pressure drop per 100 feet of duct run, and branch lines at 0.05 to 0.07 inches of pressure drop per 100 feet.

4. Air Distribution: There should be a minimum of one supply register for every 150 square feet of conditioned space. Return air grilles should be located in each room, preferably near the floor, to ensure proper air circulation. Aim to have at least one supply register or diffuser for every 150 to 200 square feet of floor area, and one return grille for every 200 to 300 square feet. If you have high ceilings or open floor plans, additional supply and return outlets may be necessary.

5. Thermostat Placement: Place thermostats in a central location away from direct sunlight, heat sources, or drafts. It is recommended to mount them on an interior wall, approximately 5 feet above the floor. For energy efficiency, it's recommended to set the thermostat to 78°F (25.5°C) during summer cooling and 68°F (20°C) during winter heating. Adjust according to personal comfort preferences.

6. Equipment Efficiency: Follow energy code requirements for minimum equipment efficiencies. For furnaces, look for efficiency ratings of at least 80% AFUE (Annual Fuel Utilization Efficiency), while air conditioners and heat pumps should have SEER (Seasonal Energy Efficiency Ratio) ratings above 13 in most regions.

It is important to note that these rule of thumb guidelines are meant to provide initial estimations and should not replace detailed calculations or professional HVAC design. Each construction project may have specific requirements that should be addressed by a qualified HVAC engineer or contractor.