Heating Load Calculations for HVAC Engineers

Accurate heating load calculations prevent undersizing (inadequate comfort) and oversizing (cycling, inefficiency). This guide presents ASHRAE methodology for residential and commercial heating load determination.

Heat Loss Components

$$Q_{total} = Q_{transmission} + Q_{infiltration} + Q_{ventilation}$$

Transmission Heat Loss

Through envelope:

$$Q = U \times A \times (T_{in} - T_{out})$$

Where:

$U$ = overall heat transfer coefficient (Btu/(h·ft²·°F))
$A$ = surface area (ft²)
$T_{in}$ = indoor design temperature (typically 68-72°F)
$T_{out}$ = outdoor 99% or 99.6% design temperature

Below-grade losses:

Basement walls: Use F-factor method
Slab-on-grade: Use F-factor or C-factor

Infiltration Heat Loss

Air change method:

$$Q_{inf} = 0.018 \times V \times ACH \times (T_{in} - T_{out})$$

Where:

$V$ = building volume (ft³)
$ACH$ = air changes per hour

CFM method:

$$Q_{inf} = 1.08 \times CFM \times \Delta T$$

Typical ACH values:

Tight construction (ICF, SIPs): 0.15-0.25 ACH
Average construction: 0.35-0.50 ACH
Loose construction: 0.60-1.00 ACH

Design Temperatures

Indoor: 68-72°F heating

Outdoor (ASHRAE 99.6%):

Chicago, IL: -7°F
Denver, CO: -2°F
Atlanta, GA: 17°F
Phoenix, AZ: 34°F

Degree Days

Heating degree days (base 65°F):

$$HDD = \sum (65 - T_{avg,daily})$$ for days when $T_{avg} < 65$°F

Annual energy:

$$Energy = \frac{24 \times HDD \times UA_{building}}{\eta_{equipment}}$$

Practical Applications

Residential: Use Manual J (ACCA)
Commercial: Room-by-room or block load
Safety factor: Design equipment capacity, do NOT add to calculated load

Related Technical Guides:

References:

ASHRAE Handbook of Fundamentals, Chapter 18: Residential Cooling and Heating Load Calculations
ACCA Manual J, 8th Edition