A load calculation answers one question: how much heat does this building gain in summer and lose in winter at design conditions, so that the equipment can be sized to match. It is a sum of many small formulas rather than one, and the shortcuts the trade uses are estimates of that sum. This page is for the estimator or technician in a US HVAC contracting business who needs to know what the HVAC load calculation formula actually computes, when the square footage shortcut is acceptable and when it is not, what free load calculation software leaves out, and what commercial HVAC load calculation software adds. The pricing calculator on this site uses the shortcut deliberately, for pricing, and says so.
What the load calculation sums
Heat gain and loss through each surface: walls, roof, floor, windows and doors, each as its area times its U-value times the temperature difference, with solar gain through glazing added by orientation. Infiltration and ventilation air, as airflow times the temperature and moisture difference. Internal gains from people, lights and appliances. Duct losses where ducts run outside the conditioned space. The residential procedure the trade uses is the ACCA's Manual J; commercial work uses more detailed procedures with hourly weather and occupancy schedules. The result is a sensible and latent load in Btu per hour at design conditions.
The square footage rule, and what it is for
Dividing conditioned area by a fixed number of square feet per ton gives a nominal capacity in seconds. It is an estimate of the sum above for a typical house in a typical climate, and it is wrong by a lot for a house with unusual glazing, insulation, orientation or infiltration. The pricing calculator on this site uses it to price a replacement, because pricing needs a capacity now; it tells you to confirm with a real load calculation before ordering equipment, because the price survives a half-ton error and the comfort does not.
HVAC load calculation software free versions
Free tools typically implement a simplified residential procedure: fewer construction types, default infiltration, no duct loss modelling, and no room-by-room output. They are useful for a sanity check on a replacement and for learning the method. They are not a substitute for the full procedure when the ductwork is being redesigned, when the house has been renovated, or when the equipment is a heat pump in a cold climate where the heating load decides the size.
Commercial HVAC load calculation software
Commercial loads are dominated by internal gains, ventilation requirements, occupancy schedules and solar gain through large glazing, and they vary hour by hour. Commercial software models the building by zone with hourly weather and schedules, sizes the equipment by zone and the system by the coincident peak, and produces the ventilation calculation the code requires. It is a design tool for an engineer or an experienced estimator; a residential program will not do the job.
Where the load meets the price
The load gives the capacity; the capacity, the efficiency tier, the ductwork and your labor and margin give the price. The pricing calculator on this site does the second half from your own costs. Do the load calculation before you order, price the job with the calculator, and put the capacity and the method on the proposal so the customer knows the system was sized for their house.
Questions people ask about hvac load calculation formula
Is there a single HVAC load calculation formula?
No. It is a sum of surface, infiltration, ventilation and internal gain formulas at design conditions. Manual J is the residential procedure that organises them.
Is HVAC load calculation software free good enough for a replacement?
For a sanity check, yes. For sizing when the ducts change, the house was renovated or a heat pump carries the heating load, run the full procedure.
Who should run commercial HVAC load calculation software?
An engineer or an estimator trained on it. The inputs are schedules and ventilation rates that a residential estimator does not usually hold.