American Edge Engineers ·
What a load calculation is
An HVAC load calculation works out how much heating and cooling each room needs at the design weather conditions for the building's location. It sets the size of the equipment, the airflow to each room and, from that, the size of the ducts.
It is a small piece of engineering with a large effect: equipment chosen without it is often too big or too small for years. If you need one on its own, without a full design, see our HVAC load calculation service.
When the code asks for one
In places that use the International codes, the residential code asks for heating and cooling equipment to be sized from a load calculation - for homes, usually ACCA Manual J for the loads and Manual S for the equipment. The mechanical code asks for design loads on commercial buildings to be calculated by a recognised method, such as the procedures in the ASHRAE Fundamentals handbook, and the energy code ties equipment sizing to those loads.
Codes are adopted and amended locally, so check what your building department wants to see with the permit application. Many ask for the calculation report with the mechanical drawings, or for the load and equipment capacities to be shown on them.
When you need one even if nobody asks
A load calculation is worth having whenever the equipment size is being decided, not only when a reviewer asks for it:
- Replacing equipment - the old unit may never have been the right size
- Checking a contractor's quote that looks too large or too small
- A renovation, extension, new windows or added insulation that changes the loads
- A tenant fit-out that changes how a space is used, for example from offices to a clinic
- Rooms that are too hot, too cold or too humid with the existing system
- An energy code compliance report that needs the equipment capacities
What the calculation accounts for
A room-by-room calculation adds up the heat that passes through walls, roofs and floors; the sun through each window, which depends on its size, shading and the direction it faces; the heat and moisture from people; the heat from lighting and equipment; and the outdoor air brought in for ventilation or leaking in through the building. Each is worked out at the local summer and winter design conditions.
That is why two rooms of the same size can need very different amounts of cooling: a west-facing room with a wall of glass, or a room full of equipment, can need several times what an interior office does.
Why rules of thumb get it wrong
| Rule of thumb | Load calculation | |
|---|---|---|
| Basis | Floor area only | Each room's walls, windows, orientation, people, equipment and outdoor air |
| Typical result | Equipment that is too big, or too small in the hardest rooms | Equipment and airflow matched to each room |
| Oversized system | Short cycling, poor humidity control, higher first cost | Avoided by sizing to the calculated peak |
| Undersized system | Rooms that cannot hold temperature on the hottest or coldest days | Avoided by checking every room against design conditions |
| Permit | Not accepted where a calculation is required | Report can be submitted with the drawings |
What to prepare
The more of this you can send, the faster and more accurate the calculation:
- Floor plans and building sections
- The building's address or city, for the design weather conditions
- Window and door schedules, or sizes marked on the plans
- Wall, roof and floor construction and insulation, if known
- How each space is used: number of people, and any heavy equipment
- Details of the existing equipment, if it is being replaced
Load calculations in our projects
Every system we design starts with the loads. In a two-storey house in Mill Basin, Brooklyn, room-by-room loads zoned the house by floor and sized one 5-ton VRF heat pump. For a medical office in Astoria, Queens, they set a 3.5-ton fan coil with a damper and thermostat for each room. In a warehouse with offices in the Bronx, they led to two 10-ton VRF systems, one per floor.
If you need the loads for one building or tenant space, order an HVAC load calculation; if you need the whole system designed, start with our MEP design service.
Next step
Learn how our MEP Design & Engineering service works, or send us one drawing set for a free project review and decide after you see our work.