AC Tonnage Calculator: Screening the Cooling Load of an Office

What it calculates

The calculator gives a screening estimate of the cooling load for an office floor or zone: the total heat gain in BTU/h, the equivalent tons of refrigeration (TR) and kilowatts of cooling, and the floor area each ton serves. It also breaks the load down by source so you can see what is driving it.

It is designed for the stage before an HVAC engineer is involved: comparing buildings, checking whether a landlord's installed capacity is in the right range, or deciding how much plant space and budget to allow.

Inputs

Conditioned carpet area is the floor area the system cools. Use carpet area, not the chargeable area on the lease.

Peak occupants is the number of people in the space at the busiest time.

Exposure to sun and outside air chooses an envelope allowance: low for an interior or shaded floor with little glass, average for a typical intermediate floor with some windows, and high for large west or south glazing or a top floor under the roof.

Lighting load is in watts for each square foot of floor. If you have used the lighting calculator, divide its connected load by the floor area.

The assumptions panel holds the envelope allowance itself, the equipment heat for each person, the heat from each person, the fresh-air load for each person and the margin.

Method

Each heat gain is worked out in BTU/h and added up:

• People = occupants × heat from each person.
• Fresh air = occupants × fresh-air load for each person.
• Lighting = area × lighting load × 3.412.
• Equipment = occupants × equipment watts × 3.412.
• Envelope = area × envelope allowance.

The total is increased by the margin, then divided by 12,000 to give tons of refrigeration. One watt is 3.412 BTU/h, and one TR is 12,000 BTU/h or 3.517 kW.

Assumptions

The envelope allowance is the roughest of these. Heat through glass depends on orientation, shading, glass type and the hour of the day, and a heat-load calculation treats each façade separately. The three presets are deliberately broad; change the figure if you know the building is better or worse.

The ventilation rate behind the fresh-air figure is set by the project's HVAC design under ASHRAE 62.1 or NBC 2016 Part 8. A different rate or outdoor condition changes this load directly.

Limitations

• It is a single peak figure. A heat-load calculation checks the load hour by hour, because the peak on the west façade comes in the late afternoon while the peak on the east comes in the morning.
• It does not split sensible and latent load. Equipment selection needs both, because dehumidification is a separate duty from cooling.
• It does not size zones. Cabins, meeting rooms and server rooms have very different loads and need their own units or zones.
• It ignores special rooms. Server rooms, kitchens and labs need their own calculation.
• It does not choose equipment. The choice between split units, VRF systems and chilled water depends on the building, the load profile and the landlord's infrastructure.

Worked example

A 5,000 sq ft office with 50 people on an intermediate floor with some windows, using the default assumptions:

The total is 289,788 / 12,000 = 24.1 TR, or about 85 kW of cooling, with each ton serving about 207 sq ft. Fresh air is about 19% of the load before margin.

Double the occupancy to 100 people and the load rises to about 33 TR, around 151 sq ft for each ton, without any change to the building. That is why a single area-per-ton rule of thumb can be badly wrong in either direction.

How engineers use it

An HVAC engineer starts from the same heat gains, but calculates each one properly: the envelope from the actual wall and glass construction, orientation and shading; the occupancy and equipment from the layout; the fresh air from the ventilation design and the local design weather. The calculation is run for each zone and for each hour, and the system is sized on the coincident peak.

A screening figure like this one is still useful to an engineer early on: it shows whether the landlord's existing capacity is likely to be enough, how much space the plant will need and what order of budget to allow, before the layout is fixed.

When a professional design must replace it

Use the calculator to compare options, to check a landlord's installed capacity and to set an early budget allowance. Replace it with a heat-load calculation by an HVAC engineer before any equipment is selected or ordered, before the ductwork or piping is designed, and whenever the building has unusual glazing, occupancy or process loads. Where the project must comply with ECBC 2017, the engineer also checks the equipment efficiency and controls against its requirements.