Evaluating Cooling Needs

An undersize air-conditioner * just can’t keep up on those really hot days. An oversize unit can make a room even more uncomfortable, however, because it cools in short, energy-wasting bursts, and then shuts down. Meanwhile, the indoor humidity level climbs and the air begins to feel clammy. Properly conditioned summer air, remember, is drier, as well as cooler.

Whether it’s a room or central air-conditioner, a cooling unit should be just large enough to cope with prolonged hot spells. At those times, you can expect it to run almost constantly, controlling humidity as well as temperature. This means that the unit’s output, in Btus, must roughly equal the sum of the heat gains of your house or room. If you’re shopping for a central system, get bids from several installers and let them compute the gains, as explained below. For a room unit, you can do the figuring yourself without too much trouble (see below right).

Some manufacturers size their equipment’s output in tons rather than Btus. To convert tons to Btus, multiply the tonnage by 12,000. Chances are you won’t be able to get an exact Btu-for-Btu match between a model’s capacity and your home’s heat gains. Generally it’s safer to go to the next smaller size. With a slightly undersize unit, indoor temperatures might rise somewhat on really hot days, but continuous dehumidification will maintain a tolerable level of comfort.

Calculating room cooling needs. Retailers size room units by the number of “rooms” you want to cool—figuring 6,000 Btus for the first “room,” and adding 3,500 to 5,000 Btus for additional “rooms.” Other factors come into play, too. How big is a typical room? And what about exposure, insulation, the number of windows and doors, heat gains from appliances, and the number of people who will use the space?

Use the chart to calculate your Btu needs. Move up to the next larger size for hot spots and for spaces with ceilings over 8 feet high. If you want an exact computation, ask a contractor for a cooling-load estimate form, along with the Btu factors they normally use.

Cooling Needs

Cooling Area / square feet

Capacity / Btus


















Calculating heat gains. To calculate peak cooling demand, expect contractors to go through an even more lengthy and detailed process than that for heat losses. Besides assigning Btu gains to the exterior factors illustrated above, they also need to know about hot or cold spots inside, how many appliances you have, and even how many people are in the family.

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