Why the bigger rooftop unit is not the safer choice

When a rooftop unit fails in July, the instinct is to replace it with the same size or a size up. The same size is a guess. A size up is usually wrong. Here is why.

What oversizing does

An oversized unit satisfies the thermostat quickly and shuts off. Then it starts again a few minutes later. That short-cycling wears compressors, never runs long enough to pull moisture out of the air (yes, even in Arizona during monsoon), and uses more energy per hour of cooling delivered than a unit that runs steadily.

How often the old unit was wrong

On our replacement programs, about one in four units comes off the roof larger than the building needs. The original tenant had more people or equipment, or the original contractor added a safety factor to a safety factor. Twenty years later the building is different and nobody re-ran the numbers.

What a load calc costs

A Manual N or Trace calculation on a commercial building takes our engineer a few hours. It is included in every replacement proposal. The savings from downsizing one 20-ton unit to 15 tons is roughly $4,000 in equipment and a few hundred dollars a year in electricity, forever.

When bigger is right

When the load calc says so: a new tenant with a server room, a restaurant where an office was, a space that added people. The calc tells you. The nameplate does not.

One in four of our replacements is a smaller unit than what came off. The owner pays less twice.