Is My Air Conditioner Too Big for My Home?

Written By: Angie's List

Get your air conditioning unit sized to your specific home

Size matters when you install a new A/C unit, and not necessarily the way you might expect.

In fact, a system that’s too large or too powerful for your home can waste money, energy and comfort. Heating and cooling experts recommend performing a load calculation before replacing an A/C system.

“Lots of things are better if they’re bigger, like TVs or speakers, but not air conditioning,” says Rich Morgan, owner of Magic Touch Mechanical in Mesa, Arizona. “An A/C unit is supposed to dehumidify the house as well as cool it. A unit that’s too large will turn on and off very quickly, bringing the temperature down very fast but not removing the humidity from the house. So you’re losing comfort as well.”

How is load calculation determined?

During a load calculation, technicians take into account the size of each room, duct condition, the home’s orientation toward the sun, shading, window type, insulation, building materials, tree shade and regional weather conditions. Assembling this information at the home usually takes an hour or two.

Using this data, a computer program calculates the recommended tonnage for a new system, usually by using the “Manual J” protocol established by the Air Conditioning Contractors of America.

Patrick Boykin, owner of Roussos Air Conditioning in Panama City, Florida, says an HVAC contractor should perform a load calculation for any new system, rather than just install the same size as the previous one, even if that size seemed to have been working fine.

"A lot of factors can change the size your home will need,” he says. “If you’ve done significant updates to the insulation and windows, or replaced the roof with a new material, that can change things a lot,” he says. “And just in the last 10 years, HVAC systems have become much more efficient.”

Though several load-calculation methods exist, Boykin says most contractors and municipalities follow the Manual J protocol.

Load calculations save money

Andreas Laux, president of Mighty Ducts Heating and Cooling in Santa Ana, California, says proper load calculations save customers significant money, both in energy bills over time and in lower upfront purchasing costs.

“You could take a pretty nice vacation in the Bahamas with the money you’d save if you sized these correctly,” he says. “Out in the field, about 80 percent of the homes I see are equipped with oversized units.”

Load calculations measure both heating and cooling needs, using the same set of data. HVAC companies describe a unit’s strength in terms of tons, with each ton representing 12,000 British thermal units (BTUs).

“With heating, you’re adding BTUs, and with cooling, you’re removing them,” Laux says. “A single BTU will raise one pound of water by one degree. One BTU is basically a match.”

In terms of both heating and cooling, Laux compares a properly sized system running for longer periods of time to a car keeping a constant speed on the highway. “You run most efficiently if you have a lot of highway miles,” he says. “But with an oversized system, it’s like doing a lot of city driving, with stops and starts. You use less energy with smaller equipment with longer periods of operation.”

How much does load calculation cost?

Laux says he never performs load calculations as a separate service, since it’s part of a larger purchase and considered part of the cost of purchasing and installing a new unit. But if someone were to ask for it separately, he says he would charge about $250.

Morgan says he charges $400 for separate load calculations, which he usually performs for builders or engineers rather than individual homeowners. When installing a new system, he considers the calculation part of the overall cost.

“We do it for our own protection,” Morgan says. “We’ve been in business for 17 years, and I’ve never once had to pull out a system because we put the wrong size in. That’s because we did the due diligence before the job.”


Click here to see the original article on Angie's List.

This article was written by Paul F. P. Pogue of Angie's List.

Want more A/C tips? Check out Angie's List guide to air conditioning.

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