What size AC or furnace do I need for my house?
The right size comes from a load calculation for your house, not from square footage or the size of your old unit.
Why can't I size an AC by square footage?
The U.S. Department of Energy points to ACCA Manual J to find your home's heating and cooling loads, and to Manual S to pick equipment that matches them. Ask the contractor who quotes your job to do one and show you the results.
Two houses with the same square footage can need different systems. A 2002 DOE fact sheet on right-sizing lists what Manual J looks at:
An older Omaha home with original windows and little attic insulation can need more capacity than a newer, tighter home of the same size. DOE warns against "casual sizing estimates based on home floor area or contractor 'experience.'"
- Wall, ceiling, window, and floor area
- How well each is insulated
- Air leakage in the house and the ducts
- Which way the house faces, roof color, and how many people live there
Is a bigger AC or furnace better?
No. DOE says rules of thumb too often lead to oversized systems. According to DOE, an oversized system:
If a quote is for a bigger system than you expected, ask how it was sized.
An undersized system has the opposite problem: it may run constantly and still not keep up on the hottest or coldest days.
- Costs more
- Wastes energy
- Turns on and off too often, which adds wear and can lead to early failure
- Can do a poorer job of controlling humidity
Should my new system be the same size as my old one?
Not necessarily. Trane's homeowner guide says to be leery of anyone who sizes a new system based only on a rule of thumb or the size of the existing unit. Your home may have changed since the old system went in. New windows, added insulation, a finished basement, or an addition all change the load.
What is a Manual J load calculation?
Manual J is the residential load calculation method published by ACCA (Air Conditioning Contractors of America). It estimates how much heat your house gains in summer and loses in winter based on its construction, insulation, windows, and air leaks. Manual S then uses those loads to pick equipment. The DOE fact sheet recommends you "insist that documented sizing calculations be performed on your home."
ENERGY STAR's hiring tips say the same thing in plainer terms: a contractor should inspect your home and system, size the equipment based on your house, insulation, and windows, and check your ducts for leaks and airflow.
What should I ask a contractor about sizing?
1. Did you do a Manual J load calculation for my house? Can I have a copy? 2. What size system does it call for, in tons for cooling and BTUs for heating? 3. Why is your recommended size different from my current system, if it is? 4. Did you check my ductwork? Can it handle the airflow this system needs? 5. If I'm considering a heat pump, how did you size it for heating and for backup heat?
If you have another quote for a noticeably different size, ask the contractor to explain the difference with their numbers.
Does size affect Omaha rebates?
It can. OPPD's HVAC Smart rebate requires the system to condition the home's main living space (systems serving only sunrooms, garages, or outbuildings don't qualify). OPPD's terms say systems smaller than 2 tons get 50% of the standard rebate unless the system conditions the entire home. Ask the contractor whether the size they propose qualifies for the full amount.
What if my current system isn't keeping up?
A system that can't keep the house comfortable isn't always the wrong size. Low refrigerant, a dirty filter, a failing part, or leaky ducts can look like a sizing problem. Have it checked first. See Omaha AC repair, furnace repair in Omaha, or repair or replace your HVAC.
Sources
- Proper Sizing of HVAC Systems (U.S. Department of Energy, Building Science Education)
- Right-Size Heating and Cooling Equipment, fact sheet, January 2002 (U.S. Department of Energy)
- How Long Does It Take to Install an AC Unit? (Trane)
- 10 Tips for Hiring a Heating and Cooling Contractor (ENERGY STAR)
- Residential Single-Family HVAC Program Terms and Conditions (OPPD)
Common questions
What size AC do I need for my house?
It depends on your home's insulation, windows, air leakage, orientation, and ducts, not just square footage. A contractor should do an ACCA Manual J load calculation to find the right size.
Is a bigger AC better?
No. The U.S. Department of Energy says oversized systems cost more, waste energy, cycle on and off too often, and can control humidity poorly.
Should I replace my AC with the same size?
Not automatically. Your home may have changed since the old system went in. Ask for a load calculation instead of matching the old unit.
What is a Manual J calculation?
It's the residential load calculation method from the Air Conditioning Contractors of America. It estimates how much heating and cooling your home needs based on its construction, insulation, windows, and air leaks.