You don't see anything. Wave hands!
By Tom Moor
Oops! Looks like this image is too cool for alt text!
Posted 8/1/23

Today’s HVAC question comes from Thomas in Columbus, Ohio. Thomas writes:

“ team,

I’m thinking about buying a new HVAC system, and I have some questions about sizing. How do I know how big my furnace and air conditioner should be? I want to make it as efficient as possible. Will a larger system heat and cool my house faster than a smaller system?”

The easy answer to Thomas’ question is no, bigger is not always better. But at, we want to give you all the information you need to make the right decisions about your HVAC system’s health. When it comes to system sizing, that means understanding how improper system sizing can cost you money and how HVAC technicians determine the right-sized unit for your home.

Measuring The Angle And Length

Improperly Sized HVAC Units Can Cost You Money

When your system is larger than it needs to be, it is much less efficient than it should be. This can cost you hundreds and even thousands of dollars over the lifetime of your system. Oversized systems use up more energy than necessary to heat and cool your home, and often do not provide the comfort level you seek.

HVAC systems use up the most energy when their cycles are starting. Since the energy used to start a cycle for an oversized unit is higher, these units are less efficient and cost more money to run. Because they are larger than necessary, they also tend to short cycle, or turn on and off repeatedly, which wastes more energy than a properly sized unit, costing you money long-term.

Improperly Sized Furnaces & AC Units Won’t Deliver Optimum Comfort

Air Conditioning: Your AC unit is not just for cooling your home, it also helps dehumidify the air you breathe. When a system is oversized, however, it doesn’t have an opportunity to dehumidify the air. Short cycling keeps your system from running long enough to allow condensation to drop off the evaporator coils, removing humidity from your air and keeping your home comfortable.

Furnace: If your furnace is oversized, you may experience uneven heating in your home. Your unit will cycle on and blast hot air into your home quickly. But wait a minute, that’s good, right? Not exactly. Because the unit is larger than the space, it may heat rooms so quickly that the thermostat does not have time to adjust. This could result in some rooms being uncomfortably warm while others are still cool — not optimum.

How You Know You’ve Got The Right Fit

If you’re buying a new unit, the best way to make sure your system is properly sized is to consult with a licensed HVAC professional. A good HVAC contractor will determine the proper size of your unit using Manual J load calculations, the Air Conditioning Contractors of America’s heating and cooling size protocol.

The ACCA Manual J load calculation protocol are used in conjunction with software programs like RHVAC, Wrightsoft Right-J8, and Adtek AccuLoads. These programs use information about the home like the home’s size, orientation, air leakage, insulation, appliances, lighting, and window areas and locations to calculate the individual room-by-room heating and cooling requirements for a home’s HVAC system. Once entered, the software analyzes all of the factors and suggests the optimum size for the home’s HVAC system.

Is Your Furnace & Air Conditioner Sized Correctly?

If your unit is still in good health, but you think it may be oversized, the best thing to do is find a contractor to perform a Manual J load calculation test.

For new homes, ask your contractors how they are sizing your equipment. If they say they’re basing it off of your home’s square footage, tell them that’s not good enough. Since proper HVAC unit sizing is about more than square footage, ask them to show you the Manual J load calculations before choosing your system.

Here’s The Bottom Line:

Remember, oversized HVAC systems can cost you more and just don’t deliver the comfort levels that you and your family deserve. If you’re looking for a new HVAC system or you’re building a new home, make sure you and your contractors consult the latest Manual J load calculations to find the most efficient, money-saving system for your home.

Are you a contractor with tips for finding out if an HVAC system is oversized? How did your contractor calculate your HVAC system’s size?