DISCOVER WHAT MAY COST YOU 50% MORE IN UTILITY BILLS—AND DETERMINES 100% OF THE HOME’S COMFORT.
Buying the wrong size of air conditioning unit may seem like a small mistake, but it will affect you in a BIG way! That’s right. Making this one mistake will cost a homeowner much more in utility costs than necessary and, more importantly, will have a 100% impact on the home’s interior comfort. That one mistake is “oversizing the air conditioning unit (AC).” The size of the AC unit is important because if the unit is too small, it won’t cool the home properly, and if it’s too big, the AC unit will cost more to buy, more to operate, and may result in clammy, uncomfortable interiors with potential long term moisture problems. So how does one know what the right size is? Here’s what you need to know.
Busting The Myth That Bigger Is Always Better
A common myth persists that when it comes to AC units, bigger is better. But—don’t believe people who tell you that a bigger AC unit will be more comfortable and more efficient. Contractors promote the myth, and to consumers, it seems logical and therefore believable. Unfortunately, however, nothing could be further from the truth. That may be true for other things, but when buying an AC unit, “bigger” is not better—“right size” is better.
If your air conditioner is too big for your home:
• The unit won’t run long enough to pull the humidity out of the air, creating an unpleasant, clammy house. No one wants to have moldy, smelly clothes or feel sticky inside their home.
• Increased moisture indoors may accumulate and cause mold issues over time.
• It will cost more than necessary to buy, install, and power.
• It will cycle on and off frequently, causing uneven temperatures inside and potentially more wear and maintenance.
Think about how you buy shoes: a size too small will cause blisters and pain, while a size too big will never stay on your foot. The idea is the same in trying to find the size of the AC unit that will fit your home’s unique design, square footage, location, number of occupants, and other factors specific to that house that will have an impact on the load calculations for the AC unit.
Methods Used For Sizing Your AC Units
1. Using Square Footage
This method, which is unfortunately one of the most common used by contractors, is the worst way to size an AC system. The most common formula is requiring 1 ton of cooling power for every 600-800 square feet. Tighter built and spray foam insulation homes would use a formula of maybe 1 ton per 1,000 square feet of house. Using square footage alone to determine the size of the unit that will meet the proper cooling (or heating) loads required by the home completely ignores the major role of factors such as climate, insulation type, window values, air infiltration, and many others that would affect load calculations. Clearly, using the square footage is one sure way to over-size the AC unit. And there is no square foot rule of thumb for determining heating equipment size because the estimate would be even more inaccurate.
2. Manual J Calculation Is The Best Way
The best method by far for the correct sizing of an HVAC (Heating Ventilation and Air Conditioning) system is called the Manual J Calculation. It is a procedure developed by the Air Conditioning Contractors of America (ACCA) that uses specialized software to determine the correct size required based on the information that is input. And this is where it gets tricky. Everyone has heard the expression “garbage in, garbage out.” Well, this idea is especially applicable because if the Manual J information is input incorrectly, either by mistake or deliberately, the resulting size calculations will be incorrect.
The following is an example of the type of information used in the Manual J to determine the correct size of an AC unit for a specific home. The company that installs your AC is supposed to input information like this about your home into a Manual J software program. This information includes:
• The square footage
• Number of rooms
• Average number of people in the home
• Window types and direction
• Construction of the roof
• Direction the home faces
• Average outside temperatures in summer and winter
The software program then does what are called “load calculations” on each room and your house as a whole to determine the proper sized unit for the home. When the contractor takes the time to do this calculation, you get an HVAC size that is right for your home.
The best advice is not to rely solely on your HVAC contractor for your Manual J. This will eliminate an obvious conflict of interest. What that means is that since the HVAC contractor is the one selling you the HVAC system, and these are sold by the ton, it stands to reason that the more tonnage required, the higher the profit for the HVAC contractor. This is not intended to imply that HVAC contractors are dishonest, but rather simply to point out the obvious conflict of interest. After all, inspecting the information input on the Manual J can easily identify fraud or devious intentions. While some inaccurate inputs may be honest mistakes, other outright attempts to manipulate the Manual J to justify an oversized air conditioning system can easily be spotted by a third-party home inspector. Which brings us to our final point: the third-party inspector.
3. And The Best Way: Ask Your Builder For A BUILT TO SAVE™ Certified Home
Asking for a BUILT TO SAVE™ certified home works with Option 2 (above). One of cornerstones of the BUILT TO SAVE™ High-Performance Homes Program is that in order for a home to be certified as BUILT TO SAVE™, the HVAC system must be properly sized. This is a critical component that insures many of the benefits of a BUILT TO SAVE™ home: improved energy efficiency, even temperatures from room to room, and comfortable interiors. Inspections and testing are performed by third-party, licensed home energy raters who review Manual J and other applicable documents such as Manuals D, S, and T to make sure the home is in compliance with the BUILT TO SAVE™ program requirements and that the information is accurate. In short, you don’t have to know how to do a good Manual J—you just have to know a good professional who does. If your builder offers a BUILT TO SAVE™ home, you can rest assured you will have an excellent home with a properly sized HVAC system for years to come.
For more information or to find a BUILT TO SAVE™ builder affiliate, visit http://www.builttosave.org or call 956-971-9700.