What size air conditioner and heater do I need?

What size air conditioner and heater do I need?

We love it when our customers ask us this question, because it is important to buy the right size. Spoiler alert: the right sized air conditioner and heater will provide consistent comfort year-round, cost less to operate and live beyond the normal life expectancy of 12 to 15 years.

Air conditioners are designed to perform two jobs:

  • Remove heat – Air conditioners remove heat and transfer it outside leaving you with colder air inside your home.
  • Control humidity – Less moisture in the air helps you feel colder, even if the temperature is the same.

It is tempting to buy a larger air conditioner believing it will be more powerful and cool your home faster. That is like buying a shirt that is twice your size just in case you need the extra capacity.

In reality, air conditioners that are too big often cool well because they quickly meet the temperature set on the thermostat. Sounds great, right? The problem is the oversized system will turn off before it has a chance to remove the moisture in the air (control the humidity). The shorter run times will lead to an uncomfortable, clammy feeling in your home and unnecessary wear and tear on your system.

Air conditioners need to run for at least 10 minutes before they begin to dehumidify.

A high humidity level will feel hot in your house even if your thermostat reads 68 degrees. The air will feel muggy and moist. The air will even smell differently because of all the moisture that is trapped in the air.

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Mold, mildew and dust mites thrive in high humidity environments which will lead to allergy issues.

When a system that is oversized turns on and off too frequently, it is like driving your car around the neighborhood by stomping on the gas pedal and then stomping on the brake pedal, then stomp on the gas pedal and stomp on the brake pedal. Imagine repeating this cycle all day long. You’ll waste gas, wear your brakes out, and damage your engine and transmission.

Of course, you would never drive your car that way. This may seem like a no-brainer, but too often air conditioners are “driven” this way all day when they are not sized properly.

When air conditioners are sized properly, they are like driving a car while it is coasting at a steady speed, the car sips gas and the parts are not at risk of damage while you’re cruising the neighborhood.

Signs your air conditioner is too big

  • The first thing you’ll notice is the air feels moist and clammy.
  • The air smells musty.
  • The system turns on and off frequently, but runs for less than 5 minutes.
  • You found mold or mildew around your vents, on your walls or crown molding.

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  • Family members complain about how their head feels stuffy when at home, but the symptoms are less severe when they are away from home.
  • Damage to your home’s hardwood floor, paint, artwork and furniture. Windows and doors are hard to close since they have swollen and do not fit into the frame as well.
  • Higher energy bills.

Air conditioners are sized for the hottest day of the year, so an air conditioner that is too small will likely perform well when the outdoor temperatures are mild. But you’ll notice a difference when it is extremely hot outside – over 90 degrees. The undersized air conditioner will not be able to cool your home properly. The system will run constantly and it will not feel cool.

Signs your air conditioner is too small

  • The air conditioner seems to run all of the time without stopping.
  • The temperature in your home does not agree to the setting on your thermostat.
  • Higher energy bills.
  • Hot and cold spots in your home.

What size air conditioners are available?

Standard, residential air conditioners are measured by their cooling capacity in tons. Our technicians are superheroes, but not because they can lift an air conditioner that weighs three tons. Ton refers to the cooling power, not the weight of the machine.

The most common sizes in tons are 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 5.0.

Methods for determining what size air conditioner and heater you need

The three most common methods used to determine the size of your air conditioner and heater include:

  1. Same size as your old air conditioner and heater
  2. Square Footage calculation
  3. Manual J Load calculation

Defaulting to the same size as your old air conditioner and heater is not recommended. There is no guarantee the old system was the right size. Over the years, the insulation has settled, the windows leak, and the building materials have aged. The previous owners may have renovated the house without considering the effect on the air conditioner and the ductwork design.

Simply replacing your old system with the same size without performing a Manual J calculation is not an approved method by the Alabama HVAC board.

The second method is the Square Footage Calculation. To perform this calculation, you’ll need to know the amount of air-conditioned space in your house that is supplied by the air conditioner you want to replace.

Formula: Square footage / 400 = Size air conditioner

Example: 1,000 sq. ft. / 400 = 2.5-ton air conditioner

Reasons why we do not recommend the Square Footage method:

  • It is an out dated method that does not consider all of the factors necessary for proper system sizing.
  • The method only considers the amount of space, not the location of the space. For example, a bedroom in the basement will require a smaller air conditioner than a bedroom of the same size that is over the garage. Bedrooms in the basement require less air conditioning capacity because less heat enters your home from the ground. Bedrooms over unconditioned space like the garage, require the air conditioner to have more cooling capacity so it can remove all of the heat generated in the garage.
  • The Square Footage method became popular decades ago when building materials were simpler. Now, we have access to so many energy efficient materials including windows, doors, and insulation. This method no longer works with all of the variables that need to be considered with newer homes.
  • The Manual J is required by law.

Drum roll please….the Manual J calculation is the best method, because it is:

  • Customized to how you use the space and the construction materials used to build the house.
  • Considers all of the factors that increase the heat load of your house.
  • The most accurate calculation.
  • Required by law. The Alabama Board of Heating, Air Conditioning, and Refrigeration Contractors requires all licensed HVAC contractors to “comply with the minimum standards required by ACCA/Manual J as described in Load Calculation for Residential Winter and Summer Air Conditioning.”

What is a Manual J Load calculation?

The Manual J load calculation determines the amount of heating and cooling the home requires to keep you and your family warm during the winter and cool during the summer.

When sizing your air conditioner, Manual J calculates how much heat is gained through the windows, walls, building materials, basement, crawl space, etc. For heaters, Manual J calculates how much heat is lost through the exterior of your home during the winter.

What affects the size of my air conditioner?

The Manual J calculation considers all of these factors when considering the perfect size air conditioner for your home:

  • Number of people – our bodies give off heat, so the number of people living in the house affect the size of your air conditioner.
  • High, vaulted ceilings – air flow volume is measured by cubic feet not square feet, so the height of your ceilings affect the calculation…not just square footage.
  • Appliances – how often you use your stove, microwave, dryer and other heat-generating appliances affect the calculation.
  • Insulation – the quality and amount of insulation you have in your walls and attic affect the performance of your air conditioner.
  • Windows – the number, type, age and direction the windows face affect how much heat they absorb.
  • Doors – the number of exterior doors, their materials and the direction they face affect the calculation.
  • Home office or game room – spaces with computers, gaming system and large TVs generate more heat which must be considered in the load calculation.
  • Exterior walls – if your duplex is on the corner of the building, you may require a larger air conditioner than your neighbor in the middle space. The home in the middle space will enjoy the conditioned air from their neighbors which will reduce their heat load…smaller air conditioner.
  • Foundation – whether your house is built on a slab, crawl space or basement affects the size of your air conditioner.
  • Building materials – the building materials on the outside of your house make a difference. Brick absorbs heat while other materials reflect heat.
  • Location of ductwork – It is easier for your air conditioner to push cool air through ductwork in your crawl space versus in your attic. The air surrounding the ductwork in your crawl space may be 70 degrees. The air surrounding the ductwork in your attic may be up to 140 degrees. The air in the ductwork may be 50 degrees, but the attic air of 140 is fighting to get in.
  • Direction of the sun – the same house rotated ninety degrees could change the Manual J calculation by 25%.
  • Landscaping – the amount of shade from trees and shrubs reduce the heat load and affect the calculation.

The list goes on, but you get the idea. Air conditioners are designed to remove heat, so the Manual J load calculation is the perfect way to determine how much heat your home absorbs which determines the amount of cooling your home needs to battle extreme temperatures.

Don’t skip this step. It’s important to invest the time in the Manual J calculation to ensure you are buying the right size air conditioner. You’ll save time and money in the long run.

Square footage method vs Manual J Load Calculation

Here is a side-by-side comparison of the same house using two methods for determining what size air conditioner:


Example 1: Replace system in townhome. 

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Unique factors for the load calculation: 

  • Two exterior walls since the home shares space with their neighbors
  • Approximately 15” of insulation
  • Only a few windows
  • System will serve the upstairs bedrooms that is over air-conditioned space


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Conclusion: The more accurate method, Manual J, is 1-ton smaller. By sizing the system correctly, the customer saved money on the installation costs and will not deal with humidity issues and high utility bills.


Example 2: HVAC inspection before customer bought a 21-year-old house.

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Unique factors for the load calculation: 

  • House is built on a slab
  • Double pane windows
  • Adequate insulation
  • 2 windows facing South-West with thermal blackout curtains

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Conclusion: Even though the current system works, it is 1-ton too large for the house. The purchase price of the home needs to consider the additional costs the oversized air conditioner will cause in the future. High humidity is impossible to avoid in Birmingham, AL so it is only a matter of time before the visible signs of an oversized system appear – mold and mildew on the around the air vents, on the crown molding, on the walls and even on clothes. Plus, the utility bills will be higher than the new homeowner may have expected.


Example 3: House built three years ago.  

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Unique factors for the load calculation:

  • Air tight construction – house was built in the formation of a continuous airtight envelope to minimize air leakage.
  • Spray foam insulation in the walls and roof line.
  • Duct work in the attic – the spray foam helps the attic maintain a temperature of only 80 degrees in the summer compared to most homes that can reach up to 140 degrees. The duct work is essentially in air-conditioned space.
  • Gas filled, double pane windows
  • Slab

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Conclusion: Wow! What a difference. If a HVAC contractor installed the air conditioner and heater based on the outdated, Square Footage method, the system would have been grossly oversized by 2 tons. By taking the time to complete the extra steps required by the Manual J method, the customer saved money on the installation costs and will not deal with humidity issues or high utility bills.


Example 4: Replace upstairs system in large home 

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Unique factors for the load calculation:

  • Upstairs space is over air-conditioned living space
  • Few windows
  • Well insulated

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Conclusion: And the winner is…Example #4. The Manual J method is a whopping 3-tons smaller. To put that in perspective, the average size air conditioner in most homes is 3 tons. We cannot imagine the problems the upstairs space of this home would have if they relied on the Square Footage method.

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Does the Manual J calculation require special training?

Yes. Even though Manual J is a required subject in most HVAC training programs, it is a skill that takes time to master. The key is applying the principals in the book to the real world.


We use a software program that helps us more accurately calculate the size air conditioner you need. The software is like all other tools – garbage in, garbage out. We blend the results of the software with our years of experience and your unique home.

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In our internal training program, technicians learn the theory from the Manual J Residential Load Calculation book. Then, we apply the principals in the field with our customer’s houses. After the technician proves they understand how to perform the load calculation by hand…the hard way, we teach them how to use the software. Technicians then test the principals and software on every structure they can find – our office, their house and their Grandma’s house. If the size from the calculation is not the same size that is currently installed, we dive deeper to ensure we are considering all of the information that affects the size of your air conditioner and heater.


I have to admit…my favorite part of HVAC school was Manual J. I love math, so this subject was right up my alley.


That’s a lot of information about the air conditioner…what about my heater? In the south, the driving factor for sizing your system correctly is your air conditioner. The heater that matches your air conditioner will perform well since we rarely experience extremely cold temperatures for long periods of time.

Bottom line:

As you can see, a lot of variables go into determining the proper size of air conditioner for your home. The Manual J load calculation considers all of the ways your comfort may be affected by the hot air fighting its way into your air-conditioned space through the walls, windows, ductwork and doors.

A system that is not sized properly will require you to pay more money in the long run including the initial cost of the equipment, more repair costs and higher utility costs.

Ask a professional

If you’re searching for the perfect sized air conditioner and heater, consider asking a professional to perform a Manual J Load Calculation. Calculating the right size requires the perfect blend of science, math and an understanding of your home’s unique characteristics.

Don’t waste time and money…contact Brown. We’ll send a highly trained service technician to help you get back into your comfort zone.