Deciding what size of air conditioning system a home needs isn’t an easy task. The cooling requirements for a home vary dramatically based on where you live and also the age, size, and condition of your house. Determining the exact cooling requirements of your home is essential for ensuring that you get an AC unit that is big enough to work effectively. You also don’t want a unit that is too big, as it will use much more electricity and cost you more upfront. This article will show you the different factors involved when determining what size air conditioning system you need and also the process that HVAC technicians use to calculate a home’s cooling requirements.

AC Requirements by Square Footage and Climate

The size of your home and how hot the local climate is are two of the main factors that will dictate what size of AC system you need. A home’s cooling requirements are obviously much higher in hotter places like Houston than they are in places where the summers are typically milder. The fact that our area tends to be extremely humid is an especially important factor to consider since AC units are less effective in more humid conditions. Your AC system may need to be bigger than if you lived in a drier climate.

The general rule of thumb is that you need approximately 20 to 25 BTUs of cooling for every square foot in the home. However, this is really just an average, and homes in Houston and the Gulf Coast area often need anywhere from 25 to 45 BTUs of cooling per square foot due to our hot, humid climate. It really depends on how well insulated the home is and how much heat it tends to get from the sun.

This means a 1,500-square-foot home could need anywhere between 37,500 and 75,000 BTUs of cooling for the air conditioning system to work effectively. That may seem like an extremely wide range. It varies so much because every home is different and has different cooling requirements.

BTU requirements will also vary based on whether your home has high ceilings in any of the rooms or if all rooms have standard 8-foot-high ceilings. High ceilings obviously increase the total volume of air in the home that needs to be cooled, which means that the BTU requirements will be higher than they would in a home with standard-height ceilings.

Understanding AC Tonnage

Air conditioners and heat pumps are always measured in tons, and residential units range in size from 1.5 to 5 tons. One ton of air conditioning is equal to 12,000 BTUs of cooling. When determining how many AC tons a home needs, it’s important to always round up rather than rounding down. Opting for an AC or heat pump that is slightly larger than you need will always be better than choosing a unit that is slightly too small.

An undersized unit will typically work just fine in the less hot, less humid parts of the year. However, it will struggle and not be very effective during most of the summer. This means that your AC system would have to run almost all the time to ever have a chance of keeping your home sufficiently cool. As a result, it would use far more energy and also undergo much more strain and wear and tear. This increased strain means that an undersized unit would likely end up needing lots of additional repairs and not last nearly as long as a properly sized unit.

Let’s say that an HVAC technician determines you need around 40,000 BTUs of cooling. A 3-ton AC would be too small, as it would only produce 36,000 BTUs. A 3.5-ton AC produces 42,000 BTUs, so it would probably be sufficient. It may still be better to choose a 48,000-BTU 4-ton unit just to ensure that your AC system works effectively on even the hottest, most humid days of the year.

Evaluating Insulation and Air Sealing

When calculating the specific cooling needs of a home, an HVAC technician will evaluate the level of insulation in the exterior walls and attic. They’ll also need to check to see how many air leaks the home has and how tightly sealed its envelope or exterior structure is. A home that is poorly insulated and has lots of air leaks will have much higher cooling requirements, as these factors will always contribute to it getting much hotter and more humid compared to a house that is sufficiently insulated and sealed.

The reason that the level of insulation in the attic is an especially critical factor is that attics always get extremely hot in the summer. On days when the temperature is between 90 and 100 degrees, the temperature in an attic will often rise to 130 or 140 degrees by the middle of the day, even if the attic is sufficiently ventilated and has a decent level of insulation. Even though heat rises, all of the heat that builds up in the attic will force its way into the home and make it much hotter if the attic floor isn’t fully insulated.

Insulating the attic will make the home much more energy efficient and help reduce its cooling and heating costs. If your attic isn’t well insulated, adding more insulation will typically mean that you won’t need as large of an AC unit as you would otherwise. As such, insulating your attic would normally end up paying for itself by saving you money on your new AC and also lowering your energy bills.

HVAC Load Calculations

When you hire an HVAC contractor to install air conditioning in your home or to replace your AC unit, the technician will first evaluate your home’s cooling requirements and perform something called a Manual J load calculation. The Manual J formula was developed as a way to fully and accurately calculate how many BTUs of cooling or heating a building requires.

When performing the load calculation, the technician will first simply determine the total square footage of the building and factor in the ceiling height so that they can get a base for how many BTUs the home would typically need. They’ll adjust the original number based on the insulation factor and the level of air sealing.

The next step is to total up the number of exterior doors and windows in the home and then add another 500 BTUs to the calculation for every door and window. After that, the technician will add or subtract based on how much shade or direct sunlight the home gets.

The number of people in your family or household is important since people generate heat. A technician will need to add another 600 BTUs to the calculation for each person. All of the appliances and electronics in a home also radiate heat, so a technician will also need to account for this.

At Punbar Air, we understand the importance of having an air conditioning system that works to keeps you cool. This is why our certified technicians take the time to calculate a home’s cooling or heating requirements before getting started on any HVAC installation project. Whether you’re looking to replace your AC or you need another cooling or heating service, you can always count on us to get the job done right. We also offer indoor air quality testing, air flow testing, and insulation. To schedule a consultation and learn what size AC will work best for your home in Houston, give us a call today.


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