Ensuring that your HVAC system is sufficiently sized for your home is essential in terms of both your comfort and energy costs. While you may think that opting for a larger system than you need will make your home more comfortable, it isn’t the case. An undersized or oversized system will never work as effectively. It will also cost more to operate and have a shorter lifespan compared to a properly sized unit. In this article, we’ll discuss all of the factors HVAC technicians take into account when sizing cooling or heating systems and why each one matters.

1. Square Footage of the Home

The first step when determining what size of AC, heat pump, or furnace a home needs is to calculate the total square footage of the conditioned space. The conditioned space refers to only the living areas and not the garage or anything other part of the house that isn’t connected to the central HVAC system. Calculating the square footage is important since there are general recommendations on how many BTUs of cooling or heating a home needs for the HVAC system to work effectively and efficiently.

2. Climate Zone

The BTU-per-square-foot recommendations we just mentioned vary quite drastically for different parts of the country. In Florida, you obviously need many more BTUs of cooling per square foot to keep the home sufficiently comfortable during the hot, humid months compared to what you’d need in a cooler climate. Similarly, your BTU requirements for heating will also be much lower compared to places with freezing cold winters.

The US Department of Energy splits the country into seven distinct climate zones, and each zone has its specific BTU recommendations for cooling and heating. Tampa and all of Florida except for the very southern tip near Miami is in climate zone 2. In this zone, the general recommendation is that an HVAC system needs to produce 35-40 BTUs of heating per square foot and 45-50 BTUs of cooling per square foot for it to work effectively.

Both of these recommendations are important for homes with separate air conditioning and heating systems. If you plan on installing a heat pump, the technician will just focus on the number of BTUs of cooling your home needs. Even though a heat pump can heat as well as cool, it’s mainly important that the unit is large enough to cool your home effectively.

One other thing to understand is that a climate zone isn’t just about how hot or cold the location is. How humid the area is also makes a huge difference when determining cooling requirements. This is because the effectiveness of air conditioning decreases by quite a lot in humid conditions.

3. Level of Insulation

These BTU recommendations are really just a starting point or a basic rule of thumb. If you were to compare two homes of the same size that are located in the same climate zone, one will still often have much higher or lower cooling or heating requirements than the other. One of the main reasons for this is that not all homes are as equally insulated.

For an older home that is poorly insulated, both the cooling and heating requirements per square foot will usually be quite a bit higher than the numbers we mentioned in the previous section. Especially important in terms of the cooling requirements is whether the attic floor is sufficiently insulated. Attics tend to get extremely hot during the summer. Even though heat rises, much of this heat will still get pushed down into the living spaces if the attic floor isn’t properly insulated. This leads to the home getting much hotter from the afternoon until early evening, which means a larger AC or heat pump will be required to ensure the HVAC system can keep the house sufficiently cool.

4. Air Sealing

This factor is right in line with the previous one and refers to how well sealed the exterior structure of the home is. It usually isn’t a factor for homes built more recently since newer homes are usually sealed quite well and have very few air leaks. The majority of older homes, on the other hand, typically have a large number of gaps in their structure that allow air to get in and out.

These gaps make the home quite a bit warmer in the summer since they allow hot air to seep in from outside. They also result in the home being cooler in the winter since they allow lots of the warm air from the heating system to escape and colder air to get inside. Consequently, the heating and cooling requirements will be quite a bit higher for an older home that is poorly sealed compared to a newer house with fewer air leaks.

5. Attic Ventilation

This factor is less important if the attic floor is sufficiently insulated, but it can make a major difference in terms of cooling requirements if the attic isn’t well insulated. A well-ventilated attic generally won’t get nearly as hot in the summer since air can flow through it to prevent as much heat from building up inside it. If the attic isn’t properly ventilated, the temperature inside it can often get to 140 degrees Fahrenheit or higher. The temperature inside a properly ventilated attic will typically stay almost exactly the same as the temperature outside. That means the house won’t get as hot as it would if the attic is poorly ventilated or has no ventilation.

6. Number, Size, and Location of Windows and Doors

Having lots of skylights and/or large windows, especially ones that face north or west, can be both a good thing and a bad thing. The good part is that sunlight streaming through the windows and skylights will help keep the home warmer in the winter so that it may not need quite as big a heating system.

In the summer, this heat gain is obviously a negative since it will contribute to the home getting much hotter. Therefore, a larger AC system is required. To account for this heat gain, the Manual J calculation that technicians typically use when sizing an HVAC system says that you need an additional 500 BTUs of cooling for every exterior door and window in the home.

7. Occupancy

This factor mostly applies when calculating a building’s cooling requirements. Human bodies are quite warm, and they continually release lots of heat and humidity into the air. Because of this, the Manual J calculation also says that you need an additional 500 BTUs of cooling for every occupant. While this usually isn’t a major consideration for residential HVAC systems, it can definitely make a big difference for commercial buildings with many more people inside.

8. Direct Sunlight or Shade

One last factor that the technician will evaluate, especially in terms of cooling requirements, is how much direct sunlight the building gets during the day. A home with lots of large trees that keep it shaded throughout most of the day will stay cooler. As such, it may not need quite as large a cooling system compared to a similar-sized home that receives lots of direct sunlight throughout the day.

If you need to install a new cooling or heating system or replace your existing system, Air 24/7 Air Conditioning & Heating is the resource you should trust. Our team has more than 30 years of experience and can help if you need a new air conditioner, heat pump, or furnace. We offer an extensive selection of new units from the best, most-trusted brands in the industry. We also specialize in maintenance and repairs to ensure your system continues working as effectively as possible. Contact us today to schedule an AC or heating installation consultation or any other HVAC service in the Tampa area.


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