You only have to include the basement in your AC unit size calculations if you intend to use it as an added living space.
Determining the size of your air conditioner (AC) unit is probably one of the most important and complicated parts of the entire installation process.
There is a fine balance between having a powerful AC and having one that is overly powerful for the space. This is why you should only include your basement square footage in your calculations if you will be using the space regularly.
Why the size of your AC is so important
Adding an air conditioning system to your home will certainly make the space a lot more comfortable when those warmer months roll around. But it is also a big investment with many important considerations.
Sizing your home air conditioning system correctly is probably one of the most important aspects in ensuring that the system is as effective and efficient as it can possibly be.
Selecting the right size is not merely about the physical dimensions of the unit; rather, it involves a meticulous assessment of your home’s specific cooling requirements.
An AC system that is too small might struggle to cool the entire space adequately, leading to discomfort and inefficiency.
On the other hand, an oversized system can result in frequent cycling, leading to unnecessary energy consumption, higher utility bills, and premature wear and tear on the equipment.
To ensure that your AC system functions at its best, a thorough evaluation of factors such as the size and layout of your home, insulation levels, windows, and local climate conditions is also important.
Do you include the basement when sizing an AC unit?
Figuring out the correct size for your home AC unit is complicated, because there are so many different factors to consider.
In Layman’s terms, ACs work by extracting the warm air from your space, running this warm air over cold coils to cool it down, and then releasing the now, much cooler air back into the space to systematically lower the temperature of the air in the entire space.
In order to do this, the air conditioner uses a considerable amount of energy, and this energy is measured in BTU (BTU is also converted into tons for bigger whole-home HVAC units sometimes).
The problem is that an air conditioner which is too powerful for a space will waste energy, while an air conditioner that is not powerful enough will never be able to cool the space adequately.
For this reason, you should only include your basement when sizing your AC unit if you plan on actively using the space, just as you would with any other living area in the home.
A completed, air conditioned basement can give you a considerably larger, comfortable space to entertain (especially when the sun is really beating down outside). Moreover, it can even help with the humidity and mold control in your below-ground level basement.
However, if you are just using your basement space for storage, it will likely not need much cool air and including this space in your sizing calculation could result in an oversized system.
Though there are also many other things that you need to consider before making the final decision.
How do the professionals do it?
Typically you can divide the total square footage of the spaces in your home that you want to include by 600 to get a rough estimate of the BTU (British thermal unit) that you will need to cool the space.
However, an HVAC professional’s estimate may differ from this figure slightly.
Many professionals use the Manual J Residential Load Calculation to determine which size AC unit you need in your home.
This calculation takes factors like ceiling height, building materials, number of windows and insulation into account. But it will still include your basement in the total size of the space if you choose to include it.
The practical aspects of installing an AC unit in the basement
Before you decide whether to add your basement square footage to your AC unit size calculation or not, you should consider some of the practical hurdles of adding this type of system.
If your basement is unfinished or it does not already have ductwork installed, adding new ductwork to this space to make it part of your home HVAC system can be an expensive and time-consuming process.
This is not always necessary if the space is only used once in a while. If your basement serves only occasional or specific purposes, such as storage or a recreational area that is not in constant use, the necessity of incorporating it into your central HVAC system may be reevaluated.
In such cases, installing a standalone or localized AC unit could be a more cost-effective and efficient solution.
This approach allows you to address the cooling needs of the basement without the need for extensive modifications to your home’s existing ductwork.
Standalone AC units for individual rooms or zones offer flexible cooling, enabling independent temperature control in the basement without the need for extensive HVAC system renovations.
You can always add the basement AC later on
If you find yourself going back and forth on the decision to add your basement to your home HVAC system, it is important to remember that you can always add to this system later on.
In fact, you can even choose between upgrading the system if the basement is too large for the current system, or installing a smaller, second AC unit that only cools the basement.
In this case, ductless mini-split units or portable ACs are popular choices, because they do not require any additional ducts.
Until then, you can keep your basement pretty cool, dry and comfortable by using a dehumidifier, like this Simsen one, which covers up to 7000 cubic feet of space, and a fan, like this Dreo tower fan, which can move the air around.