Is my air conditioner too small?

Contrary to popular belief, it is actually better to have an AC that’s a bit smaller than your load compared to one that’s too big. We’ll get to why in a minute.

Guest Post from Bob Wells of HVACTraining101.com

Isn’t it annoying that your expensive air conditioner that cost you an arm and a leg is not doing its job properly? It sort of cools the house, but it's still a bit sticky, it runs noisily, and uses a lot of energy while doing so. All this despite being in perfect running condition. If this sounds familiar, it means your AC wasn’t sized correctly to fit your house’s load.

What is “Load” and What Does it Have to Do with Air Conditioning and Heating?
The main function of your HVAC unit is to keep the occupants of your house comfortable. In order to do this effectively, the HVAC system has to match the house’s thermal load, which is a measure of the heat gain and loss for the home. The load consists of two components - the temperature of the air and the moisture content or humidity of the air.

The load of a building is determined by several factors, such as the building construction, air leakage, orientation to the sun and the “R” value of the insulation. The number, size, and placement of rooms and the number, size, and placement of windows and doors play a big role as well. Finally, the types of windows and doors (thermal efficiency); number and arrangement of floors; and the climate are also important considerations.

An AC that is too big will cool down the air very quickly, but it will not run long enough to remove sufficient moisture or “humidity” from the air. This will make the temperature fall sufficiently to make you feel cool, but you’ll also get a clammy feeling. One issue with the insufficient moisture removal resulting from oversized cooling equipment is mold growth. If humidity levels stay too high, mold may take root, possibly in unseen spaces like basements, walls, and crawlspaces. There can be other types of moisture-related damage as well. This is another reason to right size your air conditioner.

Let’s dig into what determines how big your AC needs to be.

House Size
Surface area has a large impact on the house load.
The bigger the house, the more AC it will need - but only kind of. It’s an obvious principle but the reality is more complicated.

Many HVAC professionals just use a rule of thumb to guess the load of your house, such as 500 square feet per ton. The rule of thumb method usually leads to oversized equipment. And that often means an increased initial cost and higher monthly utility bills. It also leads to increased maintenance and shortened equipment life because the equipment cycles off and on too frequently.

You should know that a ton in HVAC parlance refers to the rate of cooling, and has nothing to do with the weight of the unit. One ton of cooling is equal to 12,000 Btu/h (British Thermal Units per hour), the rate of cooling required to freeze one ton of ice at 32°F in one day.

When it comes to HVAC, size definitely matters! But it’s not as straightforward as just measuring the square footage of your house. As you’ll see below, there are other factors that go into determining how much cooling (or heating) your house needs.

Air Sealing
Here’s a scary thought: the drafts in your home during winter through the windows and under the doors are costing you big money! This is true in the summer as well, but let’s think about winter first because it’s easier to visualize.

When cold air enters through the cracks, it lowers the temperature inside the house and forces the HVAC system to work harder to maintain the temperature at the thermostat set point. The same issue occurs in winter when your AC is running flat out to cope with cool air escaping and moist air coming inside. If you live in a place where the grass stays green without watering it, lots of humidity is sneaking in through those cracks too.

It’s not just doors and windows that are to blame for leaking air. Ducts deliver heated or cooled air to your rooms. But some ducts may excessively leak air or not deliver the right amount of air to each room. This means higher operational costs - from a few percent to over 50%.

The best way to right-size your home’s air conditioning unit is to plug leaks and holes which allow air to leak through. This will immediately decrease the workload on the air conditioner. A licensed HVAC technician can easily identify and seal these leaks in an HVAC system. A Home Performance contractor can seal and insulate the holes in the house itself. An energy auditor can help you find these opportunities as well. In fact Nate recommends beginning with an energy audit to find opportunities rather than just trying something only to be disappointed.

It’s best to be sure your house is sealed tightly. Some flow of outdoor air is important but you don’t want this left to chance. Ventilation is important but should be incorporated into your HVAC system and under the system’s control. But this should be carefully planned so that energy isn’t going to waste.

Insulation
When you handle a hot pan, you use a dry cloth or oven mitt to prevent burns. The cloth is an insulator – which means it doesn’t conduct heat very well. Similarly, a well-insulated house has layers of materials in the walls and roof to keep the cold in during summer and the heat in during winter. After air sealing, your home should be insulated enough to slow down heat flowing in or out of your home.

Solar LoadThe solar load on your house will determine how much cooling you need in summer and the warmer months as the solar radiation heats up your house. It can also reduce your heating load in the winter by warming up the interior through solar radiation.

The orientation of your house will affect how much sunlight enters your home, raising the air temperature inside. The orientation must be taken into account due to changing solar heat gains throughout the day according to the season. Passive solar building design uses this principle to maximize the amount of solar heat provided to a building in winter, while minimizing it in summer.  

The size and positioning of the windows as well as blinds or curtains, impact the sunlight entering your house. Properly-designed awnings and shades can go a long way to reducing your cooling load during hot, sunny days, particularly if you live in a warm climate.

If your house is shaded by trees or nearby buildings, it leads to cooler rooms for part of the day, which is another way to reduce your cooling load. You can enhance the shade on your house by consulting experts from the Society of American Foresters or Extension.org in order to find out what trees work best in your location.

In addition, using light-colored materials on the rooftop will reflect more sunlight, thus preventing heat from seeping in from above.

Climate
The climate of the location where you live depends on its latitude, elevation and proximity to the sea. Climate will determine the humidity, temperature highs and lows, number of hot and cold days as well as seasonal variations your AC has to deal with.

In humid climates more energy is required to remove excess moisture from ventilation air. You may notice that the temperature at which you feel comfortable differs with varying levels of humidity. Temperatures as high as 78 or 80F can feel comfortable if humidity is low enough.

The ideal HVAC system is sized so that your AC runs non-stop on the hottest days, as this means it is not oversized. An AC that starts and stops frequently is too big for the thermal load that it is dealing with.
The concept of design temperature comes into play for HVAC design. The 99% design temperature for a house is the outdoor temperature that your location stays above or below for 99% of all the hours in the year, based on a 30-year average. On the other hand, the outdoor air where you live is going to be colder than the 1% design temperature for only 1% of the hours in a year. That happens to be about 88 hours per year.

Temperature
The cooling requirements of your house will change throughout the day. At night, the outside ambient temperature is lower. This is largely due to solar radiation being absent. Once your thermostat is set to a set point that is comfortable for the occupants, the HVAC needs to ramp up to meet the cooling needs during the high temperatures and ramp down during lower temperatures.

The sizing of the HVAC should take into account the diurnal temperature variation and be sufficient to cool the house to a comfortable temperature at peak temperatures while running continuously.

Occupancy and Use
An interesting factor that you might never think of as affecting the thermal load of your house is you! A person in a room gives off heat, just like any other warm body. We all noticed that on the really hot days (“Don’t touch me - it’s too hot”) and the really cold ones (“Wanna cuddle?”).

The more occupants in a building, the higher the cooling load. However, the number of occupants in a house changes on a daily, weekly and even yearly basis. The load additional load per person is about 400 BTU/hour considering normal residential activity levels.

Think about your average weekly routine. On weekdays, after you are done preparing for work, you are out of the house for around 9 hours. During that time, the house could be empty and thus have a lower cooling load. On weekends, the whole family might be away, leaving the house empty.

With the passage of time, your house’s occupancy will change. Children grow up and leave the house, or guests stay for a few months. While you cannot plan for every eventuality, your AC should be able to cope with at least a reasonable occupancy level. Most calculation methods assume that the number of occupants equals the number of bedrooms plus one.

Conclusion
Designing the perfect sized HVAC system is not as easy as following some rule of thumb. But if you pay attention to some factors such as climate, occupancy, solar load and insulation, you can calculate the optimum system size that will meet your house’s cooling and heating needs for the whole year. A competent HVAC contractor is going to do a lot more than just measure the length and width of your house.

They should run what’s called a “load calculation.” The most common one is called “Manual J”. Insist on getting one and seeing how they calculated. These are far more useful if they do a blower door test to discover how much it leaks, as this can account for 30-70% of heating and cooling load.

When it comes to the comfort of your house, there should be no compromise. But rather than going for an oversized AC like the majority of homeowners, opt for the smart move and right-size your air conditioner. This often saves on initial costs and creates a far more comfortable home.

Author Bio
This is a guest post by Bob Wells, a retired HVAC tech who now dedicates himself to sharing knowledge on his website HVAC Training 101. Bob worked over 30 years in the field, 23 of which he ran his own contracting business. He’s dedicated to keeping up with the latest developments in the field and helping others to learn the trade better and advance their own careers.
Bob is on Twitter with the handle @hvactraining101 and you can also find him on Facebook.