Heating and cooling systems are designed to meet loads based on average low and average high temperatures. What happens when the the outdoor temperatures exceed the design temperatures?

Heating and cooling systems are not designed to meet the most severe weather conditions for a location because those conditions rarely occur. Weather data is used to determine the low and high temperature design conditions: 99% of the outdoor temperatures fall within the design conditions.

These design conditions come from weather data compiled and published by The American Society of Heating, Air-Conditioning and Refrigeration Engineers (ASHRAE). When designing heating and cooling systems, we find the design conditions in ACCA’s Manual J, Table 1A “Outdoor Design Conditions for the United States”. Cooling system design conditions for cities across the southeastern United States include the following:

The cooling design condition is the highest anticipated temperature in the summer. Stated another way: the outdoor temperature is below the cooling design condition temperature 99% of the time.

How It Works
When the outside temperature rises above the cooling design condition, your cooling system may have to work constantly to cool your house. It wasn’t designed for such high temperatures though that doesn’t mean your house will be hot.

Figure 18-1 in ACCA’s Manual J has a footnote that reads, “On average, the outdoor temperature [in Atlanta, GA] is… above 91°F for less than 156 hours per season.” So, on average, a cooling system in Atlanta is too small 156 hours out of the entire cooling season.

You might not even notice because those 156 hours don’t happen all at once. They happen every now and then. For example, the outdoor temperature might rise above the design temperature for a few hours in an afternoon.

Even if your cooling system can’t achieve the indoor temperature to which your thermostat is set, it should come close. In the summer, as the outdoor temperature rises further above your cooling system’s summer design condition, the indoor temperature rises further above your thermostat setting. Still, that shouldn’t be a lasting condition… just 1% of the time.

Unfortunately
To further complicate things, heating/cooling systems are not always designed correctly, not always installed correctly and not always maintained properly. Even if your system is designed correctly, it may not be able to perform as designed because of leaking ducts, clogged air filters and other common (self-inflicted) problems.

The image above is a plenum at the end of an air handler in the basement of a house. The unit flows conditioned air (heated or cooled) into the silver box (the plenum) where it’s supposed to flow through the attached ducts to various parts of the house. As you can see, there’s no possible way the collapsed duct in the upper left-hand corner is flowing the air it’s intended to flow. So, even if the rest of the system is working as intended, this common problem is choking the air handler and at least some part of the house isn’t comfortable.

Let’s use Atlanta as an example. Atlanta’s summer design temperature is 91°F - a cooling system in Atlanta should be able to keep its home comfortable up to 91°F outside. At temperatures above 91°F, the cooling system can’t maintain the thermostat setting though it should be close and those hot outdoor temperatures don’t last long.

Leaks, obstructions, poor design, etc. have the effect of lowering the outdoor temperature at which your cooling system can keep the house comfortable. The obstruction in the image above is in a house on the south side of Atlanta. The cooling system isn’t going to be able to keep the home comfortable up to 91°F outside.

Let’s guesstimate the combination of leaks, obstructions, etc. means the cooling system can only keep the home comfortable at outside temperatures up to 87°F. That’s a big difference because we spend many more hours above 87°F in the summer than we do 91°F. So, the home is not able to maintain a comfortable indoor temperature through more summer hours and the cooling system is wasting costly energy through leaks, obstructions, etc.

But, What If….
Should you close off vents to channel the cool air to one part of the house? No. Your ductwork is designed to deliver a certain amount of air flow to each space. Shutting off airflow to other spaces reduces air flow through the cooling system. Slowing the air flow can result in humidity freezing on the air conditioning coil in your air handler - a frozen coil.

Should you remove the air filter to allow better air flow? No. Your system is designed to account for the resistance of the air filter and removing it just increases the likelihood that you’ll get dust, pet hair and other gunk in your air handler, especially on the air conditioning coil – which isn’t easy or cheap to clean.

Should you set the temperature on the thermostat lower to get more cool air? No. If your air conditioning system isn’t cooling your house to the temperature to which you’ve set your thermostat, setting the thermostat even lower won’t help.

There May Be Some Relief (in Georgia)
Georgia adopted the 2015 International Energy Conservation Code with Georgia amendments as its latest version of the state’s energy code. The energy code and amendments took effect on January 1, 2020.

In the process of developing amendments to the energy code, we acknowledged that many home owners choose to purchase and have installed equipment which can moderate its capacity. In simple terms, these systems can run at more speeds than just “off” and “on”. We included the following language for owners who choose to use modulating equipment:

R403.7 Equipment sizing and efficiency rating (Mandatory). (The beginning of the section left unchanged) For automatically modulating capacity heating and cooling equipment, the system shall be deemed to comply with appropriate portions of Manual S provided the lowest output capacity of the equipment is less than the peak design load as determined by Manual J.
(Effective January 1, 2020)

This language means you can put in larger heating and cooling systems than those determined by following the design criteria in the building code so long as the lowest speed of the equipment is less than the design loads required by the building code - the design temperatures for heating and cooling. So, you can put in a larger cooling system as long as its lowest capacity is less than 91°F… if you’re in Atlanta.

Your larger cooling system might have summer design temperature closer to 95°F rather than 91°F. It runs at a lower speed during most of the year when you don’t need it to operate at full capacity. Instead of having 156 hours a year when the outside temperature is above 91°F, you might have just a few hours each year when the outside temperature is above 95°F.

That’s a possible, code-compliant solution to meet your cooling loads though you’ll want to take care to have your system inspected and maintained if you install a larger, variable-speed or multi-stage system. The savings you hope to achieve from one of these systems may not be realized if the duct work installation is poorly sized and leaky. This is especially true if you install modulating equipment onto an existing duct system.

Also, you may need to discuss this option with your heating/cooling contractor. Even though this amendment has been effective in Georgia since the first day of 2020, not everyone is aware of it.

In Conclusion
It’s critical that air conditioning systems are designed, installed and maintained properly. It’s easy to design a system without taking into account the right variables. Design assumptions (like the level of insulation or air sealing in a home) that turn out to be wrong can mean a home owner makes a significant investment in an air conditioning system that’s not able to deliver comfort and efficiency.

We’ve shared these notes so you won’t lose faith in your air conditioning system if it’s struggling to maintain your comfort on unusually hot or cold days. Hopefully, this has given you a better understanding of how air conditioning systems work so you’ll know when to ride out the severe weather and when to call a service technician.

As always, we’d like to hear from you. Leave a comment below to let us know if you’re experiencing a cooling system that’s not keeping up, you’re considering upgrading your system or any other concerns. We invite you to offer guidance too!

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