What Outside Temps Are Too Cold for Your Heating System?
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 aren’t 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”. Heating system design conditions for cities across the southeastern United States include the following:
The red numbers in the image above are the winter heating 99% dry bulb temperatures used to design heating systems. The value for Frankfort, KY is taken from the neighboring city of Lexington since data for Frankfort isn't provided.
The heating design condition is the lowest anticipated temperature in the winter. Stated another way: the outdoor temperature is above the heating design condition temperature 99% of the time.
How It Works
When the outside temperature drops below the winter design condition, your heating system may have to work constantly to heat your house. It wasn’t designed for such low temperatures though that doesn’t mean your house will be cold.
Figure 18-1 in ACCA’s Manual J has a footnote that reads, “On average, the outdoor temperature [in Atlanta, GA] is below 23°F for less than 85 hours per season… per season.” So, on average, a heating system in Atlanta is too small only 85 hours out of the entire heating season.
You might not even notice because those 85 hours don’t happen all at once. They happen every now and then. For example, the outdoor temperature might drop below the design temperature for an hour and then warm up.
Even if your heating system can’t achieve the indoor temperature to which your thermostat is set, it should come close. In the winter, as the outdoor temperature drops further below your heating system’s winter design condition, the indoor temperature drops further below 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 winter design temperature is 23 degrees Fahrenheit - a heating system in Atlanta should be able to keep its home comfortable down to 23 degrees. At temperatures below 23 degrees, the heating system can’t maintain the thermostat setting though it should be close and those cold temperatures don’t last long.
Leaks, obstructions, poor design, etc. have the effect of raising the outdoor temperature at which your heating system can keep the house comfortable. The obstruction in the image above is in a house on the south side of Atlanta. The heating system isn’t going to be able to keep the home comfortable down to 23 degrees.
Let’s guesstimate the combination of leaks, obstructions, etc. means the heating system can only keep the home comfortable at outside temperatures down to 30 degrees. That’s a big difference because we spend many more hours below 30 degrees in the winter than we do 23 degrees. So, the home is not able to maintain a comfortable indoor temperature through more winter hours and the heating system is wasting costly energy through leaks, obstructions, etc.
But, What If….
Should you close off vents to channel the hot 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 heating system. This can overheat and damage the system.
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 – which isn’t easy or cheap to clean.
Should you set the temperature on the thermostat higher to get more hot air? No. If your heating system isn’t heating your house to the temperature to which you’ve set your thermostat, setting it even higher probably won’t help – it’s not like a transmission that can shift gears. On systems like heat pumps that have back-up heating strips, the reserve heating capacity should already be in use if the system isn’t meeting the thermostat setting.
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.
COMMENTS: We’ve enabled comments for this post to encourage discussion and learn from you. Please review and adhere to the blog comment policy in our Terms of Service if you want your comment to be posted. Requirements include no anonymous posts (first name and last initial is fine). Please include your email address - your email address will not be displayed or added to our email list. All comments are moderated - they will not appear immediately.