When you air condition your home to 75 degrees, and if it is 90 degrees outside, this is only a 15 degrees temperature difference. Each degree of this temperature difference represents a percentage of the total cooling load. Raising the thermostat 2 degrees lowers the temperature difference between the outside and inside by 2 degrees.

Since there are only 15 degrees total, a 2 degrees change represents a large percent of the total load. And, most hours of the day are not 90 degrees!

For all the summer hours where the temperature difference is only 10 degrees or even 5 degrees, 2 degrees represents a very large percentage.

In the Midwest area you could save about 10 percent to 15 percent per degree, for the first few degrees you set up from 75 degrees. One way to visualize this is to look at a chart of the average cooling load for each summer day. Cooling degree day data charted for Indianapolis will show us a pyramid of increasing daily cooling loads, starting in April, rising to a peak in July and decreasing back to zero in October. If a thermostat is set low enough, a home might air condition from April to October, or theoretically on all the days that have one or more cooling-degree day.

If the thermostat is set higher, a home may only cool from June to mid-September. A very conservative family, one that "...turns on the A/C only when road tar is bubblin' and the fish are swimming upstream through the front door," may only cool for a few weeks each summer.

A higher thermostat setting should reduce the number of days you air condition, but more importantly, it also reduces the temperature difference on all the other air conditioning days. Setting the temperature up a few degrees essentially eliminates the bottom rows of cooling load shown on chart A.

An Example: Indianapolis

For a better insight into air conditioning run time, let's look at the number of hours at each temperature. Chart B is a chart for Indianapolis during the summer of 2003. Notice how many hours are in the moderate range of 71 degrees to 80 degrees.



In this chart, there are 1,052 hours at 75 degrees or above. If you raise your thermostat from 75 degrees to 77 degrees, you will "lose" the 149 hours at 75 degrees and the 112 hours at 76 degrees. This is 261 hours, which is about 25 percent of the total hours over 75 degrees.

What happens if your customer is cooling lower than 75 degrees? In Indianapolis, in 2003, you would add to the 1,052 hours (75 degrees and above), 10 percent more hours to cool to 74 degrees, 28 percent to cool to 73 degrees, and 42 percent to cool down to 72 degrees. It is important for heating contractors to know what a significant difference one or two degrees really makes in the summer.

What is it worth? If your customer's cooling costs are $200 per summer, and if they save 25 percent by cooling to 77 degrees instead of 75 degrees, that's $50 per summer with no initial investment.

On the other hand, lowering the temperature 3 degrees from 75 degrees to 72 degrees could increase cooling costs 40 percent to 50 percent, which is an additional $100 per summer. These are estimates for central Indiana, where electric rates are low. In many areas, these numbers could be double.

Will we always see these savings with a temperature set up? Two conditions that will change the above estimates considerably are large solar loads or high latent loads. If a home has large areas of east and west, unshaded glass, even if the thermostat is raised to 80 degrees, the radiant gains could be such a significant portion of the cooling costs that the reduced temperature difference may not influence the bill as much. Likewise, if there is a very high latent load or there are high internal gains, the thermostat changes may be less noticeable.

Another obvious factor that changes these savings estimates is the weather for the area you live in. Looking at the summer cooling hours for Atlanta, there are twice as many cooling hours as Indianapolis. The savings are different, but they are still very significant for each degree you set up from 75 degrees.

In 1998, Atlanta had about 2,141 hours at 75 degrees or higher. Of these hours, the hours at 75 degrees were about 9.5 percent, at 76 degrees about 10 percent, and the hours at 77 degrees were about 8.5 percent. If you lowered your thermostat, you would begin adding the hours at 74 degrees, 73 degrees, and 72 degrees - and this added about 10 percent, 14 percent, and 10 percent, respectively. Lowering the thermostat to 72 degrees may have increased some bills by 34 percent.

Four Favorites

Considering all the variables, it is difficult to give exact savings in a list of general tips. However, it is important to understand how significant some measures can be.

On average, I would estimate that adjusting your thermostat could change air conditioning costs about 10 percent per degree you change, for the first few degrees from 75 degrees. This is significant.

When asked for my most helpful tips for summer air conditioning, I have four favorites:

1. Shade solar gains.

2. Set the thermostat up a few degrees.

3. Insulate ductwork in the attic.

4. Replace your 8-SEER air conditioner (which now runs at 6 SEER) with a 12-14 SEER.

Welklin is an energy consultant for Cinergy/PSI, an electric utility in Indiana. He works with builders, HVAC contractors, and residential customers as a consultant on energy usage, high-efficiency HVAC systems, and residential comfort. Welklin can be reached at 317-776-5357, or dwelklin@cinergy.com.

Publication date: 08/23/2004