AC 20-Degree Rule Of Thumb Explained: Real Or Nonsense?

Oh, the infamous AC 20-degree rule. Whenever we get 100+ degree days in the summer, some local news will start throwing around the ’20-degree rule for air conditioner’. Do the HVAC experts agree with this rule of thumb (input from HVACR School)? Or it’s just a bunch of nonsense?

Alright, let’s first explain what the 20-degree rule states. In simple terms, the rule spells out like this:

“Never set the thermostat temperature more than 20 degrees below the outside temperature.”

There it is, easy to remember, easy to comply with, right?

In practice, that would mean that:

• On a 95-degree day, you can only set the AC thermostat to 75 degrees.
• On a 100-degree day, you can only set the AC thermostat to 80 degrees.
• On a 110-degree day, you can only set the AC thermostat to 90 degrees.
• On a 120-degree day, you can only set the AC thermostat to 100 degrees.

As you can see, the difference in temperature should never be more than 20 degrees. Even here, we see there is something wrong with the 20-degree AC rule. On a 120-degree crazy hot day, you can only cook like a crab in a sauna-like humid 100-degree indoor temperature? That’s a bit off, right?

Well, what they say happens if you set the AC thermostat more than 20 degrees below the outdoor temperature? Example: 72 degrees on 100-degree day. Here is what they say it happens if you violate the 20-degree rule (with commentary):

• “Your utility bills will skyrocket!” That’s actually true. You can see up to a 100% increase in electricity bills but what’s the use of AC if you can’t use it for air conditioning on the hottest days?
• “The AC will break down!” No, it won’t. We will follow this with an insightful commentary from HVACR School further on, but in short, air conditioners are engineered to withstand high outdoor temperatures quite well.
• “You will reduce the AC lifespan.” Yes, running any device on high operating output can reduce the unit lifespan. However, the whole point of AC is to protect you from high indoor temperature. You would certainly not use it lightly and sparingly on very hot days.
• “You will catch a cold!” This is my favorite one. On a 100+ degree day, setting the AC thermometer to 72 degrees is now a health hazard? Not a doctor here, but we don’t see many people getting cold in the summer.

Truth About 20-Degree Rule For Air Conditioners

Alright, here’s the deal (HVACR School emphasizes this):

HVAC techs work hard to adequately size air conditioners for your home (using the Manual J by Air Conditioning Contractors of America or ACCA for short). The Manual J allows for up to 15% AC “oversizing”; this will not lead to short cycling like actually oversizing a unit would do.

The size of AC units heavily depends on the location (and thus outdoor temperatures; here is the detailed Manual J location appendix). Here are 2 examples put forward:

• In Florida, the AC sizing is based on 90-degree to 95-degree days. With 75-degree indoor temperatures, we get AC units designed for 15-20 degrees of outdoor-indoor temperature difference.
• In Las Vegas, Nevada, the AC sizing is based on 106-degree to 108-degree days. With 75-degree indoor temperatures, we get AC units designed for 31-33 degrees of outdoor-indoor temperature difference.

From this example, we see that AC units are sized appropriately for the location. That means that in some locations AC units are designed to best handle a 15-degree difference, at other locations a 20-degree difference, and in very hot climates like Las Vegas, they are designed to handle more than 30 degrees of difference.

Now, it is good to understand these are the optimum outdoor-indoor temperature differences. That doesn’t mean you can’t run an AC unit, optimized for a 20-degree difference, on a 110-degree or even 120-degree day (indoor temp 75 degrees; 35 and 45 degrees difference, respectively).

Yes, your AC unit might struggle on these very hot days, but that’s the whole point of AC. It struggles with heat so you don’t have to.

If the 20-degree AC rule of thumb was actually true, all air conditioners would be a bit useless on 100+ degree days.

You could at most get down to 80 degree indoor temperature. In practice, however, you can always set the AC thermostat to 75 degrees or even 72 degrees, and use the full capacity of the AC unit exactly when you need it.