What Is Supercooling?
Hearing the term supercooling might give you a flashback to a long past science class, but don’t worry: You won’t need to brush up on high school physics or don safety goggles to take advantage of it.
In the HVAC industry, supercooling—also called “subcooling” or “precooling”—is the practice of running the air conditioner at optimum times to significantly chill an environment. Then during the non-supercooling period, the AC system is run less or even not at all.
This creates a reduced financial and energy burden while maintaining the same (or even better) comfort level provided by running the air conditioner in a more traditional fashion.
What this looks like, in practical terms for most people, is running your AC at night to lower the temperature of your home below the temperature you would normally set your thermostat.
If you normally have the thermostat set to 72°F during the day, you dial the thermostat back at night to a much lower temperature such 65°F or even 60°F. The colder the better—within reason.
Then, during the day you set the thermostat to a higher temperature and coast on the supercooling you’ve done.
What Are the Benefits of Supercooling?
While we just mentioned the financial and energy benefits, let’s take a closer look at what they are and who they apply to.
Practically everyone can benefit from supercooling, but depending on variables like your local climate and what kind of energy pricing scheme your local utility company uses you may only realize some of the benefits, say, being more comfortable without necessarily saving a ton on your actual utility costs.
Your AC Works Harder at Optimum Times
The first supercooling benefit applies to everyone: When you run the AC at night, the system isn’t fighting the sun.
When the sun is shining on your home and the daytime temperatures are higher, the AC system is working not just to remove the existing heat from your home—and you, your pets, your appliances, and just daily activities like cooking or running your computer to play a game all generate heat—but to remove heat introduced by the sun.
At night your home radiates heat into the environment and the sun isn’t pouring through the windows adding more heat. So the AC can run more effectively, removing more heat from your home.
And removing heat isn’t just about making the air around you feel cool. Removing heat also cools the structure of your home and all the stuff inside it, too.
Significantly cooling your home at night essentially “sucks” heat out all your furnishings and even the walls and floors. Come the next day, your well-chilled home will act like a thermal sink, keeping you more comfortable with less strain on the AC.
It is in no way an exaggeration to say that, when you’re employing supercooling as an HVAC strategy, your goal is to chill your home right down to the “bones” of the building.
You’ll Take Advantage of Off-Peak Electrical Rates
The extra demand that increased AC usage puts on a power grid during hot weather is significant. Some utilities will simply increase the cost of electrical energy during the summer to compensate for the demand, but the majority of them either have across-the-board peak and off-peak rates—or they have a program you can opt into to further incentivize their customers to use power during off-peak times.
The chart above is from Nebraska Public Power District and is a good stand-in for the kind of on and off-peak rates you can expect across the United States. Not only does the utility have an off-peak rate in the morning and late evening and a traditional peak window during the 2-7 PM block that coincides with high AC demand, it has a super off-peak rate at night.
Every hour you run your AC at night that helps you avoid running your AC during that on-peak rate window is a 300% savings as you’re spending ~5 cents per kilowatt-hour instead of ~20 cents.
Even if your local utility doesn’t have anything like a deep discount “super off-peak rate” you should still expect to save easily around 50-150% on nighttime off-peak rates.
You’ll Put Less Wear and Tear on Your HVAC System
Labor and materials just keep going up in price, and anything you can do to extend the life of your costly HVAC system is ideal to put off service calls or even a full system replacement. With replacement costs in the “I should have bought a used car instead” range, every extra year you get out of a system is a welcome one.
Supercooling is a great way to minimize hot-weather wear and tear on your HVAC system because your AC and the associate hardware like your furnace blower will run steadily through the night when temperatures are lower, instead of cycling on and off all day when temperatures are higher.
You’ll Sleep Better
Using your AC more efficiently and saving money with off-peak rates is great and all, but here’s a perfectly comfort-centric reason to use supercooling to chill your house at night: you’ll sleep better.
While people often enjoy sleeping cozy snuggled up in lots of blankets, nobody really likes sleeping hot. The optimum sleeping temperature for humans is between 60°F to 67°F, according to experts.
Sleeping in a room above that temperature can lead to sweating, disrupted sleep, and general discomfort. It’s such an issue for some folks that there are entire product lines like the ChiliSleep and BedJet devoted just to optimizing sleep temperature.
The best part about using a system like supercooling is that it allows you to chill your homes down nice and low to get that restorative sleeping-in-a-cave coolness. Everything else, like saving money, just might be the cherry-on-top part as far as we’re concerned.
How to Supercool More Effectively
Obviously, the heart of supercooling is just to turn the thermostat way down before you go to bed and turn it back up when you are awake to coast on the deep cooling you did overnight.
But there are always ways to optimize an optimization, no? So if you’re considering supercooling, consider using some of these tips to make it more frictionless and more effective.
Use a Smart Thermostat for Easy Scheduling
Fiddling with the thermostat by hand is so last century. While you can use a standard programmable thermostat to program a schedule, it’s so much more convenient to use a smart thermostat.
Further, some smart thermostats come with additional sensors you can place in critical areas, like the ecobee SmartThermostat. If you’re worried that your supercooling plan might lead to, say, the room you keep your dog in while you’re at work getting too hot, you can use the smart sensor to monitor that space.
Schedule the Fan During the Day
You might be used to your HVAC blower fan only running when the heating and cooling is active, but we’d encourage—especially when using supercooling—to take advantage of the fan during off-time to just circulate the air.
By scheduling the fan to blow during the day, you’ll move air through your home and keep the temperature stable so individual rooms don’t end up stuffy or hot while other rooms stay much cooler.
Settings vary between thermostats and systems, but it’s common to find a setting like “Run Fan for X Minutes Every Hour” or such. If you have a newer system with a variable speed fan, you can usually enable a comfort mode where the fan blows at a low-and-slow speed all day to ensure uniform air temperature.
Minimize Heat Gain During the Day
In addition to your nighttime supercooling routine, you should also use tried and true techniques for keeping the heat out. Every bit of heat your AC system doesn’t have to work to remove is an all-around win and will make your supercooling efforts more effective.
Close the blinds and draw the drapes on any windows that get direct sun exposure. If appropriate for your location, consider installing heat-reducing film on the windows and/or installing awnings to keep the heat out before it bakes the interior of your home.
Minimize Air Gaps and Loss Points
Again, this is just good overall practice regardless of whether you’re supercooling or not, but look for any points where air is leaking in or out of your home or points where the insulation is poor.
Most utility companies have free or very low-cost programs where someone will come out to your home and do a blower test and give you tips on where you need to seal or better insulate your home. Not only are such visits usually free but they’ll typically also give you free LED bulbs, help you insulate exposed pipes, and otherwise give you extra cost-saving benefits.
Replacing weather stripping on a door or using caulk or spray foam to seal up gaps around windows or utility vents is a great way to keep all that cool and conditioned air inside your home.
The finer points of buttoning up your home tight to keep the cool air inside aside, the heart of supercooling is to chill things as cold as you can stand them every night. The colder you make the home at night the longer you’ll be able to go throughout the heat of the day without the AC turning on. So remember, if you feel the need to grab a throw blanket during your late-night Netflix binge, you’re doing it right.
