Use Your Fans and Cross Ventilation to Cut Down on AC Use this Summer. Here’s How

My home office, originally the master bedroom, is the worst room in my apartment for cooling off in the summer. It feels like the roof does nothing to keep the heat out during heatwaves.

It gets worse. The room is too big for a single central air conditioning vent to cool adequately. Not far from my desk, there’s always a fan running. (Needless to say, 85ºF isn’t ideal for working.)

But a week ago, I wore a hoodie to warm up after ventilating the space all night with open windows in cooler weather. It was fantastic. Had I kept the windows closed, my thermostat and air conditioner would’ve tried in vain to reach the same temperatures as two box fans.

Let’s explore how cross ventilation works and some ways you can use it to cut your cooling bill.

How cross ventilation can cool your home

Cross ventilation works because the wind creates a pressure differential outside your home. The upwind side of your home experiences high-pressure airflow, while the downwind side creates a low-pressure zone. The pressure differences exist whether or not your windows are open, so you might as well harness them.

By opening your windows, the high-pressure air has a more direct path to the low-pressure zone — it’s easier to flow through a home than around it. It’s a great way to bypass your home’s insulation, which prevents thermal energy from entering and exiting your home. Once your home is cool enough, closing the windows and curtains ensures the indoor air will warm more slowly.

If you want to use natural ventilation for cooling, the outside air must be colder than the indoor air.

For example, I close my windows and rely on my AC after the outdoor temperature tops 75ºF. This is usually when my thermostat kicks in — around 72ºF in my basement. I keep the windows closed during hot weather — unless the nighttime temperature drops below 70ºF before I head to bed.

Cross ventilation and air conditioning can coexist

If you’re using an air conditioner or heat pump, you’ll see the best energy efficiency when every window is closed during operation. This is because open windows and doors create a huge hole in the building envelope, which separates indoor and outdoor air.

Limiting the amount of air the AC unit must condition means it won’t work as hard as when inside air constantly exchanges with outside air. In mild weather, free cooling is effective enough to offset the solar heating that plagues my home when temps go up.

You have to remember to close the windows daily, but the extra effort is generally worth it. Before I started regularly using cross ventilation as a remote worker, my AC would run at least twice as much. I’ve seen substantial savings these past few years.

Effective natural ventilation depends on many factors

According to Energy.gov’s Energy Saver office, the best candidates for natural ventilation are coastal and arid climates. On the coast, you can harness cool sea breezes to help make your home more comfortable.

While these regions are the most likely to experience huge nighttime temperature drops, you can also use wind ventilation in other places. It probably won’t be as effective, especially if your local climate is humid, but it’s still a good option for bringing in fresh air.

Unfortunately, the effectiveness of cross ventilation as a cooling method diminishes as the outside temperature rises. Keeping the windows open all day may eventually allow hot air to enter your home and strain your AC.

Likewise, poor air quality days aren’t great for open windows, so it’s worth setting up an alert if you live somewhere susceptible to air pollution.

It’s not always easy to create a cross breeze. Passive ventilation works best where the wind blows directly into a window and through the house in a straight line.

Opening the windows won’t cut it in a home where the air currents must navigate complex hallways and rooms to find an exit. Every obstacle causes turbulence, so the cold air may pick up a lot of heat as it tries to flow into an adjacent living space.

For example, my home’s best windows for a cross breeze are on the upper level, which is more difficult to cool than the relatively stable basement. This often means a temperature difference of at least 10 degrees in the summer — a comfortable 72º F downstairs and a balmy 85º upstairs.

Unfortunately, the airflow needs to move around several walls to connect them. Without my window fans, there’s a swirling turbulence in the hallway instead of a pleasant breeze through my office.

Make your own breeze with a window fan

Using a window fan to create cross ventilation is straightforward. The simplest method is exhaust ventilation, which uses a single fan that pushes air outdoors on the opposite side of the house from where you want the air to come in. This creates a negative pressure zone that pulls air through your home and out through the fan.

While this method is effective, it may not move air fast enough for a strong breeze, especially in large homes. Low air movement also means potential cooling effects can disappear into turbulence before reaching the out-blowing fan.

Fortunately, we can mostly overcome this by adding a second, in-blowing fan in the other window in a process called balanced ventilation. The in-blowing fan creates a high-pressure zone that more smoothly flows into the existing low-pressure zone.

Although I use a pair of Lasko box fans from Walmart, you can also buy purpose-built window fans, like this model from Bionaire, with reversible airflow, thermostats and extender screens for sealing windows.

Using smart home tech to control window fans

Let’s run through the basics of my window fan ventilation setup:

• Box fan 1 brings air in through the bedroom window.
• Box fan 2 pulls air through the hall and into the office before blowing it outside.
• Each fan connects to a smart plug for easy control — I group them using a virtual switch so they run in unison.
• A Sonoff temperature sensor in the bedroom triggers the fans to run as long as the indoor temperature is warmer than outside. It also keeps the room from getting too cold.
• I use a temperature deadband and a time delay to keep the fans from constantly power cycling near the target temperatures.
• A window sensor in the bedroom prevents accidental activation when the window is closed while also triggering the automation when I open a window.

While I don’t have a smart thermostat, you could incorporate one into these routines to keep the AC off while the ventilation process is active.

For this to work, you need a smart home hub capable of using temperature as an automation trigger. Your best bet is something like Home Assistant, SmartThings or Ezlo. Unfortunately, I couldn’t find this option on Alexa or Google Home with my current smart home devices. I can’t access Apple Home, so I can’t verify how it uses temperature in smart home routines.

Although window fans work well in my situation, you probably don’t need them if your home already handles cross breezes like a pro. Still, you can use smart home sensors to notify you when it’s time to close the windows and blinds for the day.

Fresh air ventilators

While window fans are probably best for renters and people on a budget, you can also outfit your home with a fresh air ventilator accessory for your HVAC system. Provided you have a compatible system, Ecobee smart thermostats support cross ventilation through their Free Cooling feature. You can also get similar functionality from whole-house ventilation products like AprilAire.

I have no experience with whole-house ventilation systems, so I recommend contacting a local HVAC installer if you want to add one to your home.