Open the Windows or Run the AC?

This summer, I'm going to try to tweet regularly in the evening to report on our cooling strategy for the night. We will be deciding between two choices:


We installed central air a couple of years ago, because at least one of us works in the home all day, and during the hot, humid part of Ontario's summer it was hard to be productive. Our house does have lots of windows that open, so we can get good cross-ventilation. Many of the rooms also have ceiling fans. On the worst days, though, it wasn't enough.

There are times in an Ottawa summer when the nights are cool and the days are hot. I was wondering how often - and how well - it would work to open the windows at night, bring the temperature down, and then close up in the morning. Presumably if the conditions were right, the air conditioner would have less work to do the next day. Not always, though. If it's humid all night, even if it's cool, the house will get loaded up with moisture by morning. Then the next day the air conditioner will have more work to do removing the moisture than you saved by lowering the temperature. You don't want that.

Clearly the moisture in the air is a huge factor. The moisture in the air contributes to what's called the latent load on an air conditioner, whereas the simple dry-bulb temperature measures the sensible load. In Ottawa it's not at all unusual for the latent load to be larger than the sensible load on some days.

So how do you decide when the open-windows strategy will work and when it will be a mistake?

What Am I Doing?

The measure of energy content of air - including both temperature and moisture content - is called enthalpy. Enthalpy turns out to be a reasonable way to make this decision. The energy that has to be removed from air to change it from 30°C and 70% humidity to 25°C and 50% humidity can be calculated by comparing the enthalpies at those two points. If the outside air coming in at night is cooler but moister than the air inside, you can look at the change in enthalpy to find out if you're making a net improvement or making things worse. If you make things worse, the air conditioner will have more work to do in the morning when you close up and ask it to bring the house to comfortable working conditions.

I have a spreadsheet that estimates the work the air conditioner will do under both strategies. One strategy involves the air conditioner working all night to maintain conditions and then continuing to work the next day. The other one has the air conditioner shut off all night, but then it has to start with whatever conditions the house will have in the morning, bring it to comfort conditions and then maintain that for the day. The spreadsheet estimates which choice will use less energy. All I have to do is click a button to copy the hour-by-hour temperature and humidity forecast from a website into my spreadsheet. Then the spreadsheet automatically updates the calculation. Each evening I plan to try and post the recommended cooling strategy for my house.

Your House is Different

Your mileage may differ:


Watch for tweets this summer under the Twitter handle @daveushipley. I will probably use a few hashtags, too. #CoolOttawa and #WindowsOpenOrClosed seem promising. #OttawaWindowsOpenOrClosed is not in use at the moment, so I may claim that one as well.


Last Updated: 23 Oct 2022
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