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:

• close the windows and run the central air conditioner all night, or
• open the windows all night and turn the air conditioner off.

Background

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:

• Your temperature setpoints may be different from ours. We usually let the temperature float up to 26°C in the afternoon (unless we can't stand it), set it to 25°C in the morning and evening, and bring it down to 24°C overnight. That's based on the times when electric rates are high, medium, and low in Ontario. Keeping the temperature cooler at night does several things: it "pre-cools" the home so that less power is needed to cool it in the heat of the day, it makes use of more cooling energy when rates are low, and it makes for comfortable sleeping temperatures. Allowing the temperature to float up a bit during the hottest part of the day provides relief to the utility during its peak load period. We think Hydro Ottawa probably likes that.
• Your house may behave differently than ours does. Our house was built in 1962. It has good windows and doors and extra insulation in the attic, but the walls are poorly insulated. Depending on how well-insulated your house is, your air conditioner may take more or less energy to maintain a constant temperature than ours does. That would change the comparison between the energy needed to maintain the cooling setpoint all night and the energy needed to catch up in the morning if you've had the windows open. (For the HVAC geeks out there, I ran a series of experiments to estimate the time constant of our house, to try to get this comparison reasonably close.
• Your air conditioner may behave differently from ours. Ours seems to do a pretty good job of bringing the humidity down to around 50% in the normal course of bringing the house to the desired temperature. Air conditioners that are oversized don't tend to control humidity well, because their runtime is too short (they reach the temperature too quickly and there isn't enough time for water to collect on the cooling coils). If your air conditioner is oversized, humidity will have less effect on your air conditioner energy, but you will also be less comfortable. [As a side note, if you run your fan continuously during the summer, your air conditioner will do an extremely poor job of controlling humidity. To get the moisture out of the air, you need the air to stop moving across the coils when the air conditioner cycles off. That allows the moisture to drip into the condensate pan and run down the drain. If you keep the fan going, the moisture that collected on the coils will just get blown back into your house and the air conditioner will have to collect it all over again. Set your fan on "auto" in the summer.]
• You may have more or less cross-ventilation than we do. With our windows open, our house typically gets to about 2°C above the outside temperature by morning. Then it bottoms out and, as the day begins to warm, the inside temperature begins to rise, too. If you have fewer operable windows, your indoor temperature may not get as close to the outside temperature before it starts to warm up.

Tweets

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.

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