Home Energy Savings For Energy Watchers

A kind reader, Guy Kuo, recently proposed a new way to keep your home cool this summer. Here is the relevant part:

Once air conditioning season arrives, seal the house. Do not open the windows at night to cool the house. Set the air conditioning system to cool the house below normal at night and allow the house's thermal capacity coast through most of the hot day without air conditioning.

This is contrary to almost every single website about saving energy that I have read. Normally, one sees advice saying to open your windows at night to let night air cool the house. That seems perfectly logical. Night air is cooler and costs nothing to to use. Sadly, that common sense strategy is not always right. For someone in a well insulated home like we have, opening the windows at night ended up increasing our total air conditioning usage. Why?

1. Water vapor enters the house along with the night air. The increased humidity decreases comfort and drops air conditioning efficiency the next day because you have to spend energy removing water from the house before you get net cooling from the air conditoner. If one opens windows and lets cool air in at night, one also allows a great deal of humidity into the house. That water vapor is not only in the air, but it is absorbed into carpeting and upholstery. When the next day's air conditioning begins, a major portion of the air conditioner's energy usage is spent removing all the water vapor that was introduced overnight into the house. Worse yet, increased indoor humidity means things feel hotter at the same temperature than at a lower humidity. That "common sense" night time window opening ends up increasing how long you need to run the air conditioner during the day.


2. An air conditioner compressor is more efficient removing heat when it is cooler outdoors. Running the air conditioner while the outdoor air is cool, allows the heat pump to be more efficient because it does not have to work against as large a temperature gradient as during the hot mid-day. Shifting air conditioning to the night also moves that load off-peak. In some localities, off peak electricity is also cheaper.


3. Having a cooler house at night makes sleeping a lot easier. Even with windows open, we could never get truly comfortable. Outdoor air was cooler, but there was always a mugginess and things never got cool enough until past midnight.

Air conditioning at night instead of during the day is worth trying. Your home must well insulated and with shades closed. Our experience has been that cooling the house to about 67 degrees at 10 PM, means we can very comfortably coast through a 90 degree day without hearing the air conditioner come on until 7 PM. It was truly amazing the first week we tried the strategy. A programmable thermostat makes it easy to dive the temp at night and let things coast during the day.

The first day on this regimen will require a longer air conditioner run to initially remove water vapor, but thereafter the house will remain a low humidity, cool refuge all 24 hours. Our home remains very comfortable throughout 24 hours and we only use about 1.5 hours of compressor time total. As a result of adopting night time air conditioning, our electric bill for summer vs winter is at most 5% higher and I am always comfortable.