Results from Measuring Household Temperatures for Most of the Summer

The temperature measurement system I installed in late June has been running for several months now with only minor problems and with the highs now in the 80s, two of the experiments I've been running over the summer have come to a close and I'd like to share the results.

Let's do the easy one first: attic temperature.  A month or so after I got the temperature system going we could see that our attic was getting quite warm on those hot days, often above 130'F.  I talked with my wife about it, we did some reading, and decided that adding a bit more ventilation would be good idea.  The cost would be minimal and we hoped it would lower the difference between the outside air and the attic.  Thankfully it did; the results are below.

The difference between the old and new ventilation is pretty clear: adding the ventilation did lower the temperature difference between the outside and attic air. The statistics show a nine degree cooling in the attic by adding the extra ventilation.  (For you statistics nerds, the sample size is ~25 for both conditions, and standard deviation is ~5 degrees.)

The other experiment I ran was based on a conversation I had at a cook-out over the summer.  One of the gals there said her father was a HVAC guy and that he recommended keeping a constant set-point during the summer; that is, don't turn the AC off when the house is unoccupied.  It was hard for me to believe that this would use the same amount of electricity as a more "conservative" approach of turning it off when gone but I realized I didn't actually have anything more than opinion to back up my assertion.  Time to do some science.

I decided to test this theory and added a sensor that would show me when the central fan in our house was running.  The fan only kicks on when the air conditioning (or furnace) are on and so this allowed me to measure how long the air-conditioner ran during a given 24-hour period.  I semi-randomly changed the programming on our thermostat to either hold a constant temperature all day or to turn the AC back shortly before we got home from work and school.  Here's an example of each:

The yellow line at the bottom is the state of the fan: 55'F is on, 15'F is off.  For a day when the AC was on a schedule, it turned off around 7am and would come back on around 2:30pm in an attempt to get the house down the temperature by 5pm when we came got home.  You can see the purple (kitchen) and cyan (hallway) lines rise throughout the day and then when the AC comes on in the afternoon, begin descending.

When we kept the thermostat constant all day, the AC cycles to keep the temperature in the house at the thermostat set point. The purple and cyan lines stay at an even value throughout the day.  (You'll notice the basement sensor is relatively unaffected by the AC.  This is why everybody should have a basement if they live in a place that gets hot.  Basements are the best.)

After a summer of running both cases, here are the results.

The results are much more mixed than I would expect.  I think to make any good conclusions a statistical linear regression would need to be done; I haven't done that yet and probably won't ever get around to it.  Its clear that the run-time of the AC is strongly related to the peak temperature for the day.  This should be no surprise to anybody.  It is less clear which thermostat schedule uses less energy.  For the very hot days (> ~105'F) you could make a pretty good case that turning the AC off when you're gone at work will save some energy.  For days when the highs are less than 100'F, though, it seems there is very little difference between the two cases.

These results are surprising to me.  I would have expected that keeping the AC off for seven or eight hours a day would cause it to run less in the grand scheme of things.  It may but the difference isn't huge. I may try repeating this experiment next summer, just to see how it turns out.  I guess the good news is that if you're home all day with kids or work you don't have to feel too guilty about having the air-conditioner running the whole time; its not killing your bill much worse than the rest of us.