Monday, September 19, 2011

Open Window "Policy"

The record-breaking summer of 2011 (hottest average in the nation) is seemingly over for DFW. Everyone knows this year holds second place to 1980 for the longest continuous streak of 100° days. But it now holds record for the greatest number of 100° days (70 days as of September 13). And, just as important for electric load, 2011 holds title to the most days with daily lows above 80° (40 days as of August 17).

Free Cooling
But now with the lows falling into the 60's, it's time to take advantage of some cool natural ventilation. Until this year, I've generally kept my office closed up tight and depended solely on the central a/c to do it's job. But this spring I thought I'd try taking advantage of some of the cooler days and nights in order to run the a/c less. Each morning I would throw open my office window and open the sliding door in back and let the cool breeze blow through. No fan needed, and the a/c could wait till later in the season--or the day.

The result? A clear savings in electric consumption as compared with the past two years. The graph below shows a 36% reduction in electric usage for March and April. The brown line is 2009, the light blue is 2010, and the dark green is 2011.

Opening the windows was the only major difference between these years, so this simple zero-cost action accounts for the total reduction in electric usage shown. The daily average fell from 33 kWh per day to 21 kWh. That's a monthly savings of about 360 kWh (= 12 kWh/day x 30 days), or a savings of roughly $36 a month (based on 10¢ per kWh). That's $1.20 a day I'm being paid just to open the windows(!) Or $144 per year if the savings can be replicated for two months this fall. We'll have to wait and see--it's time to start saving.

By the Numbers
Though our electronic gadgets cost plenty of electricity, we can also put them to good use to save energy. I never trust myself to judge the temperature outside accurately--I always use a thermometer. I keep a wireless thermometer in a shady place out back with the inside read-out in a convenient place near my thermostat. It makes for an easy and accurate way to determine when to open the windows and when to close them.

The weather forecast is also useful. If the projected high is in the low 80's (depending on your personal comfort zone), you can plan to leave the house open all day. Remember that fans can make the afternoon comfortable as needed.

My own rules of thumb:

Forecast High Action Plan
above 83° 5° difference needed to open windows
close at 75° to 77° *
83° or below open all day
* Based on a/c thermostat set at 82°


In a home setting, of course, the windows can open up all night, giving an even better temperature differential and additional time to cool the house down.

The Downside
Humidity. To maximize comfort and minimize energy consumption, indoor humidity levels need to be 30% to 50%. The a/c dehumidifies the air when it first comes on, but it has to work harder to do so. This is why I've built a 5° minimum temperature difference into the rules above. On any given morning the outside humidity may be well above 50%. One could pay close attention to this detail and refine the rules further--but then they would cease to be rules of thumb! The 5° differential has served me well.

The Tremendous Added Upside
There's a joy in the journey, and a lot of enjoyment to be had from God's creation! The call of birds, the rustling of leaves, the refreshment of a gentle breeze. I remember back to college when I lived in unairconditioned dorms built back in the 30's (1930's that is, and I lived there in the 80's). Some of the best times I've had with the Lord were spent reading God's Word at my desk before an open window, with a cool breeze on my face and the dawn gently breaking outside. It was in those quiet times during my first semester at UT that I truly felt the Lord calling me to missions in the future.

Why have I kept these gifts securely locked outside my windows all these umpteen past years since? Convenience. Just a minute or so of saved time. Ugh! I'm glad for the change back to a lost joy.

So change your minds and turn back, that your sins may be wiped out so that seasons of refreshing may come from the presence of the Lord. (Acts 3.19)

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