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Cold weather’s impact on your home and your bill   

Tags: Avista Utilities, Energy Efficiency, Weather

Snowy tree branch
Has this winter been colder than past winters? Sort of, but even little changes really add up
Post byDan Kolbet
Twitter: @Dan_at_Avista

Click to enlarge
The average high and low winter temperatures
in Spokane for the past three winters. Click to enlarge.
I’ve heard online and from friends and co-workers over the last several weeks that this winter (2010-2011) seems particularly cold. Thinking back on it, I recall the previous winter didn’t seem too cold at all. Yet that was in comparison to my memory of the 2008-2009 winter. So I guess it’s all relative.

This week I wanted to find out if the weather was really colder and if it was impacting customer bills, so I set off on a quest for information.

I decided to look up average temperatures with the National Weather Service and see if this winter was colder than the previous one. Then I created the chart on the left (click it for larger detail). I recorded the average high and low temperature in Spokane in September, October, November, December and January over the past three winters.

If you really love charts, then by all means, do your own analysis of the data, but for you normal people – here’s what I found: It depends.

For example, so far in January 2011 we’re averaging a low temperature of 14.6 degrees, compared to last year of 40 degrees - a pretty big difference. Of course the month isn’t even half over, so this is going to change. But December 2010’s average low was warmer than 2009 (24.5 to 19.1), but November was colder (27.0 to 30.4).

Since my fancy chart isn’t telling me much, I contacted Avista’s energy efficiency expert Tom Lienhard to find out what sort of impact temperature changes really have on a home. Turns out, it’s quite a bit.

There are really three ways you can lose heat in your home, but the biggest one is through conductive heat loss which happens through the walls, floors, ceiling, doors and windows of your home. The lower the R-value of each fenestration (wall, window or door), the more heat that leaves the home.

The colder the temperature outside, the harder your home works to maintain the desired temperature inside. Depending on the weather, a home could use 300% more energy to accomplish that consistent temperature inside.

Here’s an example of how it works:

Let’s say we have a building that has 1,600 square feet of floor space, but it has 4,480 square feet of surface area to lose heat (40x40 foot building with 8 foot high walls and flat floor and ceiling.)  The R-value all over this example home is R-20. We’re measuring heat loss in BTUs.

If the outside temperature is 7 degrees, the loss in BTUs would be: 15,372 per hour or 368,928 for a day. That’s 4.6 therms a day from an 80% efficient furnace.

On a warmer day, say 45 degrees, the loss in BTUs would be just 5,600 BTUs per hour or 134,400 BTUs for a day. That’s 1.68 therms a day from an 80% efficient furnace.

So in our little test home, a 7 degree day uses almost 3 times or 300% of the energy of a 45 degree day to stay warm. If you don’t have much insulation in your home and your average R-value is closer to 10, then you would use almost twice as much energy as listed above. 

So, in conclusion, Tom will officially do my math from now on. And when the temperatures dip down low, your home is probably working overtime to keep you toasty.

If you’re interested to getting scientific at your house too, sign up for a Home Energy Audit in Spokane County today and learn where your energy is going. Any Avista customer can also use the Avista Online Home Energy Analyzer for free.
Posted by  System Account  on  1/12/2011
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