The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, "hmm.... that's funny...." Isaac Asimov

Monday, March 21, 2011

Heating Index: How does your house perform compared to other houses?

It’s becoming increasingly clear that the industrial world’s massive consumption of energy cannot continue its present course into the world of the future. Easily recoverable and therefore inexpensive petroleum and natural gas are fading from the scene, and efforts to keep these conventional fossil fuels flowing including hydrofracking, deep water oil wells, and exploiting tar sands all are showing problems. Coal remains cheap, but large areas subject to mountain top removal mining are paying an environmental cost. Next generation nuclear power is promising, but expensive to build. And since the scary event in Japan, nuclear power will doubtless meet with renewed resistance. Renewables such as photovoltaics, wind and ethanol are still so expensive they cannot compete without subsidies and currently contribute relatively trivial amounts of energy. There is one approach left and it’s probably the best from many standpoints. This is energy conservation and energy efficiency. It’s something everybody can do. We could get much better at it.

A good place to start is with one’s house. Most of us use a third or more of the energy we’re directly responsible for in running our houses. In the northern half of the U.S., about half of that is for heating.

How does your house rate in heating efficiency compared to the U.S. average, and to other houses in your neighborhood? Hiring someone to do a complete energy audit is the best way to get a precise accounting of your home’s energy use, but you can easily do an estimate that’s likely to be reasonably accurate.

First, you need to know how much fuel you use for space heating in a typical year. To keep it simple, we’ll limit this exercise to natural gas and heating oil, the primary fuels used to heat houses. Fuels use will vary somewhat from year to year but any recent year should be OK for a decent estimate. Tally up your fuel use for a year by looking at your fuel bills. Natural gas will likely be expressed in therms. Look at how many therms you use during the months of June, July, August, and September. These totals represent non-space-heating uses such as water heating, clothes drying, and cooking. Take the average for each month, multiply by 12, and subtract this total from your yearly total. What’s left is the amount of heat you used for space heating. If you heat with oil, you’ll be measuring gallons of fuel used. If the units are therms, multiply the therm total by 100,000; this will express this energy use in Btus. If the units are gallons, multiply by 135,000, which is the approximate number of Btus per gallon of heating oil.

Now, look on the map above and estimate how many heating degree days (HDD) (1) there are where you live. Here in central New Jersey, this is about 5000. Divide your space heating total by this amount and you’ll have a heating intensity value for your house expressed as Btus per HDD. The average value for the U.S. appears to be in the range of 10,000 to 15,000 Btu per HDD. How does your house stack up?

If your value is much less than this, you are one of the reasons that we don’t have a worse energy problem. Congratulations!

If it’s much more than this, it may be because your house is bigger than average, you keep it warmer than average, it’s less efficient at retaining heat, or your heater is less efficient than most. Perhaps all of these factors are involved. There are a number of steps you can take. Two of these steps are free. One is simply to close off some rooms. Are there unused upstairs rooms, for example? If you can shut off the flow of heat to them, you’ll save a lot of energy. The other free step is turning the thermostat down, and it is tremendously effective; a one degree F drop will reduce your space heating fuel use by up to 5%.(2) And, as a recent article shows, dressing warmly can make a cooler house quite tolerable.(3) A third step is nearly free – installing a programmable thermostat that turns down the heat when you are sleeping or not at home. This can save 15% or more of your space heating fuel.

Next in difficulty and expense but often quite cost-effective are a variety of steps including installing better insulation, eliminating air leaks, and getting a more efficient heater. Even if it’s big, a tight, well-insulated house with an efficient heater can use less energy than a small, inefficient house. The heating efficiency of a house per square foot of living space is a useful metric to use if you want to look more closely at your house’s heating efficiency. I’ll discuss this metric in more detail in a subsequent post.

(1) A heating degree day represents the difference between 65 F and the average outside temperature for that day. For example, if a day’s high is 50 F and its low is 30 F, the average temperature is 40 F and there are 65-40 = 25 heating degree days that day.




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