The Science of Sustainability

Chu, Two and Btu

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Bad for the Lab, Good for the Country

Staff at Building Solutions, a home performance
company, install PV on a roof in Oakland. Next year, the renewable
and energy efficiency business will be even better.
Credit: Kate Kenke
Dr. Steven Chu, Noble-prize-winning physicist, and director of Lawrence Berkeley National Laboratory, was named as President-elect Barack Obama’s nominee for Secretary of Energy. Home Energy is a nonprofit magazine, but our offices are at Lawrence Berkeley Lab and the magazine was founded by Alan Meier, a lab scientist. People around here are saddened by the loss of Dr. Chu as director of the lab, but extremely excited about his nomination as Secretary of Energy. Dr. Chu believes in science and the important place of technology in helping us meet our energy goals and fight global warming—think cellulosic bio-fuels, nanotechnology, and yet undreamed of solutions to the present energy and environmental crisis.

Weatherization Works!

Word in energy efficiency circles is that the funding for Department of Energy (DOE’s) Weatherization Program will increase several-fold with President Obama’s proposed economic stimulus package. The Weatherization Program is managed state by state from money provided by DOE, and the funds pay to retrofit the homes of low-income families. Homes become healthier to live in, more energy efficient, and more comfortable for the occupants. For every one dollar the Weatherization Program spends, almost two dollars in energy savings results. Hundreds of thousands of homes have been retrofit so far, leaving about 99.5% of existing homes. Talk about green jobs potential! Many nonprofit and for profit organizations do weatherization work, and, basically, you retrofit the home of a low-income family the same way you retrofit a mansion. Lots more skilled people will be needed to do the work, and the jobs will provide a good income, benefits, and the possibility of future advancement. Community colleges, unions, professional training organizations, online trainers, and other players are gearing up to train the new green workforce.

How Many Btu Do You Do?

I promised in my last blog entry to explain the concept of heating-degree day and cooling-degree day. Sometimes you will hear that a home uses so many Btu or kWh per heating- or cooling-degree day, per square foot, per year. The degree days indicate the heating or cooling load on a building’s HVAC systems. A degree day is the rise or fall of one degree Fahrenheit for 24 hours. The rise or fall in temperature is measured from a baseline of 65F°. For example, if the average temperature tomorrow is 45F°, than the heating load on your heating system is 20 heating-degree days. If on a hot summer day the average temperature over a 24-hour period is 85F°, than the load on your air conditioner is 20 cooling-degree days. The number of heating-degree days for a winter in New York is around 5,000. Barrow, Alaska has about 20,000.

You can figure out how much energy you use to heat or cool your home by subtracting the baseline energy use. During a month when you are using neither your air conditioner or heater, such as in October or March (called the “shoulder” months), your gas and electric use represent your baseline. The baseline covers energy for lighting, appliances, hot water, and plug loads. Subtract out the baseline from your winter or summer energy use and you have the amount of energy to heat or cool your house. If you know the square footage of your home, and you have weather data for your area (go to www.degreeday.net to find out heating-degree days and cooling-degree days for your area), you are in a position to brag to your neighbors (or not) about your energy use.

At our house we used about 90 therms of natural gas from September 7 through December 7, 2008. There were about 480 heating-degree days (HDD) in our area during that time. Our baseline use of natural gas is about 10 therms per month, for heating water and cooking, leaving 60 therms for heating over the three-month period. Our house is about 1,200 square feet (ft2). Therefore, we used 60 therms/(480 HDD x 1,200 ft2), or about 0.0001 therms/HDD·ft2. Since one therm of natural gas contains about 100,000 Btu of energy, that equals about 10 Btu/HDD·ft2. That’s not bad, but not great either. How about you?

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Jim Gunshinan

About the Author ()

Jim Gunshinan is the editor of Home Energy, the magazine of sustainable home building and renovation.
  • Scott M. Kruse

    We measure energy in Joules since the Omnibus Trade and Competitiveness Act of 1988 (Bush). The preferred standard of measurement is SI. Increasingly HERS, Home Energy Rating System (http://www.energy.ca.gov/HERS/index.html) is being used for energy audits. A HERS of 100 is ideal, while a low number indicates you are wasting kilowatts rather than conserving negawatts. Make use of James Dulley's Energy Efficiency Index (EEI) and your monthly energy bill(s) that integrates heating- and cooling-degree days with your site, square meters (square feet) and provides a comparable index of energy use. An EEI less than 10 is excellent. An index of 10 to 20 indicates many things you can do to be more efficient. Greater than 20 indicates a genuine waste of energy. Harley (2008) 'Cut Your Energy Bills Now' Taunton Press, $10.36 from Amazon, provides a wealth of photographs and 150 smart ways to become more efficient.

  • Nancy Law

    My house is about 1200 sqare feet. My furnace is 70,000 Btu's, The temperature outside is 15 F. My house is extremely hard to heat. Is this is right size furnace for my house?

    Nancy Law

  • Dana

    You may be underestimating the amount of fuel used for hot water heating if your baseline 10th/mo was determined during the summer or non-heating season. According to this fairly comprehensive study of hot water use & heating done in California, the average household will use more than TWICE the amount of fuel for hot water heating in December as oppose to September:

    http://www.energystarpartners.net/ia/Water_heaters/Documents/WHPAGette_Final.pdf

    I refer you to the bar graph on page 16 (p.22 of the .pdf.)

    This makes sense- the reservoirs are at their warmest at the end of the summer, long after the snowmelt. The ground at distribution piping-depths is similarly at it's warmest at the end of summer. Then by the end of December the ground is cooler, and the reservoirs have radiated an enormous amount of heat into the sky.

    So, to figure the actual HEATING component of your fuel bills you'll have to insert monthly or seasonal fudge-factors to get it right. In 8000+ HDD climates the hot water factor disappears into the noise, but in coastal northern CA's ~3000 HDD climate hot water heating represents a much larger fraction of the total.

    I live in a 3 person household in central MA in a ~2400' house. The combined heat/HW/cooking runs ~8.5BTU/HDD/ft2 after tightening up & insulating our 85 year old wood-framed structure a bit. I know of several houses (both younger & older) that use twice that. There's plenty left to do- shouldn't be too difficult to get it under 5BTU/HDD/ft2, if I look at the necessary upgrades as 10 or 15 year investments in both creature-comfort and fuel cost savings. YMMV

    But is BTU/HDD/ft2 really the right metric? A 3 person household in a fairly tight 5000' house burning 5BTU/HDD/ft2 is already burning more than we are.

    Nancy: Do a Manual-J type of heat load estimate to figure out the "right" size of the heating plant. There's a pretty-good freebie software package downloadable here:

    http://slantfin.com/heat-loss-software.html

    If 15F is normally one of the coldest temps you see in a typical heating system, and your furnace is running more than 50% of the time, it's probably in the right ball-park. If it's running 100% of the time and continuing to cool off inside, it's definitely undersized. But that would be rare- Manual-J calculations tend to overestimate by ~25%, and heating contractors tend to then add some margin, then round up to the next larger size, so 100% oversizing (or more) is common. The net result of oversizing is higher fuel bills and rapid cycling (you first feel too cold, then it blasts you to a sweat, then an hour later it's feeling chilly again, repeat.) It's far better to do the heat load calculation and UNDERsize it by 15% or so- you'll be more comfortable through most of the season and pay less. If it doesn't quite keep up during an extended cold snap, simply leaving the lights on or intermittently using space heaters in the most-used rooms is usually cost effective. (Most of the time the "design day" coldest temperatures occur at night, when you're asleep under the covers anyway.)

  • http://www.homeenergy.org Jim Gunshinan

    Wow, I am catching up and seeing a lot of great responses to my blog. Great!

    Thanks for the information about alternative ways to look at my energy use and ways to reduce it. A colleague suggests we find a way to convert heating energy, cooling energy, and other home energy loads into gallons of gasoline! Then we could say "My house gets 1,500 gallons per year. What about yours?" It would be a way to mark your energy efficiency that everyone could understand.

    Nancy, the first thing I thought of when I read your post was "that house must be a sieve." I'm not basing that on anything as sophisticated as a Manual J analysis, but on a feeling that, in most cases, the furnace is plenty big. There are people living in houses in Northern Germany (called Passive Houses) that are so well insulated that they only use a furnace or water heater to heat their home on the very coldest of days, if they even have a furnace. Body heat and the heat from lights and appliances is enough most of the time. Of course, if you have your home air-sealed and well insulated you have to make sure that for health and other reasons you are getting enough fresh air and that you can still move moisture out. Some kind of controlled ventilation strategy is a must.

    So I'd suggest, Nancy, that you have a home performance contractor come out and do an energy audit of your home. You can go to the Home Performance with Energy Star Web site and find a local, certified contractor to do the audit. Best of luck!