Posted by: paul | January 22, 2009

Heating Degree Days & Normalized Therms

This is a follow-up to a previous post, Measuring the Efficiency of a House, in which I discuss measuring the energy consumption of your house per heating degree day.

The website EnergyLens has some great articles on heating degree days and some issues with using heating degree days.  Very good stuff.  I can vouch for the truth in the concerns they raise.  I have noticed that my own house does not consume a consistent amount of energy per heating degree day.  I believe that the reason is that we get decent passive solar gain, and we also have solar thermal to heat our hot water.   What I’ve noticed is that we consume more energy per heating degree day in the month before and after the winter solstice than the other heating months.  This makes sense – because in mid-winter, oil has to provide a larger portion of our heating load.  In the other months, we get more energy from the sun that isn’t accounted for when I calculate the Heating Degree Days per Therm.  The article makes some recommendations for using heating degree days to avoid some of the issues – I agree with them, particularly using yearly data and comparing proportional, rather than absolute, differences.

I proposed the term “Heating Degree Days per Therm.”  EnergyLens uses the term “Weather Normalized Energy Consumption” for the general concept, since the idea is indeed a normalization process.  They refer to Normalized Kilowatt-Hours for electric consumption, so instead of Heating Degree Days per Therm, we should probably use Normalized Therms instead.  Much better than my goofy acronym, HDDpT, aka “headpit.” A google search for “normalized therm” shows this is a standard term.

Notice too that Normalized Therms are the inverse of Heating Degree Days Per Therm.  This seems like it shouldn’t matter (and it shouldn’t if you are comparing things by ratio) but it does matter when comparing absolutes.  In June 2008, two professors at Duke demonstrated that we should really measure vehicle efficiency in Gallons per Mile, not Miles Per Gallon.  It is a really interesting read. The same concept applies to home efficiency.  Normalized Therms (Therms per Heating Degree Day) are the corrolary to Gallons per Mile, so lets go with it.

So start calculating the Normalized Therms for your house, make some changes, and see if they improve.  I’ll post some numbers for our house at the end of this heating season.

And check out Energy Smackdown, a competition to reduce energy consumption.  Cool idea!

The 1:5:10 EcoTip blog posted similar thoughts last month. (The blog’s name is because each tip is one thing to do that takes 5-10 minutes.)


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