Turning Chilly

Well, it was bound to happen eventually, but we had to turn on the furnace this past week. Daytime highs have only been in the 40s and 50s for a while now, with nighttime lows cold enough to generate a little frost on the ground a few times. I hope that we can hold out a little longer before a hard frost, so that we can squeeze out the last few peppers from the garden. Anyway, the house has definitely been cooling off. So, we've had to fire the furnace up here and there. Like many in the upper Midwest, we have forced-air heating run off natural gas. We get some supplemental heat from the glassed in, south-facing front porch. The house is, thankfully, quite tight for one of its age; it was something we specifically looked for when we were buying. Still, we've undoubtedly got a long heating season ahead of us.

Because of my interest in energy and energy efficiency, I have kept track of our natural gas usage month-by-month by pulling a few key numbers from our utility bills. Dividing the monthly energy usage being billed by the number of days in the billing cycle yields an average daily usage. Below is the house's average daily usage for the last 3+ years.

Yup, those big cyclical spikes are the winter heating season, which ramps up during the fall and extends well into spring. The low-level baseline is what we use for our hot water heater, which has little fluctuation.

At the height of winter, when the daytime highs can regularly fail to reach double digits, or even above zero, the house is using around 4 therms of natural gas per day. If a kilowatt-hour is a weird unit of measure for electricity, at least one can trace it back to its fundamentals. A therm is just a made-up unit, which one needs to search around to find an explanation for. A therm is 100,000 BTUs of energy, which is roughly equivalent to the energy content in 100 cubic feet of natural gas. I say roughly because the actual composition of natural gas can vary from day to day and month to month. Sometimes it has more ethane, sometimes more propane, sometimes more butane, each of which has a different energy content. Also, being a gas, it will expand and contract with temperature and atmospheric pressure, so the cubic foot of gas delivered to your house will likely be different than the standard cubic foot of gas they bill by. So, the gas company will multiply the amount of natural gas they have metered to you by the therm factor for that month to determine the actual energy they have delivered to you. Confused yet? Well, at least my gas utility actually calls out these calculations in the bill, even if the definition of the terms isn't given.

Of course, once they figure out the number of therms to charge you for, they then need to figure out the amount to bill you. The cost of natural gas, like all energy commodities, has fluctuated a lot in the last couple of years, trending upward. The market price will also vary a lot by season, since there's a whole lot more demand for it in the winter (just extrapolate the graph of my house to a few tens of millions of households and you'll get the idea). During the winter months last year I paid about $1/therm. This year, projections are more like $1.60/therm or more. I count myself lucky - I hear that the cost of fuel oil, quite popular in the northeast where our parents live, will roughly double over last year's price, and has roughly quadrupled over the last decade.

If ever one wanted to make a case for investing in energy conservation, home heating would be a pretty good place to concentrate. Compared to the cost of heating a home this winter, insulation is cheap. A programmable thermostat, even the highest-quality and more feature-filled model, could pay for itself in a cold month. Heck, even replacement windows start to look affordable.