So it looks like we’re going to take the plunge, and get a new boiler put in. Several things are driving this. For one, a detailed heat loss calculation has revealed that our current boiler, while reasonably efficient, is about twice the size it needs to be to heat the house. It has always short-cycled, which has a bunch of undesired effects.. uneven heat, loss of efficiency, bad for the boiler, etc. A properly sized boiler will save money and keep us more comfortable — an unbeatable combo.

The real, black-and-white measure of efficiency for an oil heat system, is the number of gallons of oil it consumes per heating degree day. This isn’t too difficult to measure empirically, using data from the oil company and the National Weather Service. Read your latest receipt from the oil company to see how many gallons were delivered at the last fill-up. Count how many days passed since the tank was last full, and divide by the number of gallons delivered to get the number of gallons burned per day. Then, go to the National Weather Service climate page, click on your state, and download the “Preliminary Climatology Data” for the time period in question. It’s published monthly. Add up the total number of heating degree days for the period, and divide by the number of days in the period, to get the number of heating degree days per day. Then, divide gallons per day by heating degree days per day, to get gallons per heating degree day. In theory, the lower this number, the better. I’ve been keeping track of our gallons per degree day usage with the current boiler, and it’ll be really interesting to see what effect a new boiler has.

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In an ideal test environment, gallons per degree day will give a perfectly accurate picture of a heating system’s efficiency for a given house. Unfortunately, there are a lot of factors that skew the measure, which make it hard to compare numbers from season to season. Among these are

• Varying thermostat settings;
• Other oil-fired appliances (in our case, a water heater);
• Solar gain or lack thereof (not accounted for in the NWS data).

So it’s impossible to get a really precise measure, but this is about the best approximation we have. In our case, in just over 4 years (including 5 winters), we averaged 0.2 gallons per degree day. Our usage has trended down over the longer term, as we’ve added insulation and tightened the house up with new windows etc. However, this past winter our usage was up somewhat (roughly 0.03 gal/hdd difference, which doesn’t seem like much, but translates to 150 extra gallons over a 5000 hdd season). I’m not sure why this is — we even had our boiler downfired (from 1.25 to 1.10gph), which I would think would reduce our usage.

Our attempts to improve the house’s efficiency have had less of an effect than I had hoped, netting us maybe a .03-.05 gal/hdd difference or so per winter. I think (hope) a new boiler will have a more pronounced effect. We’ll see I guess. In any event, it’ll hopefully get rid of the uneven-heat problems which have plagued us all along with the current boiler.

Followup 7/25… Details on our new system.