Estimating annual heating energy use

I'm just learning about this myself, so I am not an authority. But I did find this paper from 1985 that discusses Cd and other related correction factors, and explains the basis for them. So apparently this is not a new concept. I have not yet found where ASHRAE got their data to construct that graph, but I'll keep looking.

Meanwhile the paragraph headed "The Modified Degree Day Method" at the top of page 2 does a good job of describing why the Cd factor is needed and what it accounts for.

I have no problem to solve now, but that's because I spent the last few years learning from the pros here and doing the necessary measurements and calculations. But during that learning process, I never came across the Cd factor and its use. It would have been helpful to know about.

There are actually two ways the Cd factor can be useful. One is if you know or can calculate you building's heat loss, but you don't know the annual energy use (maybe it's a new house you just moved into, or you're running numbers for a customer who hasn't kept track of their annual energy use).

The other way Cd can be used is to back-calculate the design heating load from known annual energy use. Say I know I use 1200 gallons of oil per year at 70% overall system efficiency. That's 116 million useful BTU's, from which I could have back-calculated my design heating load of 100,000 BTU/hr. That would have been useful to know as one data point. I did use other methods to calculate design heating load, just not this one.

So since I haven't seen Cd mentioned here before in the few years I've been learning and posting, I thought it might be helpful to mention it in case anyone else finds it useful.

Just to clarify what Cd is actually supposed to correct for, here's an excerpt from the NREL paper I posted above:

"The Cd factor is needed primarily because degree days are used to a 65 F base temperature, a value based on work done in the 1930's when the degree-day method was originally developed. For modern buildings, lower base temperatures should be used because of better insulation, higher internal gains, and lower thermostat settings. Because degree-day data were not available for lower base temperatures until recently, the Cd factor was introduced in the modified degree-day method. Cd is a multiplier, less than 1.0, which modifies HDD65 to a smaller number of degree days, thereby approximating the effect of a lower base temperature. Appropriate base temperatures are a function of building characteristics and use, and Equation 1 will be in error for buildings that do not match the Cd assumptions."

So, obviously, it's important to understand what factors Cd is attempting to correct for, and to understand your building envelope's characteristics and uses and where it may vary from "average" Cd. In our case of a conventional 1924 residential building in Boston, we are apparently "average" enough that a Cd based solely on the number of our local HDD65's predicts our annual energy use to within 10% or so, which is surprisingly good, I think. I doubt most heating pros would promise their clients better than 90% accuracy on their predictions for annual energy consumption on a new heating system install.

But yes, there will be cases like super-insulated passive houses where the use of Cd based on "average" building characteristics is not appropriate and will give incorrect results. The NREL paper above addresses that and introduces other correction factors to account for solar gain, etc, when necessary.

There is also the temperature "bin" method mentioned by @hot_rod and @DCContrarian above. I haven't needed to use it yet, but I understand why it would be useful for heat pumps which are highly dependent on outside air temps.