Heating Degree Days NOT useful for sizing

[buffalobillpatrick]

Heating Degree Days are NOT useful for sizing a heating system at Design Temperature??? (which is how it's done)

I took the total January HDD for my location, say it was 930 / 31 days = 30*
60* indoor - 30* = 30*

I can't size a heating system for a Design Temp. of 30* I must use 0*


HDD are useful for estimating about how much heat might be needed for a typical January.

Weather Data Depot: free downloads of heating & cooling degree days

"
What is a degree day?

A degree day is a measure of relative heating and cooling energy required by buildings. It's calculated as the difference between the average daily temperature and the balance point temperature (60 degrees). When the average daily temperature is above the balance point, the result is cooling degree days; when below, the result is heating degree days.

Example 1: Average daily temperature = 80. Balance point = 60. Cooling degree days = 20 CDD. (80-60=20)

Example 2: Average daily temperature = 45. Balance point = 60. Heating degree days = 15 HDD. (60-45=15)

Example 3: Average daily temperature = 60. Balance point = 60. No degree days.

You may ask, "Why not use average temperature instead of degree days?" The problem with average temperature is that highs and lows cancel each other out. A warm day (80 average temp) combined with a cold day (40 average temp) average 60. So do two mild days of 59 and 61. But in the first case there are 20 CDD and 20 HDD while in the second there is 1 CDD and 1 HDD. Using degree days, you can see that the relative amount of energy required for the first set of days is much greater than for the second set of days. But if all you looked at was the average temperature, you would conclude that both sets of days were about the same."