cooling season days and degree-days in my area

Chuckles_2

I did the following tally of number of days in the cooling season in a given temperature range, and the degree-days in that range. It may be of some interest in thinking about ho much benefit one gets from a condensing boiler, although the actual data are only for my area.

The data are of normal (30-year average) temperatures at Midway airport, Chicago, IL. They are not actual temperatures (there is a subtle problem with the use of normals, discussed later).

Degree-day range, # of days, total degree-days in this range:

41-45, 29, 1200

36-40, 34, 1319

31-35, 28, 927

26-30, 26, 724

21-25, 26, 594

16-20, 25, 448

11-15, 28, 362

5-10, 32, 265

1-5, 66, 155

TOTAL FOR SEASON, 284, 5994

This table shows, for example, that although days with 41 or more degree days occur for less than a month (29 days), they account for 20% of the total degree-days in the season (1200 out of 6000).

If my reset curve is such that my boiler condenses only at temperatures above 30 (i.e. 35 degree-days or less), then it will NOT condense for 63 days, but this accounts for about 42% of the heating season in terms of degree-days.

Note thet the use of normals (averages) makes the situation look better than it actually is. For example, suppose my boiler condenses fine on a 40F day (25 ddays). But a normal 40F day may be 60F one year, 20F the next year. But there are a lot of dddays in the 20F day, and so a lot of fuel is consumed, but it will not condense that day! It will condense on the 60F day, but who cares? Not much fuel is being consumed that day.

Another way to mke this point is to note that the average temperature never goes below 20F (45 ddays), as you see in the data above, but anyone who thinks it doesn't get any colder than this in Chicago is in for a rude surprise!

So, if I had actual temperature data for each of the last 30 years or so, I could have done a better calculation.