Introduction; Weather Data; Degree Days - Honeywell AUTOMATIC CONTROL Engineering Manual

GENERAL ENGINEERING DATA

INTRODUCTION

This section provides engineering data of a general nature. It
is reference information applicable to any or all other sections
of the Engineering Manual of Automatic Control.

WEATHER DATA

DEGREE DAYS

Degree days are used for estimating heating and cooling costs.
The number of degree days per day is the difference between
65F and the daily average (mean) temperature. These are heating
degree days when the daily mean temperature is less than 65F
and cooling degree days when the daily mean temperature is
above 65F.
The daily mean temperature is the average of the maximum
and minimum temperatures recorded during a day. It is not the
average of all hourly readings taken during the day.
EXAMPLE:
Assume tha t for a g iven loca tion and da y the highest
recorded temper atur e is 74F and the lo west r ecorded
temper atur e is 46F. Then the dail y mean temper atur e is:
(74 + 46)F
2
Ther efore, the n umber of hea ting de gree days for the g iven
day is:
65 – 60 = 5 heating de gree days
ENGINEERING MANUAL OF AUTOMATIC CONTROL
= 60F
Table 1 gives heating degree days and Table 2 gives cooling
degree days in a normal year based on the years 1961 through
1990 for some selected cities. Table 1 includes the annual mean
temperatures for the same 30-year period and the heating design
temperatures for the selected cities. The heating design
temperatures are the lowest temperatures reached during 97.5
percent of the hours in the winter months of December, January,
and February (2160 hours). That is, temperature could be the
design temperature or less 54 hours during the winter months.
Table 2 includes the cooling design dry-bulb temperatures,
which are exceeded 5 percent of the hours in the months of
June through September (2928 hours), and annual average wet-
bulb temperatures.
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