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Rev 2.0
63-7048 1998 Honeywell Inc.
Page 8 of 22
H1008 Automatic Humidity Control with HumidiCalc+
- The Technical
Approach
We found that there is more than a casual relationship between furnace run time and
outdoor temperature. In fact, in a single stage heating system, in steady state, the
percentage of burner on-time (% on) is linearly related to the difference between the
balance point (in this context, the temperature at which it is warm enough that the furnace
never runs) and the outdoor temperature. To carry this one step further, an equation can
be derived which expresses % on as a function of four variables:
1.
Outdoor temperature
2.
Balance point
3.
ASHRAE heating design temperature
4.
Oversizing of furnace, relative to ASHRAE design temperature
Figure 5 shows two examples of this relationship.
Figure 5 – Relationship Between Outdoor Temperature and Percent On
The upper line shows an application with a balance point of 55
F, where the furnace is
exactly matched to the heat loss (0% furnace oversize) at the ASHRAE 99.6% design
temperature for Minneapolis (-16
F). In this case the curve is a straight line connecting
the balance point at 0 % on, with the design temperature at 100 % on. The lower line
shows the same application with a furnace with twice the btu output (100% furnace
-100.0
-80.0
-60.0
-40.0
-20.0
0.0
20.0
40.0
60.0
80.0
0 10203040506070809010
0
Percent On
Outdoor Temperature - deg F
0% Furnace Oversize
100% Furnace Oversize