AERCO BMS 168 - Manual (Page 53)

B-1
Appendix B
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Methods for Determining Reset Schedule and Indoor/Outdoor Reset Ratio Charts
Using the Charts to Determine Reset
Schedule
Each table in this appendix provides
data for a specific building reference
temperature. On the vertical axis of
each table are degree day
temperatures. These are the average
lowest temperatures likely to be
encountered. The engineer of your
system should have this number for
your area. The reset ratio is shown
across the top. The data in the tables is
header temperature. To determine the
reset ratio for your installation, follow
these steps:
• On the vertical axis, find the degree
day for your area.
• Select the temperature that should be
maintained in the header to maintain
the building at the desired
temperature. The system engineer
should have this information.
• The proper reset ratio is the value
found above the two selected points.
For example, for a degree day of 15°F
and a header temperature of 125°F,
the reset ratio is 1.4.
Determining Reset Schedule By
Formula
There are two steps required to
determine reset schedule with this
method. The first is to determine the
reset ratio by dividing the range of
outside temperatures by the range of
header temperatures:
T
header
/T
outside
= Reset Ratio.
for example, If T
outside
varies from -10 to
+95°F (105° range), and
T
header
varies from 125 to 200°F (75°
range), then the reset ratio equals
75°/105° = 0.714.
Once the reset ratio is determined, this
number and the lowest or highest
header temperature and corresponding
lowest or highest air temperature, are
input to the following equation to yield
the building reference temperature:
T
header
-
RR (TR-TO)+TR, where:
RR is the reset ratio
TR is the building reference temperature
TO is the minimum outside temperature,
and
T
header
is the maximum heating system
Temperature desired at the minimum
outside air temperature.
So in this example:
T
header
= 200°F
200°F = 0.714 [TR-(-10°F)] +TR
200°F = 1.714TR + 7.14°F
Solving for TR:
TR = (200°F-7.14°F)/1.714
TR = 192.86°F/1.714
TR = 112.5°F
Therefore, use a reset ratio of 0.7
(closest to 0.714) and a building
reference temperature of 113 (closest to
112.5).