AERCO BMS II BOILER - Manual (Page 73)

APPENDIX C
C-1
APPENDIX C
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METHODS FOR DETERMINING RESET SCHEDULE AND 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°F/105°F = 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).