2 of 8
Reset Ratio
Operation of a hot water heating system can generally be improved by modulating the
supply water temperature as the outdoor temperature changes. Using this approach, the
heat lost from the building is matched by the heat input to the building. The PC702-1 Boiler
Reset Control utilizes a reset ratio to set the relationship between outdoor temperature
and supply water temperature. The reset ratio determines the amount the supply water
temperature is raised for every 1
°
drop in outdoor air temperature, and it is determined
from the starting point and the system design conditions. In order for the control to
automatically determine the reset ratio, a starting point and design conditions must be
established. These two points are set by the following 4 adjustments:
• Boiler starting temperature
• Outdoor starting temperature
• Boiler design supply water temperature
• Outdoor Design Temperature
See
Settings - Step Four
for a complete description of each setting.
Reset Ratio Starting Point
The first point used to establish the reset ratio calculation is the starting point. It is a combination of an adjustable boiler starting water
temperature setting and an adjustable outdoor starting temperature setting.
Design Conditions
The second point to establish the reset ratio calculation is the design conditions. This point represents the required water temperature
during the coldest day of the year.
Warm Weather Shut Down (WWSD)
When the outdoor air temperature is warmer than the Warm Weather Shut Down setting, no additional heat is required in the building;
therefore, the heating system can be shut down. This setting has no effect on the reset ratio calculation.
BOILER OPERATION
The supply water temperature from the boiler(s) can be controlled by cycling the boiler(s) on and off. Modulation of the operating
temperature in hot water heating systems not only provides more comfort but also offers significant energy savings. The cooler the
boiler(s) runs, the more efficient it is due to less heat losses up the flue and reduced boiler jacket losses.
Differential
An on / off boiler must be operated with a differential in order to prevent
short cycling. When the supply water temperature drops below the
bottom rail of the differential, the boiler is turned on. The boiler is then
kept on until the supply water temperature rises above the top rail of
the differential. If the differential is too wide, there can be large supply
water temperature swings; however, if the differential is too narrow, the
boiler short cycles and operates inefficiently. This control can auto-
matically calculate the boiler differential in order to achieve an appro-
priate balance between temperature swings and boiler efficiency. This
also permits the control to adapt to changing loads and conditions.
Minimum Boiler Supply
Most boilers require a minimum supply water temperature in order to prevent corrosion
from flue gas condensation. The control should therefore only modulate the boiler supply
water temperature down to the boiler manufacturer’s minimum recommended operating
temperature. Some boilers are designed to condense and should be operated at low
water temperatures as much as possible for maximum efficiency.
Domestic Hot Water (DHW) Boiler Supply
Most indirect DHW tanks require a relatively high supply water temperature in order to
produce adequate amounts of DHW. The control should therefore override the reset ratio
when there is a demand for DHW and raise the system supply temperature to a
temperature that is suitable for DHW generation.
Sequence of Operation
POWERING UP THE CONTROL
After the PC702-1 is powered up, all the LCD segments are turned on for 2 seconds, followed by a software version number. The control
then displays the outdoor temperature.
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
90
(32)
Reset
Ratio
Starting
Point
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Design
Conditions
Supply W
ater
T
emperature
Time
Differential = 10
°
F (5
°
C)
165
°
F(74
°
C)
B
o
ile
r
o
ff
B
o
ile
r
o
n
155
°
F (68
°
C)
160
°
F (71
°
C)
B
o
ile
r
o
ff
B
o
ile
r
o
n
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
90
(32)
WWSD
Temperature
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Reset
Ratio
Minimum
