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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 PC705-2
Variable Speed Injection Mixing 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:
• Mixing starting temperature
• Outdoor starting temperature
• Mixing 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 mixing starting water
temperature setting and the 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 a boiler can be controlled by cycling
the boiler on and off. Modulation of the boiler’s operating temperature in
hot water heating systems not only provides more comfort but also offers
significant energy savings. The cooler the boiler 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 automatically calcu-
lates the boiler differential in order to achieve an appropriate 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 tempera-
ture. Some boilers are designed to condense and should be operated
at low water temperatures as much as possible for maximum efficiency.
MIXING OPERATION
The full range of water temperatures required through a heating season
can be provided with a standard (non-condensing) boiler by incorporating
a modulating mixing device into the system. Mixing valves or variable
speed injection pumps are commonly used to modulate both the system
supply water temperature and the boiler return water temperature. The
modulation of water temperatures improves comfort in the building and
also protects the boiler from cool return water.
Design
Conditions
Reset
Ratio
Starting
Point
Outdoor Temperature
Supply W
ater
T
emper
ature
90
(32)
70
(21)
50
(10)
30
(-1)
10
(-12)
-10
(-23)
°
F
(
°
C)
70
(21)
90
(32)
110
(43)
130
(54)
150
(65)
170
(77)
190
(88)
210
(99)
Supply W
ater
T
emper
ature
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 w
ater temper
ature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Boiler
Minimum
Boiler
Loop
System
Loop
Mixing
Device
