2
Heating Curve
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. A tekmar reset
control utilizes a heating curve to set the relationship between outdoor temperature and
supply water temperature. The heating curve 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. The heating curve is sometimes
called an outdoor reset ratio.
Heating Curve Starting Point
All heating curves begin at the heating curve starting point. This point is a combination
of an adjustable starting water temperature setting and the WWSD point.
Starting Temperature
When the outdoor temperature is cold
enough to start the heating system, the
control should provide a preset starting
supply water temperature. If the Starting
Temperature setting is adjusted, the heat-
ing curve is shifted vertically.
Warm Weather Shut Down (WWSD)
When the outdoor air temperature is
warmer than the Warm Weather Shut
Down setting, no additional heat is re-
quired in the building; therefore, the heat-
ing system can be shut down. When the
WWSD temperature is adjusted, the heat-
ing curve is shifted horizontally.
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.
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.
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.
Sequence of Operation
POWERING UP THE CONTROL
After the 253 is powered up, a software version number is displayed and is followed by a test
of all LCD segments. After 5 seconds the control displays the boiler supply water temperature.
HEAT DEMAND
The 253 requires a heat demand in order to control the boiler supply water temperature.
When terminal
T is connected to terminal C- (6 and 8), the control receives a heat demand.
The
H. Dem pointer flashes for up to 30 seconds and then turns on solid. The flashing
represents a built in delay for turning on the boiler.
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
3.6 3.0 2.4 2.0
1.6
1.2
0.8
0.4
1.0
0.6
90
(32)
Heating
Curve
Starting
Point
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
0.2
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)
2.0
WWSD Adjustment
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
2.0
90
(32)
Starting
Water
Temperature
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Starting Water Temperature Adjustment
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
3.6 3.0
2.0
1.6
1.2
0.8
0.4
0.6
90
(32)
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
2.4
1.0
Heating
Curve
Starting
point
Minimum Boiler
Supply Setting
130
°
F
0.2
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
Zone Valve Motor
End Switches
M
M
To control
C
R
