1
Item
Date
S/N
5
Pump /
Boiler 2
2
1
6
3
4
Boiler 1
No Power
7
8
9
Com
Sen
Sup
Sen
Out
Sen
24 V Only
2
1
C
R
Power
H11121
PRGM
F C
1
2
Min.
Outdoor
Supply
Target Supply
Curve
WWSD
Differential
Two Stage Boiler
Control 255
Nº of boilers
Power Supply:
24V 60Hz 3VA
Relay Capacities:
120V 10A 1/3hp
- Data Brochure
Two Stage Boiler Control 255
06/94
D 255
The tekmar Two Stage Boiler Control 255 is a microprocessor-based outdoor reset control that is designed to regulate
the supply water temperature from one or two boilers based on the outdoor air temperature. If one boiler is used, a
system pump output is available.
The control has programmable settings for minimum supply temperature, differential, heating curve, and warm
weather shut down temperature. The control has a digital liquid crystal display (LCD) window that normally shows
the outdoor temperature and can be used to display supply, target supply and programmable settings.
The Two Stage Boiler Control 255 is a cost effective answer for outdoor reset of many single and dual boiler applications.
Wiring Chamber;
Terminals for
Sensor(s)
Wiring Chamber;
Terminals for 24Vac
power supply and
control circuit
for boiler(s) /
pump
Hole for mounting screw
Hole for mounting screw
Programming
mode indicator
Item
identifiers
Item selection and
display button
Temperatures,
Heating Curve and
Differential
Display
Boiler(s)
operation
Sequence of Operation
• When the Two Stage Boiler Control 255 is
powered-up, the digital display will show all of
the display elements. The control will then
monitor the outdoor temperature and display it
on the digital display. Pressing and releasing
the Item button will toggle to the supply tem-
perature. Pressing and releasing the Item
button again will toggle the display to the
Target Supply temperature.
• If the outdoor temperature is warmer than the
WWSD point, the "WWSD" pointer will flash on
and off, and the control will keep the boiler(s) off.
• If the outdoor temperature is colder than the
WWSD point, the control calculates the desired
supply temperature based on the requirements
of the Heating Curve or on the Boiler Minimum
setting, whichever is greater.
If the Boiler Mini-
mum setting is higher than the Heating Curve
requirement,
the "Min." pointer will flash on and
off and the control will maintain the supply
temperature at the Boiler Minimum setting.
If
the outdoor temperature is cold enough to
operate on the Heating Curve,
the "Min" pointer
will turn off, and the control will maintain the
supply water temperature at the Heating Curve
desired temperature.
°
C or
°
F
indicators
Programming
buttons
Outdoor Reset Strategy
Heating Curve
As the outdoor temperature becomes colder heat losses from a building increase, requiring more heat
be added to prevent the indoor temperature from also becoming colder. This tekmar reset control
measures the outdoor temperature and, as it becomes colder, the control balances the heat loss by
making the heating supply water hotter. The Heating Curve is used to calculate exactly how hot to make
the supply water at different outdoor temperatures. It determines the number of degrees the supply
water temperature is raised for each degree the outdoor temperature falls.
•
If the Heating Curve selected is too low, the supply temperature will not be hot enough to keep the room
temperature warm during cold weather.
•
If the Heating Curve selected is too high, the supply temperature will be hotter than necessary and
energy will be wasted.
Outdoor air temperature
Supply water temperature
50
(10)
30
(-1)
10
(-12)
-10
°
F
(-23)
°
C
110
(43)
70
(21)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
3.6 3.0 2.4 2.0
1.6
0.8
0.4
1.0
0.6
90
(32)
50
°
F
(10)
°
C
Heating
Curve
WWSD
Point
1.2
F
Two Stage Boiler
Control 255
Min.
Item
Outdoor
Supply
Target Supply
Curve
WWSD
Differential
• There is a minimum time delay between a boiler
being turned off and then back on. For the first
boiler, this time is preset to 30 seconds. When
two boiler operation is selected, the delay be-
tween the boiler stages is adjustable from 30
seconds to 4 minutes.
• To obtain the best operation from a reset control, it is important to measure the system supply temperature as accurately as possible.
Whenever the control is energized, the system pump must operate to maintain continuous water flow across the supply temperature
sensor. Refer to the application drawings A 255 for plumbing concepts.