tekmar 256 Boiler Control - Manual (Page 8)

BOILER DIFFERENTIAL (BOIL DIFF)
An on / off heat source such as a boiler, must be operated with a differential in order to prevent short cycling. With the 256, either a
fi xed or an auto differential may be selected.
Fixed Differential
The boiler differential is divided around the BOIL TARGET temperature.
The contact will close when the supply water temperature is 1/2 of the
differential setting below the BOIL TARGET temperature, and will open
when the supply water temperature is 1/2 of the differential setting
above the BOIL TARGET temperature.
Auto Differential (Ad)
If the Auto Differential is selected, the 256 automatically determines
the best differential as the load changes. This setting is recommended
as it reduces potential short cycling during light loads.
WARM WEATHER SHUT DOWN (WWSD) OCC & UNOCC
When the outdoor air temperature rises above the WWSD setting, the 256 turns on the WWSD segment in the display. When the
control is in Warm Weather Shut Down, the Boiler Demand pointer is displayed, if there is a demand. However, the control does not
operate the heating system to satisfy this demand.
CAUTION
Improper installation and operation of this control could result in damage to the equipment and possibly even personal injury. It
is your responsibility to ensure that this control is safely installed according to all applicable codes and standards. This electronic
control is not intended for use as a primary limit control. Other controls that are intended and certifi ed as safety limits must be
placed into the control circuit.
STEP ONE GETTING READY
Check the contents of this package. If any of the contents listed are missing or damaged, please contact your wholesaler or tekmar
sales representative for assistance.
Type 256 includes: One Boiler Control 256, One Outdoor Sensor 070, One Universal Sensor 071, Installation & Operation
Manual D 256, Application Brochure A 256.
Note: Carefully read the details of the Sequence of Operation to ensure that you have chosen the proper control for your application.
Installation
© 2011 D 256 - 07/11 8 of 20
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
Off
On
Time
Heating Load
Differential