tekmar 263 Boiler Control - Manual (Page 10)

© 2009 D 263 - 03/09 10 of 36
DIFFERENTIAL
A modulating boiler must be operated with a differential while operating in low fire. The boiler differential is divided around the boiler
target temperature. The boiler burner ignites at low fire when the supply water temperature is ½ of the Boiler Differential setting
below the boiler target temperature. The boiler is shut off in low fire as the supply temperature reaches at least ½ of the differential
above the boiler target temperature. With the control, either a fixed or an auto differential may be selected.
When the boiler is modulating above low fire, the differential does not apply. Instead, the modulation output signal is determined
using Proportional, Integral and Derivative (PID) logic in order to satisfy the boiler target temperature.
Fixed Differential
If the user desires to have a fixed differential, this is set using the
Boiler Differential
setting in the ADJUST menu.
Auto Differential
If the Auto Differential is selected, the control automatically determines
the best differential as the load changes. This reduces potential short
cycling during light load conditions.
SOFT STOP
It is possible to thermally shock a boiler when it is shut off at high fire. The Soft Stop feature forces the boiler to modulate down to
a minimum before turning off. This is designed to prevent large volumes of cold air being introduced into the combustion chamber
of the boiler when it is shut off. This can occur in applications where the burner includes a fan.
Once all demands are removed, the control allows for the firing rate to be modulated down to the Minimum Modulation setting prior to turning
off the burner. This feature is enabled by setting the Soft Stop / Off DIP switch to the Soft Stop position. If the Soft Stop / Off DIP switch
is in the Off position, the control turns off the boiler at the current firing rate once all demands are removed.
Tem
pe
ratu
re f
all
Te
m
pe
ra
tu
re
ri
se
Time
Desired temperature
160°F (71°C)
Differential
10°F (6°C) Boiler Off
Boiler On
155°F (68°C)
165°F (74°C)
Increasing Load
Time
Differential On
Off