tekmar 422 Universal Reset Module - Manual (Page 20)

© 2007 D 422 - 08/07 20 of 32
On-Off Boiler Operation
If the heat source is an On-Off Boiler, the Boil On-Off / Mod
DIP switch must be set to On-Off.
Differential
An on / off heat source must be operated with a differential
in order to prevent short cycling. With the control, either
a fixed or an auto differential may be selected. The boiler
differential is divided around the boiler target temperature.
The boiler contact closes when the supply water temperature
is 1/2 of the differential setting below the boiler target
temperature. As the supply temperature reaches 1/2 of
the differential above the boiler target temperature, the
boiler is shut off.
• • Locate the Boiler Differential setting in the Adjust
menu.
Differential = 10°F (6°C)
Target + 1/2 Differential
Target – 1/2 Differential
Target
165°F (74°C)
160°F (71°C)
155°F (68°C)
Boiler
On
Boiler
On
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
In order to decrease temperature swings and increase boiler
efficiency, the Auto Differential feature automatically changes
the on / off differential of the boiler based on the heating
load. As the load increases, the differential will decrease to
minimize temperature swings. As the load decreases, the
differential will increase to prevent short cycling.
Off
Differential
Time
Heating Load
On
Modulating Boiler Operation
The 422 can operate a single hot-water modulating boiler.
This requires the use of the Mod (dc) output on the 422.
To operate a modulating boiler, the Boil On-Off / Mod DIP
switch must be set to Mod. The control operates the boiler
by first switching the boiler contact to allow the modulating
boiler to go through the ignition sequence (the boiler contact
may not be required on all modulating boilers). A 0-10 V (dc)
analog signal is used to modulate the boiler firing rate
from the Minimum Modulation setting using Proportional,
Integral and Derivative (PID) logic in order to satisfy the
boiler target temperature.
A modulating boiler requires all the same settings as an
on-off boiler in addition to the settings below.
Modulating Boiler 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
1/2 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 1/2 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.
Boiler Motor Speed
The Boiler Motor Speed is the amount of time the boiler
requires to go from 0% modulation to 100% modulation.
Gas valve actuating motors have a design time from
fully closed to fully open which can be found in the
manufacturer’s manual. The Boiler Motor Speed should
be set to this time.
The Boiler Motor Speed setting for a Variable Frequency
Drive (VFD) is the amount of time required to go from a
stopped position to 100% fan speed. Since a VFD has a
very quick response rate, it may be necessary to increase
the Motor Speed setting in order to increase the stability
of the boiler modulation.
• • Locate the Boiler Motor Speed setting in the Adjust
menu.