tekmar 423 Universal Reset Module - Manual (Page 24)

© 2007 D 423 - 08/07 24 of 36
Boiler Enable and Setpoint Enable Section E
If the 423 is one of many controls that can call for heat to
a single boiler or there is a boiler sequencer other than a
tekmar Stager (Boiler Control 264, 265, 268), operating
multiple boilers or multiple stages, then the boiler sensor
must be located on the return pipe of the boiler(s).
When the sensor is located on the return, the 423 provides
a boiler enable. The 423 no longer tries to control the
boiler supply water temperature directly, but allows another
operating control such as an aquastat to regulate the boiler
supply temperature.
When there is a requirement for heat from the Mixing Expansion
Modules connected to the 423, the Stage 1 / Boil Enbl contact
closes.
When there is a DHW Demand, or a Setpoint Demand, the
Stage 2 / Setp Enbl contact closes.
When the boiler sensor is located on the boiler return, the
control is able to provide boiler return protection through
the use of the Mixing Expansion Modules. This protects
the boiler against sustained flue gas condensation and
thermal shock.
*Important note: tN4 devices cannot be connected to
the Boiler Bus while the Boiler Sensor DIP switch is set
to Return.
To operate the control without a boiler sensor and prevent
the control from displaying an error message, set the boiler
sensor DIP switch to Return and power up the control
without the boiler sensor connected.
The control operation will be similar to that as having the
boiler sensor on the return except that boiler return protection
is no longer provided.
Zone Load Shedding Section F
Mixing Operation Section G
The 423 has four tN4 communication buses. The first bus
is dedicated for a boiler water temperature and is known
as the Boiler Bus. The remaining buses can be selected to
be either boiler or mixing depending on their individual Bus
settings. When either Bus 1, Bus 2 or Bus 3 are set to be
a separate mixing water temperature, a Mixing Expansion
Module must be connected to that bus.
A Mixing Expansion Module is required in order to use
each mixing temperature. The Mixing Expansion Module
operates at the percent output that the 423 determines is
required.
Each of the following settings must be set for each mixing
water temperature.
Mixing Expansion Modules
A Mixing Expansion Module allows the control to reduce the
boiler water temperature down to a lower water temperature.
A Mixing Expansion Module when used with a boiler sensor
also allows the control to protect the boiler from sustained
flue gas condensation and thermal shock.
Variable Speed Injection
The control increases or decreases the power output
to the circulator when there is a requirement for mixing.
The circulator speed varies to maintain the correct mixed
supply water temperature at the mix supply sensor. For
correct sizing and piping of the variable speed injection
circulator, refer to essay E 021. A visual indication of the
current variable speed output is displayed in the LCD in
the form of a bar graph while viewing the Mix Supply or
Mix Target temperatures.
If the boiler temperature approaches the boiler minimum
setting, the control can turn off certain high temperature
zones in order to reduce the load on the boiler. This is
known as Zone Load Shedding. Zones are shed in the
following order:
1. The second stage of any 2-stage thermostats which
have “BOIL” selected as their heat source in order of
decreasing priority. Priority is determined by the address
number of the thermostat (b:01 is the highest priority,
3:24 is the lowest priority).
2. The first stage of any thermostats on the boiler bus,
again, in order of decreasing priority. The first stage
of the highest priority thermostat (lowest address
number) will not be shed unless there is also a setpoint
or DHW call.
When the boiler supply temperature goes above the
minimum setting, the control begins restoring the load by
turning first stages back on in the reverse order that they
were shed, followed by second stages in the reverse order
that they were shed.