tekmar 257 DHW Control - Manual (Page 4)

© 2009 D 257 - 03/09 4 of 12
Sequence of Operation
When the DHW Control 257 is powered up all LCD segments
are displayed for 2 seconds followed by the version number
being displayed for 2 seconds. The control then enters into the normal operating mode and the LCD defaults to
displaying the current DHW temperature.
Powering Up the Control
Staging Operation
Proportional Staging Mode (P)
When the Stage Mode is set to P, the on/off points of the
HEAT 1 and HEAT 2 relays are determined by the Target
DHW setpoint, the Differential 1 and the Differential 2
settings. This is a manual way of staging the boilers. See
figure 1 for a visual representation.
Differential 1 & Differential 2 (P Mode)
Manually adjust Differential 1 and 2 to set the on points
of stage 1 and 2. Stage 1 will fire when the temperature
falls below Target DHW minus Differential 1. Stage 2 will
fire when the temperature falls below Target DHW minus
differential 1 minus differential 2.
Differential 1
Differential 2
Target
DHW
Stage 1 Stage 1
Stage 1
Stage 1 & 2 Stage 1 & 2
OFF
OFF
Increasing T
e
mperature
Increasing Time
Figure 1 - Staging operation for proportional logic
Proportional Error - How far is the temperature from theTarget?
Integral Error - How long has the temperature been there?
Derivative Error - How fast it is the temperature changing? •
•
•
Differential (PID Mode)
If Differential is set to Auto, the control will calculate an
appropriate differential to reduce boiler short cycling.
If a boiler short cycles, the differential is automatically
increased. If auto differential is not desirable, the installer
may select a manual differential.
Minimum On and Off Times
The minimum on and off time for each boiler is 30
seconds.
DHW Maximum
If the DHW temperature rises 2°F above the Target DHW
setpoint due to minimum boiler on times, the control will
open both the Heat 1 and Heat 2 relays.
Proportional, Integral, Derivative Staging Mode (PID)
To provide proper boiler staging and accurate DHW
Target temperatures, PID mode should be used. When
the Stage Mode is set to PID, the on and off points of
the HEAT 1 and HEAT 2 relays are determined by PID
logic and the differential setting.
What is PID?
By considering these three errors, the control can better
decide when to turn on a boiler stage. This function provides
greater boiler plant efficiency and also prevents water
temperature drop that is commonly seen on proportional
staged systems.