tekmar 421 Mixing Reset Module - Manual (Page 14)

© 2007 D 421 - 08/07 14 of 20
Boiler Temperature Control Section D
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.
Boil MIN + ½ Boiler Differential
Boil MIN
Boil MIN– ½ Boiler Differential
MIN segment on
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Boil Water Te
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To improve boiler efficiency, the boiler sensor can be
located on the supply pipe leading from the boiler and the
421’s boiler sensor DIP switch is set to Supply. The boiler
target temperature is determined using Boiler Load Reset,
in which the boiler water temperature is maintained at the
lowest possible temperature that satisfies the heating load
of the mixing device.
Boiler Minimum Protection
The control is capable of providing boiler protection from
cold mixing system return water temperatures. If the boiler
water temperature is cooler than the Boiler Minimum setting
while the boiler is firing, the control reduces the output from
the mixing device. Reducing the mixing output limits the
amount of cool return water to the boiler and allows the
boiler water temperature to recover. This feature can only
be used if the boiler sensor is on the supply or on the return
but is not available when the boiler sensor is not present.
Mix Minimum
The Mix Minimum settings are the lowest temperature that
the control is allowed to use as a mix target temperature.
During mild conditions, if the control calculates a mix
target temperature that is below the mix minimum setting,
the mix target temperature is adjusted to match the mix
minimum setting. During this condition, if the mixing
supply temperature is near the mix minimum setting, the
Min segment turns on in the LCD when either the mix
target temperature or the mix supply temperature is being
viewed.
•
• Locate the Mix Minimum setting in the Adjust menu.
Mix Maximum
The Mix Maximum set the highest water temperature that the
control is allowed to use as a mix target temperature. If the
control does target the mix maximum setting, and the mix
supply temperature is near the mix maximum temperature,
the Max segment turns on in the LCD when either the
mix target temperature or the mix supply temperature is
viewed.
•
• Locate the Mix Maximum setting in the Adjust menu.
Differential = 10°F (6°C)
Target + ½ Differential
Target – ½ 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