Manual

9 of 36 © 2009 D 374 - 03/09
Increasing Load
Time
Differential
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Off
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Desired temperature
160°F (71°C)
Differential
10°F (6°C)
Boiler Off
Boiler On
155°F (68°C)
165°F (74°C)
Boiler
Sensor
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 stage contact closes when the supply
water temperature is ½ of the differential setting below the boiler target
temperature. Additional staging occurs if the first stage is unable to raise
the supply water temperature up to the boiler target temperature at a
reasonable rate. As the supply temperature reaches ½ of the differential
above the boiler target temperature, stages are staged off.
Fixed Differential
If the user desires to have a fixed differential, this is set using the
boiler differential (BOIL DIFF) 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.
BOILER RETURN SENSOR
The boiler sensor should be located on the boiler return if the 374 is one of many controls
that can call for boiler operation or the boiler has its own control with outdoor reset and
setpoint operation. The boiler return sensor provides boiler return protection.
Boiler Enable Contact
When the Return / Supply DIP switch is set to Return, the 374 provides a boiler enable when
there is a requirement for heat. The 374 no longer tries to control the boiler supply water
temperature directly but allows the boiler to operate at its operating aquastat setting. The
boiler enable contact is closed at either 30% or 10% operation of the mixing device, which is
set using the 30% Enable / 10% Enable DIP switch. The boiler enable contact remains closed
until the mixing device(s) no longer requires heat.
Section B2: Boiler Enable
Setpoint Enable Contact
If a DHW demand or a Setpoint demand is registered, the Setpoint Enable contact is closed to provide a setpoint demand to the boiler’s
control if a setpoint input is available. When a return sensor is being used, the boiler should operate its own boiler pump.
NO BOILER SENSOR
The 374 is capable of operating without a boiler sensor if desired. In this case, there is no
boiler protection provided by the 374.
To operate the 374 without a boiler sensor, the Return / Supply DIP switch is set to Return
and the control must be powered up without the boiler sensor connected. This type of
application is typical if the 374 is drawing heat from a heat source that already incorporates
some form of boiler return protection.
Boiler Enable Contact
When the Return / Supply DIP switch is set to Return, the 374 provides a boiler enable
when there is a requirement for heat. The 374 no longer tries to control the boiler supply
water temperature directly but allows the boiler to operate at its operating aquastat setting.
The boiler enable contact is closed at either 30% or 10% operation of the mixing device,
which is set using the 30% Enable / 10% Enable DIP switch. The boiler enable contact
remains closed until the mixing device(s) no longer requires heat.
Setpoint Enable Contact
If a DHW demand or a Setpoint demand is registered, the Setpoint Enable contact is closed to provide a setpoint demand to
the boiler’s control if a setpoint input is available.