tekmar 374 Universal Reset Control User Manual

Page 9

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Increasing Load

Time

Diﬀerential

Oﬀ

ratu

re f

all

Time

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.