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Modu-Fire
®
Gas-Fired Boiler Installation
16
For example, assume the following settings:
Item Display Value
Setpoint 1 SP1 160
Low Temp Differential HyS1 -5
High Temp Differential HyS3 12
Low Temp Setpoint SPL 150
High Temp Setpoint SPH 195
The boiler will modulate to try to maintain 160
o
F.
If the temperature increases to 172
o
F which is Set-
point 1 (SP1) 160 + High Temp Differential (HyS3)
12, it will shut off. Once it shuts off, it will not re-
start until the temperature drops to 155
o
F which is
Setpoint 1 (SP1) 160 + Low Temp Differential
(Hys1) -5
. The Low Temp Setpoint (SPL) prevents
the operator from setting Setpoint 1 (SP1) lower
than Low Temp Setpoint (SPL) 150. The High
Temp Setpoint (SPH) prevents the operator from
setting Setpoint 1 (SP1) higher than the High Temp
Setpoint (SPH) 195.
Alternate Setpoint
The control must be configured for night setback.
(See the configuration menu.) When the Alternate
Setpoint contact is closed the boiler switches to its
alternate setpoint.
The alternate setpoint can be determined by several
different values. It can be a fixed alternate setpoint
SP2. It can be a fixed differential from SP1 called
dsP. dsP will “offset” the main setpoint SP1 by the
value of dsP. dsP can be positive or negative.
The Alternate Setpoint can also be determined by an
external analog source such as a 4-20 mA signal.
Outdoor Air Reset
The control must be configured for outdoor air reset.
(See the configuration menu.)
The setpoint of the boiler is controlled by an out-
door temperature sensor. As the outdoor tempera-
ture falls the setpoint of the boiler is increased.
The boiler modulates to maintain this setpoint. The
boiler functions as indicated in internal setpoint;
however, SP1 is determined by the outdoor air tem-
perature.
The setpoint SP1 is reset using a linear relationship
with outdoor air temperature. As the outdoor air
temperature drops the setpoint SP1 is increased a
proportional amount. This increase is determined
by the Heating Slope H. (See Table 1 at the end of
this section.) For example if H is set at 1 then for
every degree the outdoor air temp drops, the set-
point SP1 will increase 1 degree.
This heating curve has a reference temperature of
68°. This reference can be offset using the Parallel
shift parameter P. (See Table 1 at the end of this
section.) If P is set to 0 the setpoint SP1 will be 68°
when the outdoor temp is 68°. If P is set to 10, the
setpoint SP1 will be 78° when the outdoor temp is
68°. The minimum setpoint of the boiler is limited
by the control. In the above example the actual set-
point SP1 of the boiler will not go below 150°.
Analog Input Setpoint
The control must be configured for external set-
point. (See the configuration menu.)
The setpoint of the boiler is controlled by an exter-
nal signal. The boiler functions as described with
the internal setpoint, using the value determined by
the external analog signal.