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6.2.3 Recycle Feature
All step controls now incorporate the recycle feature which returns the step control to the no-load condition
upon loss of control power
.
6.2.4 Manual Step ‘ENABLE / DISABLE’ Toggle Switches
All units with >1 step now include manual ‘ENABLE / DISABLE’ toggle switches to provide a positive ‘OFF’
override of each step. These are used to disable a faulty step, limit total output or allow for a slow preheat.
6.2.5 Preheat Switch (Boilers Only)
The preheat switch is a manual limit switch. In order to preheat the boiler, the ON/OFF/PREHEAT switch must
be in the
down position. When the ON/OFF/PREHEAT switch is in its down or PREHEAT position, the boiler
output is limited to approximately 25% of full load power. This allows the operator to limit the boiler output
during startup cycles (particularly if the boiler is shut off at night and turned on again in the morning), which
could lessen electrical demand charges.
When the boiler has been brought up to the setpoint temperature (or pressure), the ON/OFF/PREHEAT switch is
moved slowly from its down or PREHEAT position to its up or ON position passing through the OFF position to
cause the step control to recycle to the NO LOAD position. Thereafter, the boiler can operate up to full load if
necessary.
NOTE
: The PREHEAT switch is included as an option on boilers of 9 or more steps. It is intended to be used to
prevent setting demand peaks on ‘cold starts’. It does not have to be used if so desired.
7.0 OPERATIONAL TESTING
7.1 Standard Controls
7.1.1 135-ohm Controllers (T991,T915,L91,etc.)
With the water temperature (or steam pressure) at setpoint, the step controller should bring on nominally one
half of the steps. An adjustable proportional band is provided on the controller (sensor) to enable tuning the
unit to system demand.
7.1.2 Solid State Controls
With the water temperature (or pressure) at setpoint, the step controller should bring on nominally one half of
the steps. An adjustable proportional band is provided on the controller (sensor) to enable tuning the unit to
system demand.
IMPORTANT: The timing between stages adjustable (with step control). Steam boilers normally have delay of
between 15-20 seconds while stepping up and 3-5 seconds while stepping down. Water boilers can normally
respond slower so 25 – 40 seconds up and 5 – 10 seconds down is usually appropriate. The slower the step
sequencing, the less wear on both the contractors and heating elements (yet still able to control the process as
needed) the less heat is generated in the cabinet which results in longer component life.
7.2 Outdoor Reset Controls
7.2.1 135-ohm Type Outdoor Reset Controller (Hot Water Boilers Only) (Honeywell T991B type)
These controllers have a fixed reset ratio: either 1-to-1, 1-to-1.5 or 1.5-to-1. The outdoor reference
temperature is 70°F (IE: the boiler outlet water control point temperature is equal to the setting on the
controller when the outdoor temperature, at the outdoor bulb, is 70°F). In order to test the operation of this
controller.
7.2.1.1
Check the outdoor temperature at the outdoor air sensing bulb; then determine the number of degrees
this temperature is above, or below 70°F.
7.2.1.2
Check the boiler outlet water temperature; if the outdoor temperature is below 70°, multiply the
difference between 70°F and the actual outdoor temperature by the reset ratio (1.0, .67 or 1.5); and subtract
this from the temperature of the boiler outlet water
.
