Hydronic Heating Boilers and Domestic Water Heaters
SEQUENCE OF OPERATION Continued
MANUAL RESET HIGH-LIMIT THERMOSTAT
A manual reset high limit thermostat is provided as backup for the electronic adjustable temperature controller. An adjustable dial thermostat will trip and require resetting should water temperature exceed its settings. When tripped, all controls remain powered, but 24VAC is prevented from being delivered to the ignition module’s thermostat input.
EMS/SEQUENCER/REMOTE AQUASTAT TERMINALS
The EMS/sequencer/remote aquastat terminals are provided for connection of the unit to an external energy management system or sequencer and are located along the top edge of the terminal strip located within the electrical wiring box. In addition to providing a means to externally stage fire the unit, these terminals also provide for remote shutdown and override by routing through dry contacts from external devices. Connection of the external dry contacts to these terminals requires the removal of one or more factory installed jumpers located on the terminal strip.
SAFETY DEVICE TERMINALS
The safety device terminals ensure that no power can be delivered to the ignition module unless
Two types of connections are offered: Continuous terminals provide for connection of safety devices that are desired to be constantly monitored. Intermittent terminals are provided for devices that are only monitored during a “call for heat” cycle. Connection to either of these terminals requires the removal of a
ELECTRONIC TEMPERATURE CONTROLLER
(Operator) A multiple stage electronic temperature controller is provided on the unit and it serves to maintain a user selectable temperature set point. In addition to maintaining a set point, it can provide for various desired modes of operation. Ultimately, the temperature controller delivers 24VAC to the ignition module and pressure switch when it senses heat mode is desired.
IGNITION MODULE
(Controlled by Electronic Temperature Controller)
Upon sensing the 24VAC delivered to the ignition module by the electronic controller, the ignition modules check for a shorted
pressure switch and then, provided the air switch isn’t shorted, supplies 120VAC to its blower output relay and waits for a pressure switch signal.
TWO SPEED BLOWER(S)
(Controlled by Ignition Module and Relay Board)
PRESSURE SWITCH
(Activated by Blower Turning On)
One pressure switch is provided per blower to monitor its on/off operation. Upon a call for heat, 24VAC is delivered to the pressure switch. If the blower being monitored generates adequate air pressure, the pressure switch closes to deliver 24VAC to their respective ignition module’s pressure switch monitoring input terminal.
PREPURGE
(Ignition Module Function)
Once an ignition module sees a 24VAC input from its pressure switch, it runs the blowers for approximately 15 seconds in order to purge the combustion chamber of any unburned fuel/air mixture.
TRIAL FOR IGNITION
(Ignition Module Function)
Once an ignition module has completed its prepurge period, it turns on a hot surface igniter. Once the hot surface igniter’s current draw reaches a proper level, the ignition module begins a trial for ignition period of approximately 25 seconds to allow time for the igniter to reach the ignition temperature necessary to ignite the fuel/air mixture.
IGNITER
(Controlled by Ignition Module)
One igniter per ignition module is provided in order to ignite the fuel/air mixture and its on/off operation is controlled by the ignition module.
RELAYS
(Controlled by Electronic Temperature Controller and Ignition Module)
A relay is provided to switch each blower from low to high speed and to deliver power from the primary stages to the secondary stages depending upon the electronic thermostat’s settings. The relay also serves to override the pressure switch proving function of the second ignition control (if equipped) to activate during a call for heat because both blowers run whenever any or all stages
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