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followed by a 40-second period to allow the ignitor to
heat.
The ignition control energizes the blower. The
blower pressurizes the air box (which supplies air to
the burners) and closes the normally-open contact on
the airflow pressure switch. The blocked vent pressure
switch senses the pressure in the exhaust plenum. This
switch opens if the pressure is excessive (as an indicator
of a blocked vent). When these two pressure switches
are closed, voltage is sent to the PSW terminal on the
ignition control, which allows the ignition module to
proceed with the ignition sequence.
If the airflow proving circuit is not proven,
ignition module will lock out.
Section 6.5
describes the
result of air flow faults in more detail.
When the ignition control gets the signal at the
PSW terminal, it will energize its MV terminal, which
sends power to the stage one (or on/off) main valve.
After a 7-second trial for ignition, the ignitor
switches to flame sense mode. If a flame is not detected,
the gas valve will close and the ignition module will
attempt ignition again (up to two more times, for a total
of three attempts.) If all three attempts fail, the ignition
control will lock out.
If flame is sensed, the burner will continue to fire
as long as there is a call for heat, and none of the safety
circuit is interrupted.
If there is a call for stage 2 on a 2-stage unit, that
valve will be energized.
If there is a loss of flame signal during a successful
firing sequence, the ignition control will remove power
from the gas valves, and then attempt to light up to
two more times. If successful, the appliance will fire
normally. If unsuccessful, the ignition control will lock
out.
When the call for heat is satisfied, the gas valve(s)
closes, and the blower starts a 45 second post-purge,
and the pump starts the post-purge (where applicable).
6.E Ignition Control Reaction to Air Flow
/ Blocked Vent Pressure Switch
Air flow and blocked vent status are continuously
monitored by the appliance's ignition control, at the
control’s PSW terminal.
At the start of an ignition sequence, if the ignition
control sees power at PSW for 30 seconds, but the
combustion blower has not yet been energized by the
control (F1 and F2), the ignition control will remain
in this fault mode, with the blower off. If the power to
PSW is removed while there is still a call for heat, the
ignition sequence will start again, but the main valves
will not be energized until PSW sees power during the
ignition sequence.
If the PSW signal is lost while the appliance is
firing, the control will immediately de-energize the gas
valve terminal (MV). The blower will remain on for
the post-purge period (45 seconds), and the control
will continue to monitor the PSW input. If the signal
is detected during the post-purge period, a normal trial
for ignition will begin, starting with the 15-second
pre-purge. If the signal is not detected during the post-
purge, the control will lockout with the blower off
.
6.F Operating the Burner and Set-Up
6.F.1 Set-Up for 0 to 2500 Feet Altitude
The setup must be checked before the unit is
put in operation. Problems such as failure to start,
rough ignition, strong exhaust odors, etc. can be due
to improper setup. Damage to the appliance resulting
from improper setup is not covered by the limited
warranty.
1. Using this manual, make sure the installation
is complete and fully in compliance with the
instructions.
2. Determine that the appliance and system are filled
with water and all air has been bled from both.
Open all valves.
3. Observe all warnings on the Operating Instructions
label and turn on gas and electrical power to
appliance.
*1 Difference between the temperatures at which the two switches
make R-W. Adjustable from 3°F to 10°F (1.7°C to 5.6°C).
*2 Two DPDT switches operate in sequence. Each switch
differential is fixed at approximately 3°F (1.7°C).
Figure 18. Two stage aquastat
Mighty Therm
2
(200, 300, 400) Install & Operating
