24
Installation and operation manual
–
DTHR-V9-2020
A PLC controls the burner duties by controlling the modulating gas valve, the fan speed and their
relative position to each other depending on the input signal (2-10 VDC) from the temperature
controller.
The control sequence is defined and executed following these eight steps:
1.
Standby
2.
Safety verification (1 and 2)
3.
Pre-purge
4.
Burner Ignition (pilot and main flame)
5.
Flame stabilization
6.
Modulation
7.
Low fire
8.
Post-purge
Main steps description
The control sequence begins by the Standby step if a start/stop burner request is present.
Step 1 -
Standby (S10)
The blower and the burner are stopped and waiting for a burner heating request greater than 2.3
VDC to starts. If the burner modulation signal request is above 2.3 VDC, the
safety verification
is
enabled.
Step 2 -
Safety verification (S20, S25)
The burner air proving switch
(PAB1) is verified during the two following tests:
1.
Activation of the air proving switch normally open (NO) contact.
The blower starts and its speed changes to 30%. The burner air proving NO contact should
close within a period of 15 seconds. If it closes, the test #2 is automatically performed. If it
doesn’t close,
another try will be performed after the test #2 is executed.
2.
Deactivation of the air proving NO contact.
The blower stops. The burner air proving NO contact should open within a period of 30
seconds. If it opens and the test #1 was correct the Pre-
purge stage is enabled. If it doesn’t
open, the test #1 is done again.
If both tests are not satisfied within 2 trials the
Low air pressure
alarm is activated and the sequence
goes back to the
Standby
step. A manual reset is required on the PLC screen to clear the alarm.
Control sequence
