61
Sequence of Operation
To better understand the direct-fired heater, it is easier to break the unit out into smaller individual systems.
There are two main systems, a make-up air fan, and a heater. The make-up air fan consists of a blower
and motor. The heater may be further broken down into two control systems, the Flame Safety Control
(FSC) and the Modulating Gas System (MGS). The burner mixes air with the gas (Natural or LP), which
heats the air.
Flame Safety Control
The first system to understand is the Flame Safety
Controller. The FSC is there
only
to monitor the flame,
NOT
to control the temperature. The FSC uses a flame rectification sensor mounted on the pilot assembly
to detect the presence of flame in the burner.
Flame strength and presence can be measured at the FSC by reading the rectified flame signal. Use a DC
voltage meter, attach meter leads to the test jacks (TP1 and TP2) on the top edge of the FSC. Flame is
present when DC voltage reads between 6 and 18V DC. Ideal flame intensity produces a signal of 12V DC
or higher. Flame signal may also be read on the HMI by entering
Service > Variable Values > Inputs >
Flame Sensor
.
The board monitors the flame signal voltage at connector J32 pin 6 (+) and 15 (-). The FSC receives an
airflow signal from the MUA Board, which tells it whether there is proper airflow through the unit (not
just
any airflow, but
proper
airflow). Proper airflow occurs when there is a 0.15” wc to 0.80” wc differential
pressure drop across the burner.
When the airflow through the heater produces differential pressure in this range, the FSC indicates so by
illuminating the AIRFLOW LED. The FSC controls the opening of the redundant solenoid gas valves and
the operation of the spark igniter to initiate a pilot flame upon start-up.
Figure 29 - Flame Safety Controller
DC Voltage
Flame Status
0 to 5V DC
No Flame
6 to 11V DC
Weak Flame
12 to 18V DC Strong Flame
OPR CTRL
AIR FLOW
PTFI
FLAME
ALARM
4 5 3
6 7 8
A 2
S1
S2
TP1
TP2
Summary of Contents for MUA Controls
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Page 83: ...83 CLEANING MAINTENANCE RECORD Date Service Performed ...
