THP-400A , THP-500A and THP-550 Installation and Service Manual
Control Functions
Airflow Switch Verifies that proper airflow through unit is present before ignition can
take place.
First-Stage Regulator Reduces tank pressure to an intermediate pressure, and supplies inlet
pressure to a second-stage regulator.
Flame-Safety Control Senses flame and shuts down the heater in the event of flame failure.
High-Gas Pressure Switch Measures manifold-gas pressure. If pressure is greater than setpoint,
the switch closes the gas valves.
High-Temperature Limit Opens electric circuit to flame-safety control and closes gas valves in
the event of an overheating condition.
Low-Temperature Limit Protects against the possibility of discharging freezing air into the
building in the event the burner fails to ignite. Pre-set at 40° F, this
control shuts the blower down if the temperature through the unit
drops below this setpoint.
Manual Bypass Switch Over-rides automatic shut-down of the blower during start-up if the
temperature through the unit is below the 40° F setting of the
Low-Temperature Limit control.
Manual Gas Valve Manually shuts off fuel supply at heater. For short-term, shut-down
only.
Modulating Control Valve Regulates gas supply to the burner controlled by the remote
thermostat.
Pilot Valve Supplies fuel to the ignition pilot when starting unit.
Proof-of-Closure Switch Monitors and proves that gas valve is totally closed in the normally
closed position.
Second-Stage Regulator Reduces the outlet pressure from first-stage levels to burner pressure.
Starter Interlock Proves motor-starter is engaged in the normally closed position.
Thermostat Automatically cycles the heater from low- to high-fire to maintain
desired room temperature.
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