Thermo Products MDA1-125N, MDA1-50N, MHA1-75N, MHA1-100N, MHA1-50N Appendix B Sequence of Operations

All installations and services must be performed by qualified service personnel.

APPENDIX B: SEQUENCE OF OPERATIONS

A. Sequence of Operations for Heating System

Mode of Operation

Standby (At any time the gas

valve is not energized)

Control System Action

Continous safe operation check. If flame simulation condition present or roll-out switch opens,system energizes inducer fan and circulator blower at high speed until situation correction occurs.

Start

Start trial for ignition

Thermostat calls for heat,

contacts close

Ignition control module

energized,draft inducer starts.

Pressure switch senses

adequate pressure,

contacts close.

Power is applied to the hot	On power interruption, system
surface ignitor. Ignitor warms-		shuts off, restarts when
up for 20 seconds.		power is restored.

	After ignitor warm-up, the
	automatic gas control valve
		is energized to open.
				If flame is not detected, the
Flame must be detected
				gas valve is de-energized,
within 4 seconds. If flame
				the ignitor is turned off and
detected, the delay-to-fan-
				the ignition control module
on time begins (45 seconds)
			goes into the "Retry" sequence.

The retry sequence provides a 60 second wait before ignition retry.

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