We apply 120 VAC to the HM Unit (Remember to check polarity!) Our primary step down T/X provides power to terminal “X” and “COM” on the ignition control and ”6” on the SC30 modulation control (Honeywell transformer leads are reverse of standard! … L1 hot is white and L2 neutral is black) The draft inducer high speed is energized (120Vac) The low pressure air switch closes, initiating a 30 second pre- purge The thermostat circuit closes on a “Call For Heat” HM Modulating Wiring

After the 30 second pre-purge is complete, the spark igniter is energized HM Modulating Wiring 24 VAC power is supplied to terminal “MV” energizing the first stage, low fire, of the gas valve (1.2” w.c.) The 2 minute “mid fire hold” timer “TD-1” and modulating relay “MR-1” are also energized

The flame rod has 5 seconds to recognize the flame presence and return an adequate flame signal to the ignition control HM Modulating Wiring 24 VAC power is provided to terminals 1 and 2 on the SC30 modulation control (MC) through the “MR” contact. A 2 minute delay sequence is initiated Upon completion of the 2 minute hold “TD-1” powers relay coil “SR” Contact “SR-2” closes. 120 volt power is supplied to the low speed fan connection via contact “FR-5”

At the conclusion of the 2 minute hold the SC30 modulation control will power the modulation valve “MDV” HM Modulating Wiring The valve will operate between 25% and 100% firing rate, dependant on the analog 0 to 10 vDC input signal from the temperature control

If the analog input rises above 5.3 vDC, prox., terminal 5 on the SC30 and delay timer “TD-2” are energized. HM Modulating Wiring After 45 seconds (adjustable) “TD- 2” energizes fan relay (FR) and control relay (CR) 120 VAC is supplied to the high speed draft inducer leg via “FR-2” contact. Provided the high pressure air switch is closed, 24 VAC is supplied to the 2 nd stage valve terminal “HI” control relay contact “CR-1” 2 nd stage gas valve opens to high fire position (3.5” w.c.)

If the analog input is less than 4.7 vDC, prox., terminal 5 on the SC30 is de-energized. HM Modulating Wiring Power is interrupted to the high pressure air switch (APS-2), fan relay (FR) and control relay (CR) Power is interrupted to the high fire (2 nd stage) gas valve, returning it to low fire. The fan relay opens energizing the low speed fan through “FR-5” contact

If the air pressure switch (APS-1) does not close after the inducer is energized or if the limit or rollout switch(s) are open, the ignition control will wait one minute for the circuit to close and if still open, go to lockout (1 Flash) HM Modulating Wiring Trouble-Shooting Lockout!

The thermostat circuit closes on a “Call For Heat” HM Modulating Wiring Trouble-Shooting The limit circuit is already complete … meaning the air flow switch is stuck in the closed position! If the switch does not open within 1 minute, the ignition control will go to lockout (2 flashes) Lockout!

The low pressure air switch closes, initiating a 30 second pre- purge HM Modulating Wiring Trouble-Shooting If the flame rod senses flame during the pre-purge, when no flame should be present, the blower will run but the gas valve remains de-energized until the “false flame” is removed (5 flashes)

After the 30 second pre-purge is complete, the spark igniter is energized If ignition is not recognized by the flame rod within 5 seconds the gas valve is de-energized. The blower runs for 60 seconds at which time there is a re-light attempt. After 3 such attempts the igniton control will lockout. (3 flashes) Note: After 60 minutes there is a “Soft Reset” and the above sequence will repeat HM Modulating Wiring Trouble-Shooting

Once main flame has been established and recognized by the flame rod, if you have a loss of flame, the ignition control will de- energize the gas valve within 1 second. A 60 second inter-purge will occur and 3 trials will be available before lockout (4 flashes) HM Modulating Wiring Trouble-Shooting

If the ignition control detects the gas valve is energized when it should not be or is not energized when it should, the system will go to lockout with all outputs off (5 flashes) HM Modulating Wiring Trouble-Shooting

If the high temperature limit opens during a call for heat, power to “W” is lost and the gas valve will close. The draft inducer runs a 60 second post-purge. Typically the high limit will close again once cooled 30 F. If we still have a call for heat “W” will be powered, initiating operation sequence. We should not allow the heater to cycle on the high limit as we have insuficcient air flow to safely continue! HM Modulating Wiring Trouble-Shooting