How to Identify and Wire a Heat Only Unit - Gas Furnace

In this video I try to explain how to identify and wire a "heat only" system. This particular unit is a simple 80% furnace and nothing fancy. It is about as easy as it gets as far as low voltage wiring for an HVAC technician. So many technicians that I see will mistakenly wire in "circuits" that aren't needed. They see terminal blocks on a furnace board that have letters next to them and they just go crazy wiring what they have memorized on a white board. You have to be able to identify what unit you are working with in order to know what terminals/circuits are involved. Once you have that figured out you can pick the proper thermostat and start making things work!

In order to save time, I will try to add to this later by posting a troubleshooting video on the same unit.

Here is how I would put the Sequence of Operation for this unit (generically speaking of course...contrary to my wife thinking I know everything):

Fan Only Mode
1. Power “ON”
2. Move “Fan” switch from “Auto” to “On” (the switch from “R” to “G” closes)
3. 24 Vac passes across “R” to “G” and energizes the Indoor Fan Relay Coil
4. The switches on the I.F.R. react and do the opposite of their normal position. (The N.O. switch(es) close, The N.C. switch(es) open)
5. The high voltage power passes through the N.O. switch that has just closed and powers the Indoor Fan Motor.
6. If the motor is good then the motor turns “ON”
7. Move “Fan” switch from “On” to “Auto” (the switch from “R” to “G” opens)
8. 24 Vac stops passing across “R” to “G” and de-energizes the Indoor Fan Relay Coil
9. The switches on the I.F.R. react and go to their normal position. (The N.O. switch(es) open, The N.C. switch(es) close)
10. The high voltage power stops passing through the N.O. switch that has just opened and stops powering the Indoor Fan Motor.
11. The motor turns “Off”


Heating Mode – Gas Furnace Heating

POWER UP
1. 115 VAC power applied to furnace.
2. Integrated ignition control module performs internal checks.
3. Integrated ignition control module LED will light.
4. Integrated ignition control monitors safety circuits continuously.
5. Furnace awaits call from thermostat.

NORMAL HEATING SEQUENCE
1. R and W thermostat contacts close, initiating a call for heat.
2. Integrated control module performs safety circuit checks.
3. The induced draft blower is energized causing pressure switch contacts to close. Induced draft blower remains energized for pre-purge period. Humidifier terminals are energized with induced draft blower.
4. Ignitor warm up begins after pre-purge is completed.
5. Gas valve opens at end of ignitor warm up period, delivering gas to burners to establish flame.
6. The control checks for a signal from the flame sensor within seven (4) seconds after the gas valve is energized. Gas will only continue to flow if a flame signal is present.
7. Circulator blower is energized on heat speed following a fixed thirty (30) second blower on delay. Electronic air cleaner terminals are energized with circulator blower.
8. Furnace runs, integrated control module monitors safety circuits continuously.
9. R and W thermostat contacts open, allowing the gas valve to cycle off.
10. Induced draft blower is de-energized following a fifteen (15) -second post purge. Humidifier terminals are de-energized.
11. Circulator blower is de-energized following a selectable heat off delay period (90, 120, 150 or 180 seconds).
12. Furnace awaits next call from thermostat.