HVAC Explanation Request

Hi Community,
I have what i think is a typical Gas Furnace and Central A/C unit coupled together on the same venting system.

My loose understanding is that there’s a Fan/Blower that pushes air around that’s shared by both
A condenser inside and a compressor outside that function as the A/C
A electrical ignition system that turns on the Furnace
and maybe a Draft Inducer (not totally sure what this does)

So, firstly, what am i missing from my understanding of electrical components making up this system?

Secondly, there are 2 devices so far that have been discovered associated to HEAT

  1. Seems to come on first and run for an initial period of heating
  2. Seems to come on after #1 and run for an extended period (the entire heating duration).

I’ve suspected #1 is the draft inducer and #2 is the blower motor, but would like to correct that understanding if mistaken.


Good understanding - that’s how my two forced air gas heating systems work. The furnace has exactly the electrical components you describe, plus one more that Sense can’t detect without a smartplug - an Always on of maybe 7W that powers the controller board and thermostat (if it is a smart one).

And the AC compressor will usually be on its own 240V circuit. So be careful about how you merge devices - you can roll them all into a single HVAC, but if not, realize the furnace blower is shared between heating and AC.

You may also have a humidifier (which won’t have much in the way of electrical consumption) and an electronic air filter (which could add 30W or so to the runtime consumption).

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I changed my thermostats at the beginning of the year and used Sense to see the operation of my gas furnace. I captured one cycle and overlaid the sense plot with rectangles showing the power used by each of the main components.

The inducer blower is a small blower that starts at the beginning of a cycle; it’s purpose is to confirm the flue is not blocked and there won’t be a buildup of exhaust gas in the house. The inducer blower starts first, the furnace controller detects a vacuum is created, then the igniter heats up. The gas valve opens and a thermistor confirms the burners have lit by detecting heat away from the igniter. The igniter can then go off. The furnace will wait for around a half minute for the plenum to warm up before the main blower starts; that is to reduce the amount of cold air that comes out of the vents at the start. When the thermostat is satisfied the gas valve will close. The inducer stays on for a bit longer to clear the exhaust out of the flue, and it goes off. Finally the main blower goes off when they have pushed most of the remaining warm air out of the vents.


Wow this is amazing. Thank you so much @demiller9

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