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I have a Lennox G14Q3 propane furnace (forced air). I used the Nest/furnace setup last winter without any problems with just the W and R terminals. But, these summer days I would like to push air through the house with the blower (without heat) and remove it with my HRV system.

I’ve tried hooking up the terminal G to the G terminal on the Nest (for the fan). But the Nest can’t seem to turn it on.

Note: When I test the heat with the Nest, my furnace goes through it’s standard cycle of heating up then turning on the blower.

Also, I can’t seem to get the blower going down at the furnace by shorting R and G, shouldn’t that work?

Thanks, Josh

Edit, added model and diagram pictures: enter image description here

enter image description here

EDIT 2: I realized that ACC isn't actually connected to anything. It gets 120VAC when I switch the relay, but there is still no path to the blower to be energized. Could I just take one of the speed wires from the blower motor and connect it to ACC so that it acts as an indoor blower? enter image description here

Josh Jobin
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  • Do you get 120V from ACC to N when you jumper G to R on the furnace? – ThreePhaseEel Sep 24 '17 at 13:25
  • Did you re-do the initialization cycle with the Nest where it discovers what kind of heater it's attached to, and what features it has? – Harper - Reinstate Monica Sep 24 '17 at 13:25
  • @ThreePhaseEel I get the standard 24VAC between G and R, until of course when I short them. – Josh Jobin Sep 24 '17 at 13:39
  • @Harper yes, the Nest detected the W, R and G lines. C isn’t connected, but when I did connect it, the Nest didn’t boot. I’ve read that’s because it detects incorrect wiring – Josh Jobin Sep 24 '17 at 13:40
  • @JoshJobin -- can you measure the voltage from ACC to N while G and R are shorted together? – ThreePhaseEel Sep 24 '17 at 13:40
  • Also, what's the resistance between G and C while the G terminal is open? – ThreePhaseEel Sep 24 '17 at 13:41
  • @ThreePhaseEel where would I measure ACC? Would I need to remove another cover? I know it has power because it sparks and hums a little, but maybe it doesn’t have sufficient power? – Josh Jobin Sep 24 '17 at 13:41
  • ACC should be available from the 120V terminal block on your control board (right next to where the incoming hot and neutral connect to the control board) – ThreePhaseEel Sep 24 '17 at 13:43
  • @ThreePhaseEel I am not home at the moment, but I’ll measure it later. Another couple facts, when I press “manual fan” on the furnace, it runs continuously. Also, when the Nest activates the heat, the fan comes on normally once the furnace is up to heat – Josh Jobin Sep 24 '17 at 13:43
  • Also, was your furnace subject to the [recall](https://inspectapedia.com/heat/Lennox_Pulse_Furnace_Notice.php)? Has it had the heat exchanger replaced with the modified unit already? – ThreePhaseEel Sep 24 '17 at 13:55
  • @ThreePhaseEel the voltage is 120VAC, and the resistance between G and C is 7.4MegaOhm – Josh Jobin Sep 24 '17 at 22:30
  • @JoshJobin -- did you see my link re: the recall? – ThreePhaseEel Sep 24 '17 at 23:31
  • I did, but I have only lived in this house for 1 year. I image the owners before me (who are the original owners) got it inspected. If not, I also have a CO detector in my basement. – Josh Jobin Sep 25 '17 at 00:12
  • @ThreePhaseEel I made an edit, could I just connect ACC to one of the other wires used for speed control on the motor? – Josh Jobin Sep 27 '17 at 17:38

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Alright, I figured out why it wasn't working. Had nothing to do with ACC not having a lead. I was looking at my setup closely and noticed that only 2 wires were coming up from the motor, but it needs 3. I noticed that there was a wire coming from pin 3, but it was crimped and didn't run to the motor. So I looked at the speed table and decided I wanted to run the Brown wire up. Connected it to pin 3. Now when I short R and G, the fan turns on :) enter image description here

Josh Jobin
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  • That'd do it :) BTW: you probably should replace this furnace soon -- there's no telling if the previous owners did anything re: the recall, and HX leaks in furnaces are something that's way better to be safe than sorry about. At a bare minimum, I'd have the HX pressure tested ASAP as per Lennox's instructions on the matter. – ThreePhaseEel Sep 27 '17 at 22:14