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I have a 16 year old Tempstar condensing 90+ two stage with variable speed motor. I just connected a Lyric T5 thermostat and would like to have the thermostat control the staging rather than the furnace. The manual says the 4th DIP switch on the control board must be in the ON position for two-stage thermostat operation. At present only the 2nd and 3rd are in the ON position. If I move the 4th to the ON position do the 1st, 2nd and 3rd stay in their present positions? Are any other adjustments required other than the wiring?

Thank you!

  • What model is said furnace? – ThreePhaseEel Jan 28 '18 at 7:13
  • Hi and thanks. It's a T9MPV075F12A2. I believe the switches are part of the Fan Control Module S1, but the switches aren't labeled in the wiring diagram as far as I can tell. Flipping the 4th switch to ON would be the factory setting (with thermostat-controlled staging enabled.) I just want to be sure I'm flipping the right thing! ;-) The four switches I'm looking at are located above the G terminal on a red board. – Temp Star Jan 28 '18 at 15:23
  • Hi again. No worries...that is indeed the DIP switch location and turning the 4th one to ON is the factory setting. Cheers! – Temp Star Jan 29 '18 at 15:50
  • Post that last comment of yours as an answer and I'll give you a +1 for it :) – ThreePhaseEel Jan 29 '18 at 23:21
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It turns out that it is indeed the DIP switch location and moving the 4th one to the left (the ON position) is the factory setting of 1 OFF, 2 ON, 3 ON, 4 ON. No other adjustments other than to the wiring between the thermostat and the furnace were required.

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