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Can 2 stage furnace be forced to stay in low? My airflow noise (and duct pops) is loud enough to wake me up, so I want to test if low reduces this problem enough before I rip into the finished spaces to quiet the ductwork (with flex, or insulation, or...)

  • Are you talking about keeping the blower throttled down while the burners stage normally? Or are you talking about limiting the staging of the furnace burners themselves? – ThreePhaseEel Feb 14 at 0:10
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The W2 wire in the thermostat controls the 2nd stage heating. If you disconnect that wire at either end you should not get the 2nd stage.

By the way, you might also check to see if your furnace has a fan speed control. The default setting, usually HIGH, may be too high for your installation. I know that in my unit there is a rotary switch with labels "I", "II", "III", "IV", "V" that controls the blower fan. I found that "V" sounds like a tornado but "IV" does the job.

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  • That should do the trick, the system may not work at max efficiency though. – Ed Beal Feb 13 at 16:38
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    @EdBeal Agreed but he was asking for purposes of testing. You probably don't want to run with 2nd stage disabled for the long-term. – jwh20 Feb 13 at 16:47
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    I just wanted to point out that out I did read that and it could have consequences, if the ducts are hidden in walls I have had some luck identifying the area and using a sheetrock screw to punch into the duct and pull it out so it will quit oil canning or the popping noise. This has helped many times with trunk lines in walls, where open in attics I have screwed a furring strip with spacers to pull it out. – Ed Beal Feb 13 at 17:00
  • I would have been helpful to know what sort of furnace it is. If a high efficiency gas furnace of the condensing type, lower stage heat can actually be slightly more efficient due to a higher level of condensation. Of course heat output might not be adequate! BTW, great suggestions about how to "tame" the ducts, I never would have thought of that. – George Anderson Feb 13 at 17:19
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More modern furnaces are pre-set to automatically escalate to stage 2 at some point in time, even if the W2 wire isn't used. Mine was 20 minutes, though 10 and 15 were options via dipswitch settings, as was Off. You may need to configure it that way.

Read your manual. It'll tell you the story.

I recently did some research on this topic due to the catastrophic failure of my 27 year old Carrier, which was replaced with a two-stage HE Ruud. For what it's worth, everything I've read indicates that efficiency is virtually identical between the first and second stages. Gas burns, heat comes out. The only variance is the rate at which this happens. It'll only be an issue if stage one can't keep up with heat loss to the exterior of the home on very cold days. A person could configure a simple switch on a jumper to W2 so as to manually enable stage two when needed.

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