I have a Nest thermostat wired up to a furnace and AC that was installed 5 years ago (a Byrant 987m furnace, Byrant 126b AC).

Both the AC and furnace work, but when the AC is running, air barely blows out of the vents. Obviously, AC can never keep up with house as the day gets hot.

When the furnace is running, sometimes the heat "kicks up" and the variable speed blower starts really pushing air out of all the vents. But this never happens with the AC.

For the past several years, I just assumed that the problem was that our AC probably undersized. However, I'm wondering - could it actually be caused by some incompatibility with the Nest Thermostat? Perhaps the cooling is artificially low?

If the answer is "yes, it could be", then I'm going to fish out the thermostat that came with the unit and see if that makes a difference. If not... well, I've got a big project.

  • Which terminals does the thermostat have? If it simply has R W G Y C, then each of those is an "on/off" call. Jumpering R to W calls for heat. R to G calls for fan only. Jumpering R to Y and also R to G calls for A/C. Each is either on or off. Jul 29, 2022 at 7:35
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    Has any maintenance been done to the units, filters changed?
    – crip659
    Jul 29, 2022 at 12:33
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    In a standard ducted system, the refrigerant runs through a matrix in the duct-work and the furnace blower pushes warmer house air past it to cool the air. If there's no way for the Next to call for the 2nd stage blower when in AC mode, you'll never get the higher speed fan blowing. It may be worthwhile digging into the instructions for the original Bryant setup (including their thermostat) to see if there's a way to get the higher speed blower running with the AC.
    – FreeMan
    Jul 29, 2022 at 13:32
  • @Harper-ReinstateMonica the terminals in on the nest are as you suggested - Rh W1 G Y1 C. Which indicates that it's just an on/off setup as you mentioned.
    – Terence
    Jul 30, 2022 at 14:25
  • @crip659 - For maintenance I replace the filters every few months (whenever the Nest reminds me), occasionally clean out the drainage line, and pay for the yearly "check up" our HVAC installer hits us up for. If here is anything else, I'm not doing it!
    – Terence
    Jul 30, 2022 at 14:28

1 Answer 1


With most standard residential HVAC, the fan speeds are set by connecting fan leads to terminals on the control board. You connect one lead from the fan to the COOL terminal (usually highest speed) and another lead from the fan to the HEAT terminal (usually medium or low speed depending on heat rise and airflow).

Your Bryant 987m however, has a variable speed fan that is controlled by the furnace control board. It has DIP switches on the control board that select the desired fan speed ranges for heating and cooling. Without knowing anything else about your house or the ducting, my knee-jerk reaction is that the installer didn't set the DIP switches correctly and your cooling airflow is too low.

Another possibility is that the cooling system is just undersized or in need of maintenance, causing the evaporator coil to just not get cold enough. You're passing air through it, but it isn't doing much to that air.

I wouldn't blame the thermostat - it's a pretty simple device that basically tells the furnace to Do nothing, Heat, Cool, or Blow. The logic for how to do those things resides in the furnace control board itself and the associated parts (fan, burner, cooling coils, etc.).

At the very least, I would have a HVAC technician come to diagnose the system. If it isn't providing heat/cool/air correctly or at all, at least the technician can tell you if the system itself needs repair or if it just isn't sized correctly.

  • Thanks for the detailed response! One thing I should note is that the air always comes out very cold - just very slowly. I doubt that changes your recommendation at all, but I thought I should mention it.
    – Terence
    Jul 30, 2022 at 14:33
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    Good to know. Knowing that, I bet adjusting fan speeds for cooling will be your ticket. The installer's manual for your furnace will have info on the switch locations, positions, and tables for airflow targets. Hopefully they left it for you. If not, it is available online. The owner's manual is pretty much worthless, I'm afraid.
    – Chris O
    Jul 31, 2022 at 17:07

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