I have a Bryant CNPV3017ALAAAAA furnace. There is a 28' duct run to a front bedroom that does not provide sufficient airflow in either heat or much more noticeably in cooling mode.

On the furnace board the blue wire is on heat and the grey wire is on cool.

Could I switch the blue wire to the orange wire to provide more heated airflow during the heating season and switch the grey wire to the black wire on the cool terminal during the cooling season?

Or could anyone recommend an inline duct boost fan.

Any advice would be appreciated.

  • 3
    The usual approach is to partially close ducts in other rooms to increase pressure to that room. I really don't follow your rewiring proposal. – isherwood Jan 18 at 16:35
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    Do you have access to the duct so it could be enlarged? – isherwood Jan 18 at 16:36
  • 1
    What do you hope to accomplish with the rewiring? – FreeMan Jan 18 at 16:36
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    Forget about rewiring - that may change fan speed/airflow, but it would affect everything equally and then you'd post a question "how can I limit airflow to all the other rooms". Adjust the ductwork or get an inline duct boost fan. – manassehkatz-Moving 2 Codidact Jan 18 at 16:44
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    How tight does the door seal in that room? I have seen homes that were setup with wood floors and later carpeted the carpeting in contact with the door bottom eliminates the air flow once the room pressurizes. If the home was set up properly with appropriate sized ducting and registers for each room a simple hand held anemometer can be used to balance flows by using the wind speed. Closing down high flow registers increases low flow resisters if there is a return path (in some cases cutting off the bottom of the door is needed OR adding a return in that room if maintaining the “seal” is wanted – Ed Beal Jan 18 at 16:58

Raising fan speeds during heating season can be counter productive. Shutting vents off does not mean more air will flow as we desire. In most instance power consumption increases. The best option is to increase duct size for a significate portion of the run if the duct was properly sized to begin with. Duct runs should never run over 25 feet remaining the same size. Duct runs generally should be limited to no more than 20 linear feet. A 100 square foot room generally will need a 6" duct.

  • Especially true when your ducts are in an attic where the air inside gets cold pretty fast, notwithstanding the insulation around the ducts. – daneb Jan 19 at 16:00

What you’re describing is common in setups where you have one long duct run compared to the others. As others have mentioned, one option is to restrict the airflow to the other rooms. I don’t like the idea of restricting airflow everywhere else to help a single room. Your best, cheapest option may be to run a power duct/register grille that uses a fan to suck more air out of the long duct. Any way you look at it, your HVAC setup isn’t ideal.

  • You haven't made the case why adding more hardware (and figuring out how to sync it with the furnace) is better than letting the dedicated furnace fan do the work. – isherwood Jan 18 at 21:18
  • One reason is I’ve heard from more than one HVAC professional that you should always leave all or most vents open for best furnace performance. I don’t know if this applies to only high efficiency models. At any rate, an HVAC system is designed and sized for a specific CFM of airflow and hampering that by seriously restricting the outflow creates an imbalance and may put undue pressure on the blower. – daneb Jan 18 at 21:29
  • I have put in over 100+ hvac central forced air systems and have never found one that the dampers did not need to be adjusted for proper flow in each room. The reason we have dampers or adjustable registers is because vents or registers come in distinct sizes so we are limited. We have no idea if this system is optimal or not. The worst thing you can do on some systems is to install a duct fan. A properly designed system will need adjustments multi story 2 per year to rebalance from heating to cooling seasons. An undersized run or duct could be the root problem but more often it’s dampers – Ed Beal Jan 18 at 23:34
  • Where I’m from proper dampers (aside from the one built into the register) haven’t been used in 60 years. Especially nowadays with the HE units that often have the fan running at all times, it is recommended by some HVAC professionals to not close or severely restrict airflow in most rooms to help the one odd vent. Just because you’ve done 100+ forced air installs doesn’t mean everyone agrees with you. If they did, you could have written the Bible of HVAC. That said, adjusting the dampers/registers in the other rooms is worth a try, but it’ll affect the efficiency of the system. Like it or not – daneb Jan 19 at 15:59

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