I own an old house. Some of the rooms in the house that are too far away from the furnace are not getting enough air, the airflow is too weak. I tried to close as many of the other ones as possible, but the airflow is still too weak. I am going to be installing a booster fan. Not sure which one yet, because I have a rectangular duct. Once I install the fan I don't want it to be on all the time. I want to only turn it on when the furnace fan is working. My furnace fan is always in Auto mode. What's the best way to do this?

  1. It has been suggested to me to use a pressure sensor and put it in the duct somewhere near a furnace. This way, no need to wire into the furnace.
  2. Should I just find the pins that connect to the actual fan inside the furnace?
  3. Are there pins on the control board of my furnace that I can use for this purpose?

I spent a few hours searching, but I can't find any actual solutions to my problem. Seems like such a common thing.

  • Have you considered that the culprit might be a duct that's too small? (Or badly done flex duct for that matter -- crumpled flex is crazy lossy) Jul 14, 2018 at 0:26
  • First, if you haven’t already I’d find a “good” HVAC person that understands system balancing and pay them to evaluate why your system isn’t working correctly. I put “good” in quotes because I’ve found that not every repair person is good with airflow balancing... MANY residential heating and air conditioning repair people can fix the broken furnace and make it heat, or fix broken AC and make it cool BUT frankly many don’t have a clue how to handle air flow balance problems. Ask specifically if they have experience fixing poor air-flow and system balance issues.
    – Tyson
    Jul 14, 2018 at 1:18
  • But to answer your original question, “how to sync” use a sail switch in a duct near the furnace. Sure you might be able to add relays or piggyback it etc, but then you or someone calls a repairman on a -10 or 105 degree day, and they look at all that and don’t know how it works, and think it’s the problem, or want to disconnect to test, but then can’t hook it back. Just use a sail switch and avoid connections to the current equipment for the added blower.
    – Tyson
    Jul 14, 2018 at 1:43
  • I have to agree with "Tyson" that a good balancing company is the answer . Installing a booster fan in a duct is not the answer. There are better choices to fix the problem.
    – d.george
    Jul 14, 2018 at 10:29
  • 1
    " There are better choices to fix the problem." you mean like properly re-doing the entire duct system in the house to properly balance it spending thousands of dollars or hundreds of hours? It's not seem "better" in terms of cost to benefit ratio to me. I am already trying to seal all holes in the ducts where I can find them (duct tape doesn't work very well, need to research other options like maybe metal sheets, screws and some sort of sealing componds). But it's a long airway and based on my research installing a booster in a long airway is not a bad thing at all. Jul 14, 2018 at 14:53

4 Answers 4


Using a 24v/110v relay (installed in the booster fan's junction box) run 2-conductor T-stat wire back to your furnace. hook 1 conductor to ground, and hook the other conductor to your T-stat's fan wire (usually green, but whatever wire is connected to the "G" terminal on your T-stat). On the 110v side, break the black wire to the fan and connect leads to the relay. That's it. Alternatively, you can use a pressure switch, but they cost more and you'll have to run power (110v, 14/2) from the pressure switch (which should be located in the supply plenum just downstream of the cooling coils) to the booster fan.

  • Hello, and welcome to Home Improvement. I've edited out your complaint; we would love to have you stay and give us more quality answers. Thanks. Dec 6, 2018 at 17:28
  • Ah. Preson, thank you. I don't have enough reputation to upvote but I accepted your solution. Dec 9, 2018 at 22:05

1 down vote

I would not waste my time installing duct fans , or a booster fan, since most will not be worth your time spent installing them. If I were you, I would find out what you have installed including the size and capacity of the equipment, the capacity of the duct work, the location of the supply and return registers. Without knowledge of the whole system we have no way to know just what needs to be done to fix your system. You need to find a good HVAC contractor to show you what your needs are and stop with the time consuming ideas that have no real gain. Everyone has critiqued most every comment or possible solution. Please try and find a "good and knowledgeable" company that can help solve your air delivery problems or at least tell you what you need to do to correct the problem.


Find your Cubic Feet Per minute ( each room. Width x height x length then the width, height and length of the ducts and how many turns you have, plus how many Tons your AC Unit is and the Output your unit handles ) and you'll find out where to put your booster fan.

You can easily hook the power wire up to a light switch and turn it on manually or you can hook it up to your R wire on the thermostat, but makes sure its in series with the transformer and the fan relay, booster fans shouldnt go in the duct work because of the condensation in the vents. So youll want to make a separate short vent on the outside of your duct work that runs into it, so it still grabs the ambient air but it also blows into your vents, you'll probably also want an automated trap door from your 2nd duct work for the booster, just in case you loose air when you don't want it on due to lighter weather or a bad motor

Booster fans should not be used it vents that produce heat from the furnace, that's another good reason to have it separate with an automated or manual trap door

Depending on how large your booster fan is you might want to add in a capacitor


There are duct fan controllers that can sense hot or cold an turn on the fan. The fan controller can also be used for fan speed control to balance your system.

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