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Duct booster wiring

| Posted in Energy, Heating & Insulation on September 28, 2004 12:35pm

I’m working on a remodel where it’s going to be necessary to install a couple of inline duct booster fans to get the right air distribution. Seems like the best way to wire the fans is to run them back to the motor control on the air handler, then use one hot leg of the 220 to supply the fans. That way, the fans turn on and off with the air handler blower.

Has anybody done it this way, or do you have a better idea? What gauge wire do i need to use? The fans have something like 24 ga. wire attached (I assume they draw next to nothing), so do I need to run 14 ga. romex for the feed, or can I get away with smaller wire? I’ve check the NEC 2002, and am more confused now than when I started.

Any ideas much appreciated!

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  1. 4Lorn1 | Sep 28, 2004 03:07am | #1

    In my experience, having wired a few, these booster fans come in two flavors:

    The most common use a flow sensor incorporated in the unit itself as a controller. Commonly this is a small vane that activated a microswitch. The central blower gets the air moving, just not strongly enough to get to remote registers. The vane is moved by this air flow and the booster activates reinforcing, boosting, this flow.

    With these all that is commonly needed is a feed off any existing, within the normal limitations of the code, nearby circuit capable of supporting the small added load. This can usually be fed off a nearby receptacle, service receptacle under the house or attic light box if there is an unswitched leg available. Hook the power up and it does its thing.

    Some accept hard wiring while others need a receptacle placed near their location on the duct and the unit is simply plugged in. This also provides a disconnect means during servicing.

    The other type, much rarer, works off the existing main fan control relay, watch those ratings, or an additional relay slaved to the main fan control relay. These need a run from the furnace unit to the booster. A considerable distance as otherwise a booster would not be needed. Also if these units are hard wired, as opposed to plugging in, you will need a disconnect mounted.

    These would normally tap off the furnace circuit, or dedicated circuit if it is really large, instead of any convenient, and legal, source. These second type units tend to be larger, more powerful, more commonly seen in commercial application and a lot more trouble to wire and install.

  2. DanH | Sep 29, 2004 11:59pm | #2

    You could always run 24V wiring back to the furnace and use a local control relay, assuming you have somewhere convenient to place it. If the furnace doesn't already have a 24V fan-on source you can either use a current relay on the fan motor wire or parallel a 24V transformer with the fan motor.

  3. MajorWool | Oct 08, 2004 12:56am | #3

    I have two of them and run them off the ductstat (R) controller. It is a box which mounts over a small hole in the plenum or duct and senses when either hot or cold air is being pushed through the duct by the furnace. For the fans, I screwed a small one-gang box to the fan housing and then connected to an extension cord from there, in both cases 18/3's which had just be replaced by new 12 gauge cords. I thought about trying to figure out the fan controller but then decided the risk of a big furnace repair bill wasn't worth it.

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