Installing a booster Fan

jerjones1November 2, 2009

Hi all,

Here is my project that I'm seeking some assistance with. I want to install an inline booster fan to one of my furnace runs. I'd like to wire this fan so that when the furnace blower motor comes on so does the inline booster fan. I found this post while looking for suggestions:

"The best way to safely hook it up is using a relay that is hooked to the motor winding. Start by purchasing a 120 volt relay. It needs to be a minimum of 15 amps. You can buy a single throw single pole relay.

Hook the relay control solenoid across the motor winding which will energize whenever the blower is on. Then hook the one side of the vent fan motor directly to the 120 volt AC supply you purchased."

Anyone have any Idea of how to do this or a diagram that would make it easier for a novice to attempt this? Thanks


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Need more info. how large is the booster do you know how many amps the motor is. Are you using your furnace on one speed only, or do you have air and it goes to a higher speed on air?
If you have a multi speed blower and you use different speeds ,then you cant hook a relay to the motor speed wire. If you do you will burn up you relay. Do you have a electronic board that runs your furnace and blower? later paulbm

    Bookmark   November 3, 2009 at 12:58PM
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A pressure switch would be much easier and require no control wiring.

If you wanted to control it via 24volts you could just use your existing thermostat leads to a contactor, or a duct mounted thermostat.

    Bookmark   November 3, 2009 at 9:05PM
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I like what your suggesting, but am not sure exactly what you mean by a contactor? I have looked into the duck mounted thermostats and they are not very reliable or effective from what I've read. I had a similar thought that since the fan doesn't take very much to run could you just run it off your existing thermostat? Here are the fan specs and maybe you could fill me in a little bit more on what your thinking.

Fan specs:

* Air Volume : 160 ft³/min
* Amperage : .35 A
* Assembled Depth (In Inches) : 6 in
* Assembled Height (In Inches) : 6 in
* Assembled Weight (In LBS) : 2 lb
* Assembled Width (In Inches) : 6 in
* Brand/Model Compatibility : Inductor 6" In-Line Duct Fan
* Color / Finish : Gray
* Energy Star Compliant : No
* Heat : N/A
* Voltage : 110 V
* Watts : 30 W

    Bookmark   November 4, 2009 at 11:05AM
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The previous owner of my house had a booster fan installed in the main duct in the basement. It was a simple pressure switch. Basically it is a spring loaded flap that is inserted into the duct. The flap closes a 120 volt relay to turn on the fan whenever the furnace is pushing air. The fan can be mounted anywhere downstream in the duct. It is simple to wire.

I had the booster fan removed after I had my new furnace installed. It was noisy and not very effective.

    Bookmark   November 4, 2009 at 11:33AM
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that is called a sail switch. DO YOU HAVE A ELECTRONIC BOARD THAT RUNS YOUR FURNACE? The reason i ask this question is.
If you have a electronic board there will be a eac. and a Hum. post on your board one of those will be to run 110 volts and maybe enough amps to run you booster fan. they are only hot when the blower is on. Or you can use one of these post to snap a relay. later paulbm. Ps i don't think they make the sail switch any more

    Bookmark   November 4, 2009 at 10:17PM
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Duct booster fans are noisy.
For about the same money you will be better off increasing the duct run size or adding a second.

    Bookmark   November 4, 2009 at 10:23PM
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To achieve what you're trying to do, upsize the ductwork.

Don't even bother with booster fans. In many ways it's like a garden hose. You can use a larger pump but there's only so much water(air) you can move through a hose (duct). Traditionally contractors don't do ductwork right, using the smallest they can get away with rather than calculating what's needed.

    Bookmark   November 5, 2009 at 2:15AM
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If you do run a booster fan I would run the fan control through a light dimmer switch so you can adjust the fan speed/ noise or shut it off all together. My parents have one to the front room so when the wood furnace is in use it pushes a little more air to the living room that used to always be too cool but summer temps were fine. It is run off the lead for the wood furnace blower and from there to a dimmer switch by the furnace and then to the blower. It only runs about half speed and is not noticeable on noise unless on full speed which is obnoxious. It makes a big difference but only is active with the wood furnace.

If you need more air in the room summer and winter I think I would opt for increasing the duct size or adding some ducts.

if the room does not have a return then adding one or increasing the size of the current one can make a difference also.

There are so many variables that figure in to making the best decision. I would start with a infrared camera run through. I solved one rooms problems by taping the edge of the carpet at the baseboard and foaming under the base trim. I had huge leakage and a can of foam solved it. I was about to add a duct to the room which would have made the room warmer but not solve the problem.

Before adding air or modifying the system take the time to make sure there is some other problem that can be remedied to solve it.

    Bookmark   November 5, 2009 at 9:02PM
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