Installing / Wiring Booster Fans to HVAC


Old 08-28-05, 01:57 PM
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Question Installing / Wiring Booster Fans to HVAC

I have a question. I am trying to install booster fans in my attic to help circulate air better through our vents. I want these booster fans to be controlled by the controller for the main blower in the basement. I can run a wire from between the attic and basement, but how do I hook them up?

I have an HVAC system with a dual speed blower. It doesn't look from the conroller board that there is already an output for another fan(s). I have found out which output from the thermastat controlls the blower and it is a ~27V AC output that is high when the blower is on, and 0 when it is off. I believe that I should be able to just connect this to an appropriately rated relay and the unswitched power and then have 120V AC switched with the blower. Is this right? Are there any special considerations?

Of course, I may be able to just connect it in with the actual output that goes to the blower, but I am not sure if that is wise.

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Old 08-28-05, 02:48 PM
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I did a quick Google check and it appears that they usually hook up either directly off the blower fan feed or a separate 110 source thru an in line thermostat which either senses temp change or air flow (the site I looked at said air flow, which is obviously different than a temperature change) . Of course they cover any liability by saying that you should consult local codes etc.
Old 08-29-05, 10:43 AM
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Consult the "experts" at an HVAC supply-house. You may be able to install a "sail-switch" responsive to the air-flow that circulates when the "main blower" is operating.The "sail-switch" can control a low-voltage switching-circuit for the 120 volt motors.
Old 08-29-05, 01:49 PM
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be wary of hooking anything directly up to the fan motor outputs until you have determined exactly what type of voltage is produced. I too have a two speed motor in my furnace, but it is a DC motor. A lot of newer furnaces use a small DC drive and motor because it is easer to control the speed. Just be aware of this.
Old 08-29-05, 02:50 PM
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When you say booster fan"s" I can't help thinking that you may have come up with a bandaid solution to a more serious problem.
I might assume that you are trying to increase the overall output to all or most of the areas this unit supplies.

I might suggest that before you consider these add-on fans you come back with a detailed description of your equipment and your problem.
If you are dealing with a unit that is too low in capacity the fans will not do much.
If trying to deal with inadequate airflow, a more long term solution may not be that difficult to achieve.

My experience with booster fans is mainly in removing them from systems. When they burn out or sieze up they impede what little air flow there was before being installed.
All these little fans have sleeve bearings and have a very short life compared to the average furnace fan motor.
Old 08-31-05, 08:29 AM
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Post My Reason

The reason/background for installing the booster fans is:

I live in a 3 story (includes finished basement) townhouse with the HVAC unit in the basement.

There is one room on the top floor that is always very cold in the winter. The rest of the house is fine, but that one room is very cold. This means it is a problem of getting the air to that room, not of total output of the heating unit. I do adjust the vents, but this just isn't enough. When I go to that room and feel the air coming out, there is hardly any airflow. When I go into the attic and see how the HVAC ducts are routed, I find that the duct for this room is at the very end and farthest from the HVAC unit and it seems that the air is just not getting all the way to the end of the line. I thought then that a booster fan would help. I have heard of other people in my neighborhood who have made similar changes and have said that it has made a big difference.

Thanks for your advice!
Old 08-31-05, 09:24 AM
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I'm also interested in hearing how an HVAC person would solve this. I have the same problem. Ten years ago, an HVAC person tried to address this by installing a sheet of metal inside one of the ducts, but it didn't help much. He did assure me at the time that the furnace was appropriately sized for the house. So I've always just addressed it with a small 500-watt space heater in the cold room. I might move this over to the HVAC forum, but since we started it here, and Greg has already posted here once, maybe I'll just leave it here for a little while anyway.
Old 08-31-05, 05:52 PM
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Red face Sorry for hijacking this thread.

What started out as an electrical question turned out to be a ductwork answer.

What you would do to solve this with a ductwork change would have the same effect as installing a booster fan.
You would need to increase the size of the duct that supplies the problem area and increase the volume through this duct by increasing the pressure at the furnace plenum.
You would increase the pressure by reducing the volume going to other areas and/or increasing the volume of the fan.
Increasing the volume of the fan would involve either increasing the fan speed if the specifications allow or replacing the blower assembly with a larger capacity one.

Sometimes it's very tricky in an existing home to solve a problem that should never have occured had the home's hvac been properly engineered.

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