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Old 07-18-2016, 06:51 AM
David_Nelson's Avatar
David_Nelson David_Nelson is offline
Join Date: Feb 2006
Location: Austin, TX
Posts: 442
Default Evening Project: Put that old HVAC / Furnace squirrel fan to good use

With summer heat solidly upon us, a bit of moving air can sure help. If you've an old squirrel fan from a HVAC / furnace lying around (or know of somebody who's getting their HVAC/furnace replaced), here's a fun little project to convert the blower into a handy and portable fan. Just to clarify, this is not the fan that sits outside in the condenser unit.

Depending on the blower design, you may have to build some kind of a support to keep the blower stationary. For the one pictured below, I built a cradle out of some scrap cedar pickets and 2x4's. The 2x4's were ripped down the middle and bevelled to increase the friction between the cradle and the blower. The entire cradle assembly is glued and screwed. The blower wedges itself (there's about a 3/8" gap between the bottom of the blower and floor of the cradle) and it's surprisingly stable. With the cradle, I can rotate the fan from about 30* downward (great for when working under the RV) to straight up (great when hanging those new lights in the hangar).

I like adding some chicken wire, an on/off switch, a couple of plugs, and a carrying handle. I'm thinking of adding a SPST On/On switch to select between High and Medium fan speeds as High can be little noisy (but it sure does blow some air!).

As for wiring things up, there should be a schematic either on the motor or the blower housing. Follow that and you should be good. If you're not comfortable, ask around to some of your buddies for some help.

Some motors have bearings that require a bit of oil. The galleys are usually capped with small plastic caps, located on the motor's end caps. The plastic caps pull right out. It's probably been forever and a day since they've been touched so a couple drops of light machine oil (3-IN-1) is not a bad idea and then replace the caps.

Secure any loose wires (which I've yet to do on this one) and make sure all screws, nuts, bolts, etc are tight.

As most things in life, there's a trick to these things. You can't just wire it up, turn it on, and let it go. Doing so runs the risk of the motor overheating and possibly failing. Of the ones I've repurposed, I've always had to choke the exit a bit to get the airflow up and amperage draw down. Sometimes there are small dimples on the exit flange that are good guides other times it takes some experimenting by placing a small piece of plywood / board / aluminium sheet / etc. Just be careful of the moving parts. Once the proper exit size is determined, permanently affix the material of choice in place. In this example, I used a piece of aluminium left over from an elevator skin that I borked.

And there you have it - for a few bucks, you've got a fan that is portable, blows a good breeze, and can be positioned to blow in just about any position.

RV-7A Tip-Up
NW Austin, TX
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Old 07-18-2016, 08:24 AM
Bob Martin's Avatar
Bob Martin Bob Martin is offline
Join Date: Jan 2005
Location: Charlottesville, Virginia
Posts: 1,278
Default Fans

Nice pic and write up.
I too have done several of these.
I like to install fixed to a HF dolly. Your adjustability seems good though.
The fans I have used had several different colored wires coming out of the motor and it seems each one is for a different speed. So I connect power and use an Amp probe to see the current draw and label wires according like, low, med 1, med2, high....with associated amp draw. With out an amp probe, one can just listen and feel the speed to determine low to high. I use male / female end connectors and can select the desired speed for the use.

I am curious about your choking off the exit air to get a more desirable cfm's or amp draw? I have not done this and the fans seem to run cool and blow lots of air, so if I can or should modify this I would like to know more about it.
Bob Martin
RV-6, 0-360 Hartzell C/S, Tip up, 1200+TT
James extended cowl/plenum, induction, -8VS and Rudder. TSFlightline hoses. Oregon Aero leather seats.
D100-KMD150-660-TT ADI2- AS air/oil seperator. Vetterman exhaust with turndown tips.
Louisa, Virginia KLKU N94TB
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Old 07-18-2016, 09:37 AM
Marc Bourget Marc Bourget is offline
Join Date: Apr 2009
Location: Stockton, California
Posts: 338

David said: " Of the ones I've repurposed, I've always had to choke the exit a bit to get the airflow up and amperage draw down."

I'm confused and/or missing something - so the request to "eddycate".

I know there is a factor called "fan curve" that plots the output of fans. But other than it's existence and some intuitive sense, I'm a confessed ignoramus.

My sense prompts me to question "airflow" especially when associated with current. As I see it, an unrestricted fan would flow the most mass and load the motor more. Are you restricting the exit (which may increase the airspeed) to lower the mass through put and therefore the "work" the motor must perform?

Gosh, I appreciate all the knowledge that visits this forum!

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Old 07-18-2016, 12:46 PM
JKevin JKevin is offline
Join Date: Aug 2015
Location: West Liberty, Kentucky
Posts: 112

Originally Posted by Bob Martin View Post

I am curious about your choking off the exit air to get a more desirable cfm's or amp draw? I have not done this and the fans seem to run cool and blow lots of air, so if I can or should modify this I would like to know more about it.
These fans (meaning the blower wheels) are meant to blow against a certain amount of static pressure (generally about 0.5" water column pressure). If this pressure is too high or too low, the blower assembly will have a shorter life. High static pressures cause the motor to draw too many amps and overheat, low static pressures tend to make blower wheels fail prematurely.

Probably not an issue with this application, but that's why
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Old 07-18-2016, 01:00 PM
219PB 219PB is offline
Join Date: Aug 2014
Location: Victoria, Tx
Posts: 519

I will add a little clarity. All fans, pumps, etc operate on a curve. Choking the discharge will decrease the amp load. Opening the discharge draws more current.

Leaving the discharge unobstructed could result in the motor drawing too much current. The only way to know for sure is to check the actual motor current against the nameplate.

On the other end of the spectrum, if you totally block off all of the air on the discharge, the motor current will be at its lowest but could eventually overheat the motor due to lack of airflow for cooling.
Paul Bonorden
RV-6A Flying
My Builders Website
Victoria, TX
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Old 07-18-2016, 01:57 PM
artrose artrose is offline
Join Date: Jan 2011
Location: San Antonio area
Posts: 86

Lots of folks do this with old blower motors and most get away with it, but they should understand a couple things when they do this. Forward curve direct drive fan/motor combinations like this are normally designed assuming a discharge restriction, (i.e. ductwork). When you remove the discharge restriction from a forward curve it will take the advantage to move more air. Moving more air means more work is required from the motor, and motor amp draw will increase. Increasing the amp draw too much can cause the motor to overheat, or eventually cause it to fail for other reasons. (a possible fire hazard.) This usually doesn't happen with these fractional horsepower applications, but it can. The safe way to do this is to know the max amps and service factor from the motor nameplate. Using your amp clamp, make sure you are not drawing an operating current beyond the service factor of the motor. The wire color standard for single phase multi speed motors is Red for low speed, Blue for medium, and Black for high.
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Old 07-18-2016, 03:40 PM
Marc Bourget Marc Bourget is offline
Join Date: Apr 2009
Location: Stockton, California
Posts: 338

Kudos to J, Paul and Art for the excellent explanations.
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Old 07-18-2016, 04:16 PM
Scott Hersha Scott Hersha is offline
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 2,018

I used a smaller version of this, from a JennAire cooktop, in my Homebuilt basement paint booth (for priming - don't get started!). It worked great and there was no smell in the house. Exhausted through the sill plate to the outside.
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
RV6/Used kit purchased 2021 building
Cincinnati, OH/KHAO
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Old 07-18-2016, 05:58 PM
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Plummit Plummit is offline
Join Date: Apr 2010
Location: SoCal
Posts: 1,390

OK, a little clarification is probably due right about now. There are two types of what is known in the trade as an "indoor fan motor." The oldest type, and what is shown in this thread, are multi-tap, PSC (Permanent Split Capacitor) motors.

There is however a newer technology called an ECM motor. ECM motors are brushless motors that require a controller to run them. The controller is programed for the unit, and usually ramps-up to speed with the advantage of being able to vary the speed depending on the needs of the system.

I don't advise trying to use an ECM motor as a fan even if you also have the IFC (Integrated Furnace control).


2021 Donation Made
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Old 07-18-2016, 06:57 PM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,320

I repurposed mine with two high efficiency 20x20 filters. One on each side to keep the shop dust from immigrating to the rest of the house.

Nice post for the thrifty builder.

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