reversing fan air flow

I bought a pair of attic fans (one roof, one gable) with the thought of having the gable fan suck in air from outside to replace the air exhausted by the roof fan (soffit vents are inaccessible with shingles in place and were covered with insulation by previous home owner). Spoke to fan manufacturer about doing this and he approved of the idea. Suggested I simply mount the gable fan backward to have it pull air into the attic. That is a challenging solution as it means either mounting the fan outside the house (ugly) or building a lot of carpentry inside the attic where it is hot.

Looking at the fan motor, it is 115V 1 phase but reversible depending on how external wires are connected. Before I actually try that however (bit of work as fan company made permanent connections, not nice wire nuts), I would like to ask if reversing the motor rotation will make the fan move air in the opposite direction given the fan blade I have. It has 4 sheet metal blades, mounted at close to 45 degrees from the plane of rotation. They are nearly flat, but not quite, with the slightest bend in the center (they present an slight convex surface to the incoming air and concave to the exiting air).

Thanks.

If it is 1 phase... changing the external wires, i.e. reversing the 115v power cord will not cause it to reverse.. the internal wires to the windings need changed.. depending on the motor, it could either be very easy or very difficult to get it done properly... if not done properly, it could very well be a major fire hazard... would recommend you take it to a motor rewinding shop that can do it properly..
Can you find a fan blade that is the reverse of the one on it ???

If it doesn,t have a start switch attached to end bells, you can reverse the rotor, by swapping end bells. I did this on condensing fan motor on my heat pump / air conditioner. It only lasted twelve years before bearings wore out.

Ed: The motor has wiring diagrams on the side for CW and CCW rotation. Also wiring diagrams for high speed and low speed. A total of 9 wires. Definitely 1 phase, 115V, clearly marked on label.

Bruce: Do not understand what end bells are. Has high speed hot/neutral leads prewired connected to thermostatic switch. What I connect to thermostat is up to me. Low speed hot lead is capped with wire nut.

Yes if you change the motor rotation the air movement will flip as well.

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Originally Posted by Phil Thien

Yes if you change the motor rotation the air movement will flip as well.

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With the fan blade I have? That is my real question. Thanks Phil.

If you're concerned about the fan, if you can take the fan off the shaft and flip it, it will then be doing what it was intended to do with the opposite rotation.

Most of those fans are pretty simple. I expect it will work fine in reverse.

Mike

I apologize if this is a stupid question, but why do you need a gable fan instead of just a vent to replace the air from the roof fan?

Reversing the fan motor will re-direct the air flow, but at a reduced rate because of the curvature on the fan blade. The air move best from convex side of fan to concave side of fan. Perhaps maybe you can pull fan of motor shaft and flip it.

Quote:

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Originally Posted by Bill Orbine

Reversing the fan motor will re-direct the air flow, but at a reduced rate because of the curvature on the fan blade. The air move best from convex side of fan to concave side of fan. Perhaps maybe you can pull fan of motor shaft and flip it.

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Right but just to be clear, you have to reverse direction of the motor as well as flipping the fan prop. Flipping the fan prop only returns efficiency lost by spinning the prop in the wrong orientation.

I only bring this up because some people think flipping a fan prop alone is sufficient, but is not.

I simply don't understand why you want hot air in when you're trying to get hot air out. The heat from the roof will just be sucked into the house from the roof unless you're getting air from under the soffit.

I went ahead and did it. Reversed the rotation and flipped the blade and it seems to work quite well sitting on the patio. Thanks for giving me the confidence to do it.

To answer other questions. The exhaust fan required an excessive amount a passive intake vent square inches. I called the fan manufacturer to discuss it. We agreed the the same CFM rated fan over a smaller intake would handle my needs. I will be drawing in hot air to replace superheated air, i.e. in mid afternoon I am pulling in 90 degree air to replace the 140 degree air inside the attic. In early evening I am pulling in 75 degree air to replace something still above 90 or 100. Sounds like worth doing to me. Soffit air would be the same temperature (as gable fan air) IF they had not been blocked by the previous owner.

Understood.
Thank you very much.