Dome fans - converting exhaust to intake

[gsnz]

Hi all! I think i want to go with some dome-style fans in my van conversion. However, all of the dome fans i see are exhaust only.

Ideally i could buy a dome/can fan that does intake and exhaust, but i havent found this, so the next best hing would be two can fans that compliment eachother

Has anybody ever taken a dome exhaust fan and reversed it to become an intake fan? If so, does this require a lot of work?

 

[user 22017]

Check marine supply/store websites. Or add marine to your searches.

I don't know if computer fans are reversible, but I've seen posts about building your own ceiling fans with them.

I think ideally you want two fans anyway. (Even if you find a reversible one) One blowing in and one blowing out.

How to: https://www.hunker.com/13416073/how-to-reverse-a-vent-fan

Found a few that are reversible on Amazon. Here is one that says "this water resistant 4" solar powered ventilation comes with intake & exhaust blades.":

https://www.amazon.com/Marine-Solar-Powered-Stainless-Ventilator/dp/B00S1LTRGI/ref=sr_1_20?
Edit: Oops. It is solar powered.

 

[maki2]

The image below is an answerfrom the help comment section on Etrailer regarding putting a reverse direction switch on the Ventline brand of “dome can” style exhaust fans.

 

[tx2sturgis]

I have a Vanair fan that I installed on my pickup topper shell.

These small 6 inch fans do not use a propeller type of fan blade, which are capable of moving air in either direction, these use a centrifugal 'impeller'...which means that no matter which direction it is turning, it is trying to move air along the curved vanes and outward...centrifugal force being what it is.

They are very loud at normal speed, so I added a resistor on a switch that gives me a high and low setting, but reverse air flow is just not gonna work well...I tried it and they make a LOT of noise for very little airflow.

If you really need intake air from the roof its a good idea to look at the standard 14 inch or smaller 11 inch roof fans.

 

[wayne49]

Look around more, there are reversible fans out there.

Some less expensive than the fancy name brands.

What size roof opening? 11" or 14"?

 

[maki2]

Look around more, there are reversible fans out there.

Some less expensive than the fancy name brands.

What size roof opening? 11" or 14"?

The author is referring to one of the small, round, 6” diameter exhaust fans the type that is sometimes put into RV bathrooms to take out the smells. Not one of the large fans that get put into rectangular openings. He is correct that they are not sold as reversible models. But they can be wired to a switch that will allow a person to choose between intake or exhaust mode. However as the fan blades are designed at an angle for higher exhaust airflow the force of incoming air will never be equal to the volume of air moved when used for exhaust. But it will work in reverse with the right style of switch without rewiring the motor itself. Pretty cool to have the option of choice of direction as needed!

 

[user 22017]

Ideally i could buy a dome/can fan that does intake and exhaust, but i havent found this...

Found one, but it is not cheap. Plus, the store is in the UK (shipping charges).

Ultra low profile motorised van ventilator with an external height of just 64mm, low enough to fit under most roof mounted carrying racks. This motorised vent for vans features a reversible motor with suction / blowing action and is suitable for applications such as a van, bus, caravan etc. It is also suitable for use with animals and some light gases. This low profile motorised ventilator for vans is available in a choice of 12v and 24v.

https://www.incarconnections.co.uk/...an-ventilator-for-dog-van-white-fan-only.html
Oh geez, they have numerous low profile vents:

https://www.incarconnections.co.uk/van-ventilation-van-roof-vents-van-vent.html

 

[eDJ_]

....would it be possible to turn the fan blade over (with some modification(s) so it would draw in outside air ? (of course this could be a hassle and wear the hub of the fan blade to where it may wobble or not stay on the motor shaft after awhile)

I think what maki2 posted would be the ultimate solution if the fan motor would accept it. I would only caution to use a "good quality" switch.

I knew an Electrical Engineer (who has passed on) who demonstrated for me the reversing of a 12 volt auto heater fan motor. It ran in one direction a bit faster than in the other. Something to keep in mind.

 

[FlipperFla]

If it is a DC fan all you would have to do is reverse the polarity using a DPDT switch. Have the fan on the middle two posts , Normal polarity on the left outer two posts an reverse the polarity on the right two posts. Flip the switch one way flip the switch the other way for the other direction, in the middle would be off.

 

[jasper]

^^^^ Good one. Crisscrossed outer legs.

IF the dc motor can run by reversing the DC polarity, one of these switches can reverse the polarity. Maintained position (not Momentary) is essential:

https://www.amazon.com/IndusTec-Pos...ix=maintained+reversing+switch,aps,94&sr=8-14
Brushless DC fan motors are iffy. Some can, and some cannot be reversed by polarity.

My most crude method is an easy to flip around fan.

 

[tx2sturgis]

Hey guys, the Vanair fan uses a cheap hobby motor with brushes...it is not a brushless motor.

And also, it uses a centrifugal impeller, not an axial fan blade.

This is a pic of the one mounted in my pickup topper.

 

[eDJ_]

That fan is almost like a bathroom Vent Fan.

So....the centrifugal fan will be effective in either direction ?

(my bathroom Vent Fan is rated @ 50 cfm or cubic feet per minute)

 

[tx2sturgis]

Spinning in the normal direction it moves a good amount of air, but in the reverse direction it struggles to move much air at all...but it does make a lot of noise.

 

[tx2sturgis]

Just FYI, about a month ago I bought an 11" 3 speed reversible roof fan that will replace this one...when I get around to it... (yes, cutting will be involved!)

 

[minanimal]

FlipperFla is correct. That's a a cheap and easy fix if you have played around with wiring. If a double pole - double throw switch is installed, and it doesn't suit or it doesn't work well, just leave it in the original configuration. Also, it would be very easy to temporarily wire it "backwards" to check it out.

​

 

[user 29855]

If the motor has brushes, it won't appreciate being run in reverse. If it's been run in the original direction for a while, the brushes take on a "shape," and reversing rotation can cause portions of the brush to fragment. This can also damage the copper segments of the rotor.

 

[tx2sturgis]

These dome fans use a little 'hobby motor'...the 'brushes' are just little strips of brass or maybe copper. They are just 'toy' motors...except these run on vehicle 12v, instead of d-cell batteries like they would in a toy.

But again, the centrifugal blower design does NOT work in the reverse direction.

 

[maki2]

That fan is almost like a bathroom Vent Fan.

So....the centrifugal fan will be effective in either direction ?

(my bathroom Vent Fan is rated @ 50 cfm or cubic feet per minute)

You have discounted that it is not just about reversing the motor direction. The shape of the fan blades are often engineed to be optimal for moving air in just one direction. So that is why unless a fan is designed to be reversed you will typically see very decreased air movement after modifying a fan motor for reverse direction. “Think like an engineer”. Meaning you have to include all the factors of physics when creating products including aerodynamics of the shapes of surfaces that are moving through the air or in this case are trying to move the air itself.