Exhaust fans.

[Jerry Delaney]

Is there anyone out there who has any experience in using an automotive electric cooling fan in a greenhouse? Some are rated at 2500 cfm at 0sp. If so, what did you use as a power supply?

[Ray]

Interesting idea. Knocking 120V down to 12 is no big issue, but do you know what kind of amperage they draw?

[Jerry Delaney]

Originally Posted by Ray

Interesting idea. Knocking 120V down to 12 is no big issue, but do you know what kind of amperage they draw?

They are pretty variable, Ray. I have seen them from 10" to 16" and from around 1000 cfm to 2500 cfm. All are 12v DC and amperage from around 7.5 amp on the smaller ones to 12 amps on the larger ones. All of the motors are totally enclosed and since these things have to operate in all kind of conditions, I wanted to try them. I was able to purchase 3 @ 2500 cfm for ~$21 each. I converted a PC power supply that was 12v DC & 14 amp but the supply kicks out on overload protection. I imagine that these motors might pull a lot more than 12 amps until they get up to speed. The power supplies I purchased were only $10 each so maybe I was being too "frugal". I would appreciate any thoughts you might have on the subject, Ray.

[John D.]

If you think that the startinig load is drawing too much current and triping the over load try blocking the air flow on the inlet side as much as possible and spinning the blades so they are moving when the fan is turned on. A fan motor draws current based on air flow. If you block the outlet then pressure goes up as flow goes down motor load stays the same. If you block the inlet the fan has less air to move and no pressure against it so motor load goes down.
As an aside this is why shutting heating /cooling vents in unused rooms dosn't save as much as you expect. The cooling load goes down but the blower is discharging against a higher pressure.

[Jerry Delaney]

Thanks for the info, John. I'll give it a try. I haven't worked much with DC in years. Would putting a capacitor in line help?

[Ray]

Note: No coffee yet, so I haven't thought it through, but is it possible to power 1 fan with 2 power supplies to halve the current from each?

[Jerry Delaney]

Originally Posted by Ray

Note: No coffee yet, so I haven't thought it through, but is it possible to power 1 fan with 2 power supplies to halve the current from each?

OK. After smoking a couple of 400 watt pc power supplies, I have changed plans to one I feel will work. Initial cost to operate two of the 12 vdc 2500 cfm fans will be ~ $150 total (including the cost of the fans). My existing 120vac thermostat will supply power to the coil of a 30vdc 30 amp dc switching relay. Power for the fans will be supplied via a 35 amp deep cycle battery. Battery will be charged using a 12vdc 2amp charger which will run 6-8 hours each night. Later I may try using a solar panel to charge the battery during operation. Does this sound like a workable plan?