space heaters will work if you have a rack system. Since heat travels up I would suggest putting it on the floor about 1 foot from the rack. if you have more than one rack space it in the middle to a max of 3 racks. Get one that oscilates; turns back and forth, and has a auto temperature adjustment.
Of course my advice is based on a small space heater covering racks that are about 2 yards across and 2 yards high. And the area is basically closed off and no heat is wasted, meaning that the heat will get to a certain temp and then cut off until the floor area reaches a cold level to turn it back on.
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