Even in summer it is always good to put a cooler on. I don't like flannel coolers - they take much longer and aren't as nice and durable as wool coolers. On top of that, they take longer to wick away water. Horses who wear coolers need to be walked around until dry or left in a stall supervised. Most coolers don't come with all of the clips that blankets do and they are not as durable - so if you leave your horse alone in one and he decides to roll you might find yourself in a bit of a pickle. Other then that, there are really no "rules" about using coolers - if your horse is wet, then throw one on! I use them all the time. The only rules I can really think of are that the cooler should be 100% dry between uses, and to never use fabric softener with your cooler. The ingredients in the fabric softener take away many of the natural fibres used in wool to wick away moisture.
Ok - thank you - and I think I have the flannel cooler LOL!!