Actually, 20-28 is WAY too hot. It will produce some weird flavors. Cooper's yeast is bad tasting enough as it is (really "woody" tasting) but at too high a temperature it's even worse. If you're using a different strain, some are really terrible above 20C.
Ideally, an ale would be fermented at 16-20C, with a nice steady temperature and no fluctuations.
Remember that fermentation itself produces heat, and when I want to ferment at 17.8C, I keep my ambient temperature at about 16C. If your room temperature is 15C, that's actually about right for most ales.
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