Disgo, the reason you can leave them out when it drops to 47 is because the length of time the air is at that temperature is very brief, so it won't affect the orchids to any extent. However, when the nights start consistantly reaching 50-55, the length of time it's that low will increasingly be longer and longer. You're correct, the temp should not drop below that for phals. A couple of weeks, three's better, of cool nights should do the trick.