Just wanted to update this thread and let everybody know the openers are working great on my 14' doors. They also have a very good range and are easy on the doors. Very impressed with them.
I have a LiftMaster 8500 on my overhead door, it's 12x12, installed 2 years now by a door company that I wouldn't 1 star out 5. Less then a year the door started acting up. For some reason that I haven't figured out yet, the cables kept getting slack just when the door starts to come down, setting off the cable tension monitor, the door would move about 2" then reverse, but when the door does come down, the cables stay tight, it's just at top when the door starts to move.
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The only thing that starts the door moving down is gravity. Raise the far end of the track a little bit and oil the roller bearings. Also clean the track where the rollers roll, there could be dead spiders or other bugs keeping the rollers from rolling easily.
If the spring is adjusted correctly the door will stay up. Unhook the opener and pull the door down about 2 or 3 inches, see if you can feel it binding.I was thinking about raising the track a little, but not sure what the incline pitch guide line measurement is on 12' of horizontal track. There should be something to go by, cause the door is suppose to stay up when the door opener is unhooked. Leave it to cold weather to bring out problems.