i have the extreme 70 quart. i noticed the same problem, the lid won't stay closed when heated. it stays shut when I put the probe inside to track the mash temp, the wire helps keep it closed.
first time i used it i had problem with keeping temp. a lot of headspace in a 17.5 gallon tun and only 5 gallons of strike water in there. so i just roll out a layer of heavy duty aluminum foil on the top and now only lose one degree
and brettwasbtd: GREATEST AVATAR EVER! HTTR!
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drinking: Otto M. Gourd Pumpkin Barleywine, Jewel Thieves Apple Wine, Fresh Squee Zed IPA - bottle conditioning: LoCo Foot Barleywine, Basque cider - lagering : Schwarzbier - fermenting: apple wine, Skeeter Pee - on deck: Grodziskie