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!
"Why must it always be pandemonium?" - George Mueller/Nelson Van Alden
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drinking: Wojtkowiak Piwo, CLB's Barleywine, 8Hearted Pale Ale, O'Rob's Dry Irish Stout, DB8PT Session Ale, Wojtkowiak Grodziskie - bottle conditioning: Otto M. Gourd Pumpkin Barleywine, Jewel Thieves Apple Wine, CLB's Barleywine 1.2