It is likely the room temps are too low. Back heat is not very efficient as much is lost to the air it is likely you hit the wall as they say heat wise. Generally racks do best I room temps at or near 80. I played with back heat and found that the room temp needed to be no cooler than 75 to hold temps correctly. I use belly heat I gain a few degrees but you still need to hold correct ambient temps. 75 dead low most say warmer. Heat tapes don't heat air well so the ambient air temp still needs a second source anyway.


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