Having trouble keeping the cool side cool

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I'm not trying to be a jerk but under heat is the best heat source and I always strongly advise against lamps. I'm not even a fan of back heat in racking systems. You really do need a way to regulate your UTH, I've stopped pushing people to buy herpstats and VE systems but you can get a inkbird controller on amazon for like $40, it sounds like it might solve your heat issues.

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And an unregulated UTH can quite often lead to burns on the animal which is a whole can of worms no body wants to open. In the mean time you can purchase a dimmer switch for like a lamp at a hardware store for under $5. It's not a solution but it will hold you over until you get your thermostat set up.

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