Will a "vacuum" be a problem if I keep the small fill-lid off to run a heater through top of my 5-ga

[RhodyReds]

http://www.amazon.com/gp/product/B0...n waterer&qid=1451934798&ref_=sr_1_26&sr=8-26

I just ordered this and I am concerned that if I drop a corded heater into that hole that the waterer will leak because the lid is off. Does anyone have any pictures of how they keep this heated?

also what is better, under-bucket nipples or horizontal ones?
Thanks

 

[TalkALittle]

You don't need or want a vacuum with that waterer. The nipples only release water when depressed.