The gate is operated by the TEMP control and opens and closes with the movement of that control. If it is a KNOB then the control is lectric.. If it is a slidy thingy then most likely the gate is vac controled.
I have seen them fail in many ways.. Some always open and some always partially open and others always closed..
That damn gate motor seems to be the weak link in most modern heaters.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist