let me not step on Gregs thread here --- but its better to have it fail open -
if it was failing to open - the appliance should never fire as the proof of draft would also fail
There are lots of these sort of permanently / accidentally "stuck" open. many are only installed with other upgrades, to meet NG providers standards of conservation in equipment - others are installed as simply a good idea - until they don't work. Every upgrade nets extra installer profit also.
I know Greg will be around - but if I may the damper functions when it receives a call that the appliance wants to awake to for hi fire. This signal in interrupted, ( by the closed damper) until the damper motor gets the door fully open and sometimes draft is proven also. Then - after the draft is flowing, the signal is allowed to excite the burner - and the appliance should spring to action.