I just had a new boiler installed (New Yorker CG-6) and it works great with one exception. When the boiler first starts up and the automatic vent damper opens up, cold air comes racing down and out of the draft hood, after a few seconds, really hot air comes blasting out. After a few minutes, one of 2 things happens: The chimney gets warm and the boiler operates normally with the draft hood cooling off and no exhaust spills out OR the hood gets too hot, and the blocked vent switch trips. Why is this happening?

You have a severe negative pressure in your home. The chimney is acting as a air supply when the boiler is first started. When the chimney warms up it causes a updraft condition. You need to resolve the negative pressure situation. Contact the company that did the install. They should have fixed this situation.