Barometric damper for single family home?

Your draft hood works like a barometric damper. It allows fresh air (boiler room air) to enter the draft hood and mix with the flue gases that exit the boiler.

When the boiler has been operating for at least one minute the following should occur:

A. The base of the draft hood should be very hot to the touch (like 300 degrees).

B. The outside "bell" of the draft hood should be a little warmer that room temperature (about 100 degrees)

C. The top of the draft hood where it connects to the smoke pipe should be somewhere in between.

If the flue pipe /smoke pipe is very hot above the draft hood, you may another problem. If you were to add a draft regulator, you might introduce more fresh air into the chimney, reduce the draft at the boiler or cool the flue to the point that is condenses. None of these results are desired. There is a reason the draft hood says "do not alter" or something to that effect. The boiler and hood were tested together and perform to meet the specs given by the manufacturer.