If your basement is too tight, you'll have to supply outside air.

You don't say if you have oil heat, but if you do and you use a Beckett burner, there is an airboot kit available to provide this air from the outside. They are very popular.

Both appliances use natural gas.

To top it all off, the weatherization guy suggested that when we need a new clothes dryer that we replace our Electric one with Gas. It doesn't seem that he is aware of combustion air and vent requirements.

-Thanks for the your input.

I know it has been a long time, but I think the calculation used is to determine if you do not need any make up air or not. If you are under that then you use the separate calculation to determine the amount of make up air for combustion.

For a single supply for combustion air it is 1 sqr inch per 3000 BTUs of your appliances. For 2 supplies, 1 within a foot of the ceiling and the other a foot of the floor it is 1 sqr inch per 4000 BTUs. For example a single supply 225,000/3000 = 75 sqr inches. A 10" pipe has 79 sqr inches.