Ducting, Air Circulation and Ventilation Systems - Ducting for Floor Furnace in

I'm thinking of closing the vents and insulating the walls in my crawl space. If so, I assume I would need to provide ducting to get combustion air to the furnace. Underneath the house, the furnace looks similar to that in the picture linked below. How do I know what size ducting I would need to supply enough air? Also, would the ducting have to connect to the bottom of the furnace and be airtight, or would it just have to be close enough to ensure air will get sucked in from the outside?

http://us.st11.yimg.com/us.st.yimg.com/I/lowenergysystems_1985_840285

A louvered opening on one of the crawl space walls should suffice
http://i261.photobucket.com/albums/ii64/pflor_bucket/CombAir1-openingtooutdoors.jpg
(pic. & table above would apply here too)

I'd be careful about condensation. Just like a cross ventilated attic where ventilation helps prevent condensation.

A louvered opening on one of the crawl space walls should suffice
http://i261.photobucket.com/albums/ii64/pflor_bucket/CombAir1-openingtooutdoors.jpg
(pic. & table above would apply here too)

I'd be careful about condensation. Just like a cross ventilated attic where ventilation helps prevent condensation.

Have set up many of the floor furnaces. With it in the middle of the home. Then cut cold air floor grills in all four outside corner rooms. This lets the cold air go back to the furnace in the crawl space.

Go to http://aboutsavingheat.com/crawlspace.html
This will show you how to do the crawl space.