Is the house 21 years old or is the nook 21 years old? (If you have a breakfast nook that was 'added on' when the house was first constructed 21 years ago, then it's not an addition, eh? The nook is part of the original construction, no?) Sorry, Don, I'm not sure how much this makes matter, yet, but along with the other questions below, please clarify this. (It may be 'useful' ...)
How big is this crawl in terms of height? You have a furnace under the crawl? Or are you saying that you only have access to the crawl through an opening that's in the room your furnace is in?
What type of furnace do you have? (Gas forced hot air, Oil, hot water boiler?)
Where and how is the furnace vented? (Outside, into the crawl, through the crawl then to the outside?)
Hold off on putting in any ventilation for now. How 'bout wild animals? Could they be 'nesting' in there?