what is the determinant of leaving the old furnace in the crawl space?

One has to clean out the old and take in the new......less messes to look at and more space for the proper furnace to be stable. good luck.

Remove it.

No sensible furnace installer would abandon a furnace in a crawl space.

They need to bring it out of your house.

They or you or a scrap hauler can take it away from your premises.

He'd better not be charging for disposal then.

1. Is the new one going in the crawl space?

2. If not, is there a way to get it out?

The rats & roaches might be offended.

Get rid of it, parts can be recycled.

don’t be that guy who leaves junk in their house even if it is the crawlspace, get rid of it

if it isn't in the way, the installer's work is shortened if he abandons it in place. So he's likely to haul it out only if a) the contract requires it, or b) he gets paid for the scrap metal it contains

Unless it's dripping oil or full of soot, it really isn't going to bother anything except for taking up space. I would however, make sure it would be accessible to be removed later without having to dismantle the new one.

how hard ( expensive ) would it be to haul-it-out of there ........................