NWS,
Based on your location, I was guessing that it would be one of the garter or ribbon snakes. However after looking at
http://cfs.nrcan.gc.ca/subsite/glfc-amphibians/lizard-snake , I see that it could be something else. Glue traps can be very effective for some smaller snakes and large glue traps can be purchased for larger snakes. I see that some of your resident snakes are protected by law or regulation. So caution should be exercised.
Funnel type traps are very effective for snakes. Close all of the holes going under the AC unit except one. Use hardware cloth, metal flashing, boards, or back fill the area. Set the funnel trap over the remaining entrance.
Of course the snake may no longer even be there. It may have left five minutes before you get there. There may be other entry points around the house. It may not even leave the house. If there is adequate food and water available, it may stay there for months or years. I have seen many shed skins in some attics and crawlspaces. The snake(s) had been there for some time.
Most of the homes, around here, have so many potential entry points that I do not offer a warranty on snake exclusion. Some of the rat snakes can climb a brick wall. Any hole anywhere on the house can be an entry point.
Ron Fry