Is this a gas pipe or water pipe?

[Lisa Simkins]

Newbie here. Is this gas pipe? Special inspection for owner trying to trace water leak in wall. Based on discussion with owner I think this is an orphaned abandoned gas pipe (many reno's on house), but could it possibly be a water pipe? If it were a live water dead end pipe it could explain the water damage. The pipe came up through a really weird and nasty cold air return duct. There is a dead end in basement, and we are guessing same pipe goes vertical to second floor at this cold air return then under floor possibly to former location of gas range. (or former kitchen plumbing)

Generally how do you tell the difference in these iron pipes between gas and water?

Found a vine growing under floor boards. More evidence to recommend people to cut the vines off their houses.

[wayne soper]

for the size, type, joint paste type and age of that house, gas is likely, but hot water heating lines of that size were also used.
Any eveidence of past hot water heating system?
Old radiators or filled holes in the floors where radiators would have stood?
What is the house heated with now.
Changing from gas to oil would be a dumb idea now but was done in the past due to inscrupulous advice from money grubbing oil companies.

[Markus Keller]

Based on the cast 90 I would say its old pipe for a hot water system. Pretty odd for an old time plumber to use a cast 90 for gas. The fittings weren't / aren't cheap.