InterNACHI’s course on inspecting water heater tanks states:
A water heater tank should be installed inside a pan in locations in a dwelling where a leak from the
tank could cause damage to the structure or property.
With this standard in mind, how is a drain pan supposed to be installed underneath a water heater tank when a combustion air supply pipe goes out the bottom, through the floor and into the vented crawlspace below?
IMO, there may not be a practical way. 
Reminds me of vertical updraft furnace/air handlers in closets. No way to get a pan under there and not obstruct air flow.
Looks like a mobile home, which hardly ever have drain pans.
Is this a stick built or manufactured home? Just asking because the underneath (crawlspace) looks like a mobile.
Edit: Jeff was one second ahead of my question…
That’s what I told my client in person, but wasn’t sure if someone had a different solution. Thanks for the reply, Brian!
It’s a modular manufactured home.
We were both delayed a bit. It took a few extra seconds to figure out if we were going to add the comment of the water heater falling through the floor in just about every old mobile home.
In Florida, the Code (which is derived from IRC) requires combustion air to be drawn directly from the outdoors.
Manufactured homes are not governed by the IRC if I am not mistaken.
Good point. The manufacturer’s installation instructions should still be be followed.
A ventilated crawlspace counts as outdoor air if I’m not mistaken.
The rules for mobile home are different than houses. I can’t remember seeing a pan under a mobile home water heater though it would be a good idea.
To clarify, it’s a modular manufactured home.
Well. it’s one or the other, but not both.
I thought I saw a metal frame. Around here mobile homes (single and double wides) have a metal frame to move it around (the mobile part). Modular homes do not. Both types are made in a factory however so…I guess they’re both manufactured.
Install a floor drain so any leaking drains to the crawl.
Isn’t that a moisture barrier encapsulating the floor insulation?
That usually indicates a Mobile/Manufactured vs a Manufactured/Modular home.
I call it belly wrap.
This home did not appear to have any wheel chassis or HUD tags. It had all the characteristics that I’m used to seeing on a modular home.
One solution is to install water leak sensors. The trick is keeping the sensors maintained until the eventual leak…
or protectively replace the water heater and at that time bring combustion air in differently,
or split the difference and replace anode rods but that’s just a good idea not bulletproof.