Drew,

That 150 amp spec sounds like an electric tankless heater, not a gas one. What Dogfish is talking about is the kind that use gas for heating the water and electricity just to run the PLC (electronic brains).

There are, or used to be, electric water heaters that could be installed in a crawl space, but it needed to be a pretty deep crawl space. And it still needs to be able to gravity drain outside your foundation, which only works if your lot has sufficient slope to it.

Sg