First off, what does your Contract say about making changes? Have you talked to your Attorney? (You DO have someone looking after YOUR interests, no?)
Depending on the Contract, your Builder may be able to do as he wants (in this particular case.) Then again, maybe not. (This is why you ALWAYS, ALWAYS, ALWAYS want to have an Attorney. And you should have one BEFORE you sign your first document. I'm saying this not to, necessarily 'beat up' on you. I'm also hoping that other Posters get the message before it's too late, as MAY be the case in your case. I call it Pro-Active Education.)
You may be right about why he's changing his mind, especially if the crawlspace isn't heated. W/O having an Attorney reviewing your documents, he could blow whatever smoke he wants until you have enough 'power' to push back. For now, tell him to NOT do anything until you have your contract checked out w/your Attorney. (Then get one!) Hiring an Attorney is peanuts in cost compared to the amount of $$$ you'll pay over 30 years (assuming you have a 30 year mortgage.) You can figure out what 30 years x 12 months x monthly-payment ='s. Then compare that to the cost of an Attorney - It's peanuts.
I think I've said enough. As to where he can put the unit OTHER than the crawlspace, if your contract has 'wording' about 'changes' HE is making, that's what he can do. As for the contrary, again, I defer to your Attorney. Consider this - A Tankless Water Heater. It can be mounted on a wall. They're fine units for Hot Water. I, too, would opt for gas ESPECIALLY since if the power goes out, you can't heat up Hot Water (assuming you end up w/an electric stove. Having a gas HWH would be moot if your 'ignitor' is electric since, if the electricity goes out, you can forget about HW!)
My best to ya and hope this helps.
Jay J -Moderator
PS: Consider hiring a New Home Inspector too. He's another person looking after your interests. I'm talking about an Inspector that makes 5 visits (minimum) to the site for his fee. At foundation pouring, framing, siding + roofing, plumbing + electrical, and at finish time. You can negotiate other 'visits' w/him. It costs PEANUTS ...
PPS: You can see that I've heard enough horror stories that I'm a little 'emotional' about having an Attorney and Home Inspector ...