On Jan 7, 10:43*am, mcp6453 wrote:
My kitchen is a LONG way from my electric water heater, which is under
the floor beneath the downstairs bathroom. I'm thinking about adding a
second water heater under the floor under the kitchen sink. (A tankless
water heater is not being considered, and there is no inside cabinet
space to accommodate another water heater.)

1. Can a short body propane water heater be installed in the crawl
space? If so, does it have to be vented to the outside, or is venting
into the crawl space sufficient?

It's sufficient if you don't want to live long. Your floor is not
airtight and the crawlspace is likely not positively ventillated. The
CO will work its way up into your living space and poison you.

2. Is there any reason that I cannot put a valve going into the hot
water line at the kitchen sink that will allow me to switch between the
present electric hot water heater and the new propane one?

What would be the point? Why keep water hot if you're not going to use
it?

3. My house is plumbed with 3/4" copper, but the line going to the
kitchen is only 1/2". Shouldn't that line be upgraded to 3/4"?

Most houses are only plumbed with 1/2"

4. What is the optimum water pressure for the house? My last plumber
reduced it because it was way too high. Now it's too low.

Get the plumber back and make him increase the pressure.