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?

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?

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"?

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.

The reasons I want to use propane are that we have a large propane tank
for our gas logs. We don't use the minimum amount each month, so we can
get some of the heating energy for "free". Second, if the power goes
out, we'll still have hot water, and our power does go out way too often.