I just put one in this spring. I got it off Craigslist for ~$350 (~1/3 retail, woohoo) and then started the install. I spent roughly $100 for gastite brand pipe and fittings to extend the line off of the gas furnace. You have to use 3/4" line to get the capacity you need. I had to yard out some 1/2 NPT pipe and replace it. Then the exhaust line, Ouch, that was about $4-500 alone with the roof jack, exhaust pipe and cap for the thing because it all has to be stainless steel with seals and all that good stuff. The total install done DIY ran me about $1000 and 3 trips to the local hot water heater supply store. Total would be about $2000 if you had a plumber come out and do it. $1000 for the heating unit, then about $1000 for the gas plumbing, exhaust plumbing and installation.
The master bath shower is the furthest away from the heater and I still get hot water in less than 30 seconds or so. It is not as fast as the hot water heater was, but it's close. I could save even more energy if I put in a recirculator unit to conserve the water and put off opening the water spigot until the water was hot. Maybe some other time, not right now. That said, the wife takes some long damn showers now that we have it, and we do really like it.
Am I saving money, sure, on the gas bill. But overall, I could have bought a quite a bit of gas for the price I paid for the thing. But we think long term and do enjoy the lower gas bill now. It's not a huge savings, but I do have a bit more room in the garage now and it is pretty cool to have unlimited hot water.
The claimed savings on the gas usage for water heating is 30%. Because a pilot running all the time or even a piezo-electric pilot running enough to keep 30-50 gallons of water at 100+ degrees year round does add up. Compared to my new furnace, the savings are small, however, the bill did go down, so we have reduced our consumption. Regardless of my wife's loooonng showers. Thank goodness I don't have a removeable massage showerheasd, she'd never get out of the bathroom.
