Not sure about the LP model, but the Aquastar 125NG requires (a) a 3/4" gas supply line and (b) a 5" flue pipe all the way through the roof. These make it a more difficult process than installing a conventional replacement. They are of course more energy efficient; but with the added install costs, the payback will be many years. The 125 has a rating of 2.5 gallon per minute at 90 degree rise. That is enough for a shower or wasing machine or dishwasher, but never 2 of these at the same time. You may want to evaluate you usage and see if you need to step up to the model 240. If you have hard water, you may have to delime the heat exchanger periodically.