I have a Rinnai unit which was installed about 8 months ago. Total cost with installation was about $3300. That included the heater (about $1600 for the 9.4 gal/min unit, the biggest one they make for residential use), the intake/exhaust vent system (about $400), an upgrade to the gas line from 3/4 inch to 1 inch, and all installation. The unit works very well, is significantly more energy efficient than the 50-gal tank it replaced, and provides an endless supply of hot water. I have a 4-bedroom, 2-bath house and it will supply all the hot water for everything in the house at one time.
A few things to consider, however. (1) The actual quantity of hot water you will get from a particular unit is largely a function of the temperature of the incoming water. Mine will run everything in the house, but our incoming water temp, even in the winter, is usually above 55 degrees. If your supply temp is closer to 40 degrees, your output will go down. (2) As was pointed out above, the unit needs a minimum water flow to operate. Mine is 0.6 gal/min before it turns on. Most are about 0.5 gal/min. This isn't a problem in the cooler months (remember, I live in Arizona) but in the summer months, with the incoming water temp around 72 degrees, the unit cycles on and off during showers, resulting in some loud whining noises emanating from the bathroom. (3) If you put it in the garage or basement, in place of your old tanked unit, you will still have the same problem with time lag for getting the hot water to the outlet. (4) Tankless units are more sensitive to clogging up with calcium and magnesium deposits than a tank unit, so you should probably consider some form of water softener if you don't already have one. I installed an ionic softener during the summer to try to combat this problem; another $1500. You may or may not need this depending on the hardness of your water supply. If you don't have a softener, my installer recommended getting a small submersible pump to use in backflushing the unit every six months or so. (5) Most folks with tankless heaters and teenage daughters say they suffered from two big price shocks. The first was for the installation. The second was for the first month's gas bill when the teenage daughters discover that they can stand in the shower for
literally hours since the heater doesn't "run out" like a tank heater will. (6) Once installed, the system is easy to use since the temperature output is controlled from a digital panel on the front of the heater. Adjusting the output temp is a matter of tapping on a keypad. (7) For the larger, whole-house units, you need to consider upgrading the gas supply to a 1-inch line. The 3/4 inch line will work reasonably well as long as you don't run the unit at max capacity. If you run it at max frequently, the unit has a hard time keeping up with the demand. One dealer to whom I spoke flatly refused to consider installating any unit in my house unless I agreed to the upgraded gas line. (8) Given the relatively high price compared with a replacement tank unit, breakeven payback time can be a matter of years rather than months. Consumer Reports ran an article on tankless heaters a couple of months back and recommended some things to consider when deciding whether this would be cost effective for you. Rinnai gives a 5-year,
non-prorated warranty (and my installer increased this to 6 years total on the entire unit) with a 12-year factory warranty on the heat exchanger (the part which usually fails). (9) I opted for the bigger unit while the dealer actually recommended a smaller one. He said the 7.5 gal/min unit will supply adequate hot water for a family of 4 with very few limitations. This would have saved me about $150 but I went for the larger one on the theory that more is always better.
FWIW, this is what I got:
http://www.foreverhotwater.com/model-r94lsi.php
The ionic water softener I installed later:
http://www.easywater.com/EasyWater-W...r-Systems.aspx
This was long but I hope it helps.