Noritz, Rinnai, & Takagi are the big-three in the US, with decent track records and holding a huge slice of the market. (Worldwide Rinnai is the largest seller of gas-fired appliances, if only a niche player here, they still have a healthy chunk of the US on-demand water heater market.) But the service may be spotty depending on where you are, and they all need SOME amount of maintenance (especially in hard-water areas.) Navien buys you more efficiency/$ than some, but service is even spottier. (They work well when properly installed & understood, but sometimes it takes a course in Korean-English to get it installed & set up right.)

Getting ANY of them installed correctly required a pretty-good tech- this is way more than a tank-swap- it's 8x the gas supply, with a much more finicky burner & controls. It's far more akin to installing a modulating-condensing high-efficiency heating system boiler than it is to installing a 40 gallon tank. (I'm heating my house & hot water with a Takagi on-demand plumbed as a boiler, using an indirect-fired tank for both domestic hot water and a heating system buffer.) None of the bigger deal on-demands are suitable for off-grid living, since the require power for control & venting.

Bosch's lower end units are simpler (almost dumber 'n'a box o' rocks) and can work fine if you understand their limitations and are often suitably cheap to install (you can use B-vent on some rather than stainless, some don't need electricity to run, etc.) For off-grid I'd look at the Bosch Aquastar 1600H, which needs no electrical power, and is simpler to install. It's main drawbacks are it's lowest-modulation is on the high-side, which can result in it turning off at low flow (especially in months when the incoming water is on the warm side), and it's modest heat output (an issue for mid-winter showers when somebody else decides to run a load of laundry, but it's enough for a single-shower even in coldwater areas.) The 1600H uses the water flow to power the ignition- I've not timed how long they takes to fire up, but there's a bit of a lag- longer than some of their on-grid cousins from other vendors or their wasteful standing-pilot ignition 1600xx siblings. (I lived with a standing-pilot version that was a predecessor of the 1600H for ~15 years. I didn't love it, but it did the job, and was retired-working about a year ago. The 1600H has ~20% more output and the water-powered ignition.) It's still 3-4x the burner of a typical tank heater- plan on using fatter gas lines, and be sure the service lines/meters/regulators are up to the handling the load, or it could very expensive to install.

How are you (planning on) heating the house? Sometimes heating both space & hot water with the same burner improves the efficiency of both (which is why I went the heating-system buffer w/ heat exchanger for HW route.)

rk-miller;

in 2006 a the same time that I installed propane 15K back up generator, I switched our electric tank to a on demand Ranai propane water heater.
We have not experienced any problems, there are 3 things on the propane, if I was to guess I would say the HW is costing about  $10. a month

Chris, Rinnais are great & all, but they DO require power (more than than 100 watts when running), and the controls represent a significant background load for off-grid operation. If your goal is to be truly off-grid, every peak-watt of load counts, as well as every milliwatt of standby power.

Standard-efficiency Rinnais run 0.82-0.84EF, which is somewhat better than the 0.80EF Bosch Aquastar 1600H, but it's not a HUGE difference. When you factor in the off-grid power aspects (not to mention the upfront costs of installation & venting) it's probably worth taking the slight hit in raw combustion efficiency to avoid having to build the infrastructure to support any power-vented microprocessor-controlled on-demand HW heater. There are competitors for the 1600H- notably Marey (which IIRC requires batteries for ignition), mostly sold in Latin America, but there's far more support for Bosch in the US & Canada. The 1600H is about as efficient as it gets for something that works totally off-grid. They're not as nice as a Rinnai, but they don't totally suck either.

Posted By Dana1 on 09 Jun 2010 05:46 PM
Chris, Rinnais are great & all, but they DO require power (more than than 100 watts when running), and the controls represent a significant background load for off-grid operation. If your goal is to be truly off-grid, every peak-watt of load counts, as well as every milliwatt of standby power.

Mine is an exterior mount direct vent,   my goal was to be self sufficient in the event of a power outage due to a major hurricane, as what happened with Charlie in 2005.

with the 15K generator we have air conditioning, refrigerator, microwave, all lights and receptacles, overhead door operator and the ignitor for the  tankless Hot Water. With the burried 250 gal LP tank, we would be self suffcient for quite a while.

100 watts for us is insignificant being it probably is ony running 1-2 hrs a day

Dana 1 and Chris K. ; Thanks to you both for your input regarding on-demand hot water heaters.     Because we will be going off grid in 5 years or less, I need to purchase a unit that has next to no electrical load.  After getting your input, researching local vendors, and asking some friends in the trades about it, I will be purchasing a Bosch with a water activated ingnition system.  The efficiency is in the 80%'s  and, according to my local sources, its longevity is excellent as long as I continue to filter our domestic water (it's somewhat on the hard side.)   Thanks again for your opinions!  Russell

rk;

the rinnai is water activated too, in fact it will not come on if hot water is just a trickle

Bosch 2400 ES uses 250 watts during use and 25 watts during standby

Rinnai 2020 uses 52 watts during use and 5.5 watts during standby

Posted By cmkavala on 10 Jun 2010 05:29 PM
rk;

the rinnai is water activated too, in fact it will not come on if hot water is just a trickle

Bosch 2400 ES uses 250 watts during use and 25 watts during standby

Rinnai 2020 uses 52 watts during use and 5.5 watts during standby

...and the Bosch Aquastar 1600H uses ZERO watts during use, ZERO watts on standby, with no standby fuel losses from a standing-pilot either. 

It's ignition isn't water activated, it's water POWERED, which is what makes it ideal for off-grid use.

I wouldn't recommend the Rinnai 2020 and CERTAINLY not the Bosch 2400ES for off grid use. The 25 watts of standby is over 200kwh per year, eating up more than the entire output of a 180W PV panel in my location.  When going off-grid, standby power drains are simply intolerable.  If water heating capacity is an issue, it's more cost effective & efficient to buy two 1600Hs (but one is probably enough for 2-3 person families willing to manage hot water resources- like not running two showers AND a washing machine at the same time in winter.)  My winter incoming water is in the low 40s F, sometimes dipping under 40F, but our 3-person family managed OK with a unit that had slightly less output than the 1600H- it's enough heater.

Startup power on the blowers + ignition is also going to be 2-3x the steady-state operating power for any of the power-drafted units, which is an issue when you're running your own house-scale micro-grid.

Posted By rkillough-miller on 06 Jun 2010 12:35 PM

Does anyone have recommendations regarding brands of on-demand gas hot water heaters?  I'm looking to convert our electic hot water tank this summer along with our stove with the knowledge that in a few years we will get off the grid. 
   Looking for efficiency, longevity and cost as our overriding considerations.

this is going a slightly different direction than the other posts but i'll mention another brand I researched.  AO Smith primarily makes tank water heaters & a few years ago started making tankless.  Knowing the brand, I looked into them  On their site it says that their tankless heaters are designed more for point of use and to supplement tank heaters, not to provide all of a home's water needs - so I would probably avoid that brand in your case.