EDIT - 1/11/08 -------------------------------------------------
I've been having trouble with the Aquastar lately. It still works but doesn't like to work when the furnance is running. I assumed the furnance was drawing down the gas pressure in the line but after having the gas company out and an HVAC technician they both concluded the pressure is steady and sufficient. Contacting Bosch technical support has proven to be an exercize in patience to say the least. Each phone call (there have been 3) has taken a minimum of 50 minutes on hold. I've spoken to three different techs and have been given three different solutions. First call, very nice lady, apolgized for the wait, I told her the problems and she sent me a new control box (didn't work). The second time I called, the tech told me it was my problem and not the Aquastar. He said I had insufficient gas pressure (I didn't), said I had a vent back draft (tested, I didn't) basically he blew me off. He gave me the phone number of a local tech (wasn't local-40 miles away) that I'd have to pay for (because it was my problem) to come and diagnose my Aquastar. I decided to call back because I was very unhappy with the technical support I had just received. My intent was to make a complaint, but on my third call I talked to yet another tech who diagnosed yet another possibility. He sent me another part to replace, which still hasn't solved the problem. Which is where I'm at at the moment.
I've done some more research on the internet and it seems that Bosch Hotwater tech support is reknown for poor service. Unfortunately that's something you only find out when the unit isn't working.
I'll have to change my recommendation from a buy to a don't buy at this point based solely on customer service. Hot water is not something you can readily do without for any period of time and if you do have a problem with the Aquastar, waiting days to have it fixed is not a solution.
Another curious observation, why is there such a wait for tech support? Must be a lot of broken Aquastars out there.
END EDIT ------------------------------------------------I installed the Aquastar 1600H myself in about 3 hours. I bought it at Lowes and like the other reviewer, had to go to HomeDepot to buy the 5" class B ducting. I gave the Lowes sales ppl an ear full about that but it just went in one ear and out the other.
If you're comfortable with sweeting copper and plumbing your own gas lines its really not that difficult. My biggest problem was positioning the unit so I could vent the exhaust gases. If you don't have a straight vent stack and have to feed the exhaust into the chimney like I did, it required placing the unit really low on the wall. You are required to vent straight up for the first 12" before you can elbow the vent. Along with the minimum run vs. rise of the vent pipe, it forced me to place the unit just barely a foot off the ground. Your situation may be different but keep it in mind. Also, it requires a minimum of 3/4" inside diameter black iron gas pipe. In some cases if you're tapping into the same gas line as your furnance, you may have to install 1" gas line. This unit requires a lot of BTU's and if you choke down below 3/4" it will not run properly. So that means no yellow flexible line to make the last connection. You'll have to hard plumb the iron pipe all the way to the unit.
There are both benifits and negatives to this unit. First the benifits. Long, long, long showers. Literally, you can take a shower all day. Then the next person in line doesn't have to wait for the water to heat back up. It's ready to go. Great for a family getting ready in the morning. Second, big savings on the gas bill. My bill dropped $20 the month after I installed this unit. I figure it will pay for itself in less than two years after the tax credit.
Now for the negatives. Low hot water pressure. That takes some getting used to. The unit has a maximum 4.3 gpm, but that's really unreasonable.(Take into consideration that I'm in the Northern USA and the water in coming into the house is at 50 degrees.) At that flow rate the water doesn't heat up above 90 degrees. That makes it warm not hot. You have to turn down the flow rate to 2.3 gpm to heat the water to 120 degrees. At that flow rate / pressure, if you open two hot water spigots anywhere in the house at the same time it becomes a trickle. There is no way you could run the shower or any other appliance like the dishwasher or washing machine at the same time. If you turn the Aquastar back up to 4.3 gpm you could, but you'll be taking a warm shower not hot (in my case).
Also, at 2.3 gpm, it takes a long time for the hot water to actually get to the faucet. It probably takes 4 to 5 times longer than it used to for hot water to flow from the tap than it did with the old tank system. For example, if I'm washing my hands, by the time I'm done lathering up and rinsing, the water is just now starting to get hot. I've come to realize its not even worth turning the hot water on for short burst of hot water like that.
It takes adaptation to get use to this, like this trick I use in the morning. I turn the hot water on and brush my teeth while the water is slowly making it's way to the tap. Once it's hot, then I turn on the shower. Otherwise I have to let the shower flow for what seems like a very long time before it starts getting up to temperature.
Would I recommend this unit. It depends. If you really need the ability to run two showers or two appliances at the same time, then no way. You'll have to go with the next unit up that has a faster flow rate. There's no way this unit can handle that.
But if you think you can adapt to the low flow rate and really enjoy taking all day hot showers while saving money on your gas bill then this is the unit for you. I really enjoy the savings and long showers.
One more thing. The manual says you have to clean the unit once a year to maintain its efficiency. That means disassembling some parts in the unit. Clean them and put it back together. I haven't had to do this yet, but its just one more thing to consider before you switch to tankless hot water.
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Bosch 1600H NG AquaStar 4.3 GPM Indoor Tankless Natural Gas Water HeaterProduct Description: Bosch 1600H-NGBosch AquaStar 1600H-NG (Natural Gas)Except for the ignition system, the 1600H(hydro-ignition) is the same as the 1600P(standing pilot).The Bosch AquaStar 1600H replaces the 125HX model and is designed to vent vertically and requires a 5" type-b double wall vent. Can be vented horizontally with the addition of the AQ4. The Bosch AquaStar 1600H tankless water heaters supply one major hot water outlet at a time. They are sized for most homes and small businesses and have a 12-year manufacturer's warranty when installed professionally.Bosch 1600H-NGBosch AquaStar 1600H-NG (Natural Gas) Features:; Delivers up to 4.3 gallons per minute; No standing pilot ?Çô no electricity needed; 12-year warranty on heat exchanger ?Çô 2 years on parts; Provides ENDLESS supply of hot water; Saves up to 50% off utility costs; Compact and lightweight; Can simply hang on the wall; Modulating gas valve to control temperature; Pressure relief valve included; Vents with inexpensive and readily available 5?Ç¥ B Type Galvanized vent pipe "; Power Input :30,735-117,000 Btu; Minimum Flow to Activate (gpm): 0.6 gallons per minute (gpm); Flow Rates 90?? F rise / 2.1 gpm 77?? F rise / 2.4 gpm 65?? F rise / 2.9 gpm 55?? F rise / 3.4 gpm 45?? F rise / 4.3 gpm; Energy Factor : .80; Thermal Efficiency : 80%; Dimensions: H: 25 ??" W: 16 ??" D: 8??"; Weight: 33lbs.; Modulating Gas Valve: Yes; Ignition: Hydro-generated (no electricity); Accessories: Power vent for horizontal venting Vertical Vent Kit (H/P VVT); Hydro-powered sparked ignition, no outside power required; New technology generates pilot spark from the flow of water; Like a hydro dam producing electricity, the system ignites the burners when water is flowing without the need for electricity or batteries; Uses no energy when idle; Includes a pressure relief valve; Natural gas, hydro-generated unit; 117,000 BTU/input; 0.82 efficiency rating; 0.78 energy factor
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I am very happy with this purchase. Mounting the Tempra 29 Plus and the required new electrical box in the closet was as easy as 8 screws. Wiring it all up was a little more work. The plumbing is simple if you put it where your tank was. And losing our old 50 gallon tank gave us some new closet room for the house! The product itself is absolutely fabulous and is only about 4-5 inches deep and maybe 15"x15"ish. It looks very well made. (Those Germans always make good stuff!) Once installed, your water will easily get hotter than you can stand and I felt no fluctuation in the shower when the washing machine and a sink were also running hot. The ONLY negatives I see are 1)the dedicated line requirements and 2)a couple of our fluorescent fixtures flicker/strobe slightly when hot water is running. I feel like that might simply be due to cheap bulbs or something because the "flicker" is isolated to just two fixtures in the whole house. Bottom line: Buy it. Just be prepared for a little extra cash to go to a new electrical box, 3x50amp breakers and some 8ga wire. But in the end, you'll be happy, you water will be hot, and you won't be heating 50 gallons of water all day long for nothing any more. I can't wait for the next electric bill to see what my Tempra 29 Plus is saving me! 
