Over the past 25 years, I have purchased over two dozen takagi tankless hot water heaters for buildings (i'm a landlord), other projects, and I highly recommend them. I have also purchased a handful of Rheem and Rhiann (sp?).
The other reviewer who gave it one star gives a very fair review, in that when these units go wrong, the company warranty back-up is non existant. Takagi's US warranty people are awful!! I have had two go wrong over the years, and ended up just buying a new one instead of dealing with the cheap jerks at the company to get the shoddy one they sold me replaced. Frankly, I think about 10% of them come out of the box poorly made. HOWEVER when they are well made, and 90% of them are, they are just silent workhorses and do work trouble free for years. I have several hitting the twenty year mark without a hitch. It's just something you deal with, the occasional bad units. Why? Because the other brands, Rheem and Rhiann (sp?) are far worse--they just don't hold a candle to the ease of installation and workhorse-like reliability of Takagi. At least, that's my experience as a landlord, contractor, and person who has to deal with them when they go wrong.
I think old fashioned hot water heaters are the same. Ask anybody, and they've seen units go out after 5 years or so. The tankless NEVER go out. Unless they're bad out of the box. So again, it's just something you live with.
We purchased 4 of these units for space heating after reviewing Takagi's online information. We had installed TK2's 4 years ago and they continue to give us great service. These new TD2 units are 40 lbs vs 70 lbs for the older units. AOsmith bought Takagi about 2 years ago and completely redesigned all of the Takagi line. After installing the new units, we couldn't keep them running for more than 2 days without a code of some kind. One of them would only work with the door off. Throughout the time they were installed, our techs were in constant contact with takagi's hotline. The hotline people constantly contradict the install manuals and the online information. They were more interested in finding something we did that doesn't conform to the ever expanding list of requirements for proper operation. We were finally told that the TD2s couldn't be used for space heating. They couldn't be used in areas that get below 5 degrees. They must be drained every night. They can't be run at 180 degrees. We shouldn't rely on them for primary heat. We can't use more than 15 % Glycol in them. We finally yanked them and installed new forced air furnaces and tank type water heaters. After all of this, We were denied warranty because they will only agree to replace faulty parts that we pay them to diagnose and install. We had the units installed less than a month. They don't work as intended. We are stuck with these units. Our opinion is that they only work for domestic hot water and only with limited usage. I'd like to note that we didn't purchase these from Amazon, but we wish we had as Amazon's warranty process is much better.
I have installed 2 of the TK3pro and they are both awesome machines. I hadn't heard about Takagi being purchased by AO Smith, but I believe it, and don't think it's uncommon for a large company to buy another company out and then cheapen the new companies product line, so their own line will return to popularity (not that AO Smith has a bad name; I have only ever installed AO Smith tank-type water heaters and they are, as my friend likes to say "bullet-proof"). One the TK3's I used for a split domestic/radiator application I installed with a flat plate heat exchanger, so the TK3 only has to supply water through a 20 linear foot loop, and not deal with pressure and heating issues of pumping through an entire radiator system. The key, is the flat plate heat exchanger technology. Expensive, but awesome. As to the T-D2, I'm about to order one, but one of the reasons I went with the TK3pro over the TK3 was the heat exchanger was 'supposed' to be a heavier grade copper that had a much longer rated service span. I assume, since Takagi is saying the T-D2 is a replacement with all the features of the TK3pro, that it has the heavy copper heat exchanger as well. I'm about to call Takagi and ask. Good lick!
Have unit that is 2 1/2 years old and was installed when house built. Initial error codes were finally resolved after about 2 years of very frustrating service. Their local service agent never identified the problem and finally figured it out myself. However, from the beginning, when showering, water would go cold for about 15 - 20 seconds several times during a shower. This problem has never been resolved. They sent people out and no one could figure out what the problem was. It has been an issue since the first day and they just ignore it. Customer service will talk to you and give you things to do, settings, etc, but nothing works. Their warranty is a joke since they will not replace the unit even after service people have come and can't identify the problem. Would never recommend Takagi to anyone.