(CROSSPOST--moved to its own thread)
I have a TK-Jr that heats both the radiant flooring and domestic hot water. It usually works fine (great, in fact, considering the size); however, in the mornings, after it has been working a while to heat the house, when someone tries to take a shower, the unit shuts off. The diagnostic led blinks three times. Resetting it by cycling power resolves the problem (until the next morning). The unit is inside and I don't have the controller with the diagnostic codes.
UPDATE--Upon checking the unit BEFORE it shut off, I also discovered the LED was flashing FOUR times--a different set of problems (the LED inside the unit flashes a number of times to signal a general type of problem, the $100 external controller gives a numeric readout that gives a more specific diagnosis.)
Anyway, I got Takagi tech support on the phone. Based on the codes, hey recommended that (1) the exhaust be shortened as it was too long (about 20 feet of run and four elbows, each of which is considered 5" of run, so a total of 40" of run, but the unit is only rated for 25" max).
THey also recommended cleaning the unit, and emailed me detailed directions. I followed the directions, and sure enough the unit was really full of dust. This is likely due to the fact that there was a a lot of dust (plaster and other crap) in the air when the house was being constructed. Also, the gaskets were destroyed in the process. I made replacement gaskets from a thin piece of fiberglass insulation, and put it back together.
As of this morning, the unit has been running fine (about two days). I will have to see how it goes, but if you don't hear from me you can assume the cleaning did the trick. So be aware: the unit (and tankless heaters in general) is clearly an amazing piece of engineering, able to not only heat our house but also provide domestic hot water, in a unit about the size of a large briefcase--but its small size makes it susceptible to dust and dirt.