Takagi on-demand water heaters have many excellent features. They're extremely well engineered, quiet, compact (2.2 cu. ft.), lightweight (60 lbs.), can mount conveniently on a wall, vent easily, are "direct vented", meaning they use outside air for combustion, and use 3" PVC for both intake and exhaust. All models virtually eliminate "stand-by" heat loss. In other words, they don't store hot water, they heat water only when you actually need it.
Takagi water heaters are truly instantaneous and extremely powerful, and the following example illustrates this claim perfectly.
Five years ago, one of our techs installed a Takagi Flash T-K1S to heat his 1800 sq. ft. garage. The slab was literally ice cold due to weeks of sub-freezing weather. After filling the system with anti-freeze, he fired up the radiant system and watched 28 degree fluid leave the floor, enter the Takagi, then emerge from the heater five seconds later at 180 degrees!
Amazingly, the Takagi had raised the water temperature 152 degrees!
Of course, a mixing valve was used to temper the water down to 130 degrees before it returned to the floor, but this example shows how a Takagi performs under extreme conditions.
Based upon your BTU requirements, one of the above two units would be ideal for both floor heating AND domestic hot water. Detailed information and specifications for the T-H3j (left photo) and the T-H3 (right photo) can be found by following the above links. Both units are rated at 95% efficiency.
The T-H3 is Takagi's newest high efficiency model. It's rated at 95% efficiency and vents with PVC.
For schematics and photos of these Takagi on-demand water heaters in various radiant heating configurations, see: Takagi and radiant heat
Note: If your town or region has unusually hard water, that is, minerals in the water that could clog the heat exchanger in an on-demand water and shorten the unit's life, here's a simple and cost-effective solution: Soo-Soft Digital Water-Softener System.
The Elite is another on-demand unit used for radiant heating applications. At up to 98% efficiency it is an extraordinary heat source, but the Primary/Secondary plumbing configuration recommended by Heat Transfer Products is somewhat more complicated than the Takagi.
But, if you're game for a minor plumbing adventure and want an on-demand unit that is "one of the most efficient heaters on the market", as well as, compact, lightweight, reliable, all stainless steel, direct vented with PVC pipe, and boasts a full heat exchanger warranty of 7 years...the Elite is for you.