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, and virtually eliminate "stand-by" heat loss. In other words, they don't store hot water, they heat water only when you actually need it. They are truly instantaneous and extremely powerful, and the following example illustrates this claim perfectly.
Last December, 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 Takagi 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 Flash T-K jr (left photo) and the Flash T-K3 (right photo) can be found by following the above links. Both units are rated at 84% efficiency.
For schematics and photos of these Takagi on-demand water heaters in various radiant heating configurations, see: Takagi and radiant heat
The Munchkin is another on-demand unit used for radiant heating applications. At 92% efficiency it's an extraordinary heat source, but the Primary/Secondary plumbing configuration recommended by Munchkin 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 an amazing 12 year warranty...the Munchkin is for you.
