A tankless water heater is a copper-tube boiler with a high pressure drop and no circulator. They are manufactured for domestic water heating; so using thdm for boiler service is a misuse according to the manufacturer. They are not less expensive than a boiler used for radiant floor heating. http://www.sets-systems.com/ http://www.northdoorway.com/water/mora.htm http://www.globaltownewarehouse.com/HomeProducts/aquastar/aquastargastanklesswaterheaters.htm
A boiler can heat the floor and also heat domestic water using a heat exchange tank. The boiler is rated for these purposes and usually has a warrantee for 20 years.
If you want a complete heating system, use a complete system designed by a radiant heating manufacturer. Weil McLain and other manufacturers will provide an entire factory-warranteed packeage installed by a qualified representative. http://www.hydrothermcan.com/products/design.html http://www.radiant-tech.com/heat/radpanel.htm