The problems associated with tankless heaters does not apply to the some of the new
modulating heaters if their modulation is controlled by the water flow and temperature.

I do design a few radiant systems around the tankless water heaters.
A few of them offer very high quality products and make very simple, efficient, affordable heating systems that will heat both the house and a endless supply of domestic hot water.

     We have installed several different tankless water heater in radiant heating  and snowmelt systems. Including six different manufactures, from France, England, Germany and Korea.
     The theory that heating water instantly is more efficient than using a standard storage tank heater is not true. Most storage tanks lose less than two degrees per hour and unless the tank is outdoors the losses are simply subtracted from your heating costs. The reason no tankless heaters are manufactured in the United States is that they can not be built to meet the efficiency requirements of the US government. Any water heater built in the US must be at least 76% efficient, which is miserably low. In a heating system the warm water return temperatures cause the efficiency of the tankless heater to drop below 40%. Tankless heaters also have a pollution rating many times higher than 94%+ combo water heaters. Another problem is that tankless heaters constantly draw heated air out of your house, even when off causing your heating system to run more. 
   We only use tankless in small heating systems and for snow melt systems. Snow melt systems typically return very cold water which some tankless heaters handle very well with good efficiency, they do need to be engineered carefully to maintain proper flow rates.

THE BEST TANKLESS HEATER IS THE TAKAGI, SEE   www.pextubingstore.com
I have used these in my radiant designs since 1995, I have three of them, one in my boat, one in my bus and one in my home to back up my wood burning boiler.
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Tankless water heaters must be used with a
differential bypass or a storage tank to work properly.
Using them as an open system,
heating both home and hot water is best.

Because of the sudden rise in energy costs many people are rushing out to buy tankless water heaters.
Many will be disappointed, as they do heat very small volumes of water at a time and the fuel savings they advertise are overstated.

TO SATISFY A SMALL HOME YOU WILL
NEED AT LEAST A 140,000 BTU HEATER.
LARGER HOMES WILL NEED 200,000

To overcome the low flow problems associated with many tankless water heaters, use two pumps, one to pump water through the heater and another to pump water through the heating system or use a storage tank.