Good question! Tankless water heaters are very good at heating water at low flow rates (3 - 5 gallons per minute, or GPM). Higher flow rates will cause the burner cycle to be interrupted, and that's why residential tankless water heaters don't work well with high flow shower fixtures, soaking tubs with roman tub valves and with multiple showers running at the same time.

In this instance, I'd probably lean towards using a buffer tank for storing heated water for the radiant heating system.This tank would be connected to the tankless water heater and have a small circulating pump and an aqua-stat to control the tanks water temperature. A 50 gallon buffer tank at 120 degrees supplies about 50,000 btu's of usable heat. Our second hurdle is the different flooring surfaces. The basement will need either radiant wall panels, baseboards or radiant tubing with a 1.5 inch thick topping slab. All three of these will use a different water temperature than a system where tubing is stapled up below the subfloor and the heat is radiated through the wood or carpeted flooring. Controlling the different water temperatures is a simple as installing mixing or tempering valves in the hydronic piping.

Now to get to the cost question &mdash there are many factors in play here, but I will try to get you in the ballpark. Staple up systems are very labor intensive and could cost as much as $14 a square foot. Tubing over concrete is very simple and could be as low as $4.00 per square foot + another $2.00 for a lightweight concrete topping. The buffer tank, pump, aqua-stat and associated piping should add about $4,500.00. All in all, it's an expensive proposition but the comfort of radiant heating may be worth it!

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