When it comes to radiant heat, Europe has a nearly two-decade jump on the U.S.. More experienced, and and faced with exorbitant fuel-prices, Europeans have turned radiant heating into high technology. What Europeans seem to get that Americans don't is what we refer to as the "3-to-1 rule" of hot water heating. The rule states that for every 3 degrees (F) you lower the heating system's average water-temperatures, fuel-consumption decreases one percent.
So is born, the art of low-temperature hydronics.
How do we get a system to operate at lower water-temperatures?
The answer is 'mass' and weather-responsive control. The more water-mass that can be added to the system, by way of additional radiation, the higher the amount of heat-exchange into the rooms that occurs. This additional mass can come in the form of radiators (old fashioned cast-iron, or modern flat-panels), or additional radiant-heating -- in the floors, walls or even ceilings.
Wather-responsive controls give the heating system a brain. The control device senses outdoor temperatures, which talks to the boiler and/or mixing-valve to ratchet the system's water-temperatures up or down according to the weather. In conventional heating systems (not weather-responsively controlled), the boiler fires at about 180-degrees all winter long, whether it's 30-degrees (F) or 10-below. But because the way we size a home's boiler, you only need 180-degree water on the coldest day of the year.
In upstate New York, for example, 80 percent-plus of the heating season is spent in the 30's. Weather-responsive controls take this fact in account, and ratchet the boiler temperatures down to match the actual heat loss of the home, thereby lowering the system's average water-temperatures.
Weather-responsive controls typically reduce fuel consumption by 15-20%. Combined with a high-mass radiant system, one can expect even greater savings.
When operating at lower water-temperatures, one must make provisions to protect the boiler from "thermal shock," particularly 'low-mass' cast-iron boilers. Cold return temperatures can also produce condensation in the flue, damaging the boiler and chimney, and compromising the system's ability to draft correctly. Mixing-valves are one way to protect the boiler.
One can also employ a "low-temperature" boiler specifically engineered for the task. Condensing boilers are the ideal choice for low-temperature radiant heating when gas is used as a fuel source. In the oil-fired realm (condensing oil-fired boilers don't exist in the U.S.), a high-mass boiler, such as the Viessmann Vitola (www.viessmann-us.com) is a high-performance option.
Whichever boiler one chooses, without the brains of weather-responsive control, and the ability tp operate at low temperatures, radiant systems are simply dumb!