Editor's Note: Is Leadership in Energy and Environmental Design for New Construction and Major Renovations (LEED-NC) certification cost-effective to achieve? A recent evaluation of 11 LEED-NC-certified buildings in Colorado holds the answer. HPAC ENGINEERING author and past EGB present Peter D’Antonnio helped manage the project and write and edit the final report. In this session, D’Antonnio discusses the report, which captured hard and soft costs as well as occupant impressions, and presents other valuable public resources related to green-building performance during HPAC Engineering's fourth annual Engineering Green Buildings Conference and Expo Sept. 17 and 18 at Mandalay Bay Resort and Casino in Las Vegas. For more information, go to www.egbregistration.com.

• Article sections:
  • History of Radiant Heating
  • Radiant Comfort
  • High-Performance Heating System
  • Maximizing System Efficiency
  • Maximizing Source Efficiency
  • Boiler Piping
  • System Piping
  • System Controls
  • Multiple-Load Systems
  • Domestic Water
  • Reference

Radiant heating systems are being used more often in high-performance designs and green buildings. Many designs now utilize condensing boilers and other high-efficiency technologies, which bring their own sets of challenges. This article will examine tried and true methods of minimizing costly mistakes and maximizing performance. Radiant systems are part of a larger field: hydronics. Many of the concepts that will be presented in this article apply to hydronic systems in general.

History of Radiant Heating

Radiant heating is smart and safe. It is time tested: Our nearly 5-billion-year-old sun operates as a radiant mass. An open fire is another everyday example of a radiant heater. Perhaps this is why radiant heat is considered the first form of central heating used by humans. The Romans used it to heat their baths. Radiant heating is the predominant form of heating in Europe and is gaining acceptance in the United States. A survey conducted by the Radiant Panel Association determined there was a 37.4-percent increase in radiant-tubing sales in North America in 2004.1 Radiant systems in the early 1900s used steel and wrought-iron piping imbedded in concrete powered by coal-fired boilers. What a difference 100 years makes.

Radiant Comfort

Why is radiant heat so comfortable? Because our bodies are very effective radiators. Roughly 50 percent of a person's heat transfer is through radiation, 30 percent is through convection, and the remaining 20 percent is through evaporation. Comfort also relates to the concept of mean radiant temperature (MRT). MRT is the measure of the combined effects of surface temperatures within a space. MRT is the most important parameter governing human energy balance.

As evidence of the importance of radiant-heat exchange to the body's thermal equilibrium, physiologists have discovered that human skin has extraordinarily high absorptivity and emissivity--more than almost any other known substance, including matte-black metals. Consequently, humans are highly responsive to changes in MRT. In general, for every 1 F that MRT drops, a person must raise air temperature about 1.4 F to achieve similar comfort conditions.

High-Performance Heating System

Not all radiant systems are created alike. An extremely efficient heating appliance connected to an inefficient delivery system does not produce an efficient system. A high- performance heating system is one that maximizes source and system efficiency and minimizes life-cycle cost. It is one that conditions an indoor environment by first reducing the building envelope load, has an energy-performance focus, and includes such important considerations as adding underslab insulation.

Maximizing System Efficiency

Within a radiant system, there are four main points to consider:

• Supply-water temperature.
• Temperature differential.
• Variable-speed pumping.
• Preventive maintenance.

Reducing supply-water temperature generally increases the efficiency of source equipment. Increasing temperature differential also provides an opportunity for increased efficiency. Low-temperature hot-water heating systems historically have been designed for a 20-F temperature drop. There appears to be no logical explanation for this selection, other than that it is easier to size the pump by dividing the British thermal units by 10,000 to obtain pump flow (British thermal units per hour equals 500 multiplied by gallons per minute multiplied by temperature drop). Or perhaps the 20-F temperature drop was selected to protect against flue-gas condensation in non-condensing boilers. Increased differential affords pumping savings, pipe-size reductions, and higher source efficiency through lower return-water temperatures. For example, doubling differential from 20 F to 40 F halves the required flow rate. Variable-speed pumping does well to improve load tracking and matching building load to system load while also realizing pump electrical savings. Preventive maintenance serves to maintain performance during the life cycle of a system.