Copyright Goodheart-Willcox Co., Inc. 240 Hydronic Heating: Systems and Applications concrete, Figure 12-5. There are three major differences between low- and high-mass radiant heating systems: ■ ■ Energy retention: Low-mass radiant panels made of lightweight materials (such as aluminum) cannot retain heat energy for long periods of time and need to be constantly functional whenever heating is required. Conversely, high-mass radiant systems that use materials such as concrete or gypsum tend to retain heat energy for extended periods and maintain levels of heat transfer long after the heating source is de- energized. ■ ■ Response speed: Low-mass radiant systems will respond quicker to required changes in the conditioned space. Once a low-mass system is set to the desired heat output temperature, it will heat up and cool down much faster to reach the temperature required. On the other hand, high-mass systems tend to take longer to reach their desired temperature. ■ ■ Material type: Because low-mass heating systems typically have higher performance requirements than high-mass systems, they usually consist of higher conductivity materials such as copper or aluminum. 12.2 Heating Sources for Radiant Systems There are many different methods of generating hot water to supply a radiant heating system. The system designer needs to be aware of the advantages and disadvantages of the various types of systems. Many different factors must be evaluated before settling on the right source for generating hot water to the system. These factors include the type of heating fuel available, whether a renewable energy source should be used versus conventional fossil fuels, the size and scope of the project, the overall cost of the project, and the projected payback. Here we discuss four different types of hot water generating systems: ■ ■■ Conventional boilers ■■ Condensing boilers ■■ Geothermal heat pumps ■ Solar thermal storage 12.2.1 Conventional Boilers When selecting the right boiler for the radiant heating system, the first consid- eration is whether the boiler is designed for a lower return water temperature. As discussed in earlier chapters, conventional cast-iron boilers are designed to allow for a return water temperature of at least 140°F. Colder return water tem- peratures can cause premature boiler failure due to the condensing flue gas tem- peratures impinging upon the heat exchanger. However, a conventional boiler can be used on a radiant system with the incorporation of a primary-secondary loop system. The primary-secondary piping system requires the installation of two separate piping circuits. The first or primary circuit simply circulates water throughout the boiler and provides a constant water temperature through the boiler’s heat exchanger. The secondary circuit is used to supply the proper water Goodheart-Willcox Publisher Figure 12-4. An example of a low-mass heating system. Michael Helsel Figure 12-5. An example of a high-mass heating system.