National Oilheat Research Alliance 37 Internal tankless coils Internal tankless coils (Figure 2) are heat exchangers that are installed inside a boiler to generate domestic hot water. These devices can operate rather efficiently during cold weather when boilers are running to produce heat, but are much less efficient during warm weather. The coil is surrounded by the boiler water, which heats the outside of the coil; domestic water is then heated as it flows through the inside of the the coil. Tankless coils generally requre that boilers be operated at 180–200°F. Figure 3 shows a properly piped tankless coil and includes the following: 1. Shutoff valve—typically a globe or ball valve 2. Domestic pressure relief valve—Relieves excessive pressure caused by thermal expansion 3. Flow regulator or restrictor—This device automatically limits the flow of water so it doesn’t exceed the capacity of the coil. 4. Drain valves—These are installed to simplify periodic flushing of the coil. 5. Mixing valve—Thermostatic mixing valves blend hot water from the coil with cold water to deliver a desirable and selectable temperature and help prevent scalding injuries. The recommended maximum hot water temperature setting for normal residential use is 120°F. NORA recommends that thermostatic mixing valves (also known as tempering valves or anti-scald valves) be installed and used according to the manufacturer’s directions to prevent scalding. Tankless coils supply hot water on demand, BUT only up to a limit that is determined by the Btuh output of the appliance in which the coil is installed. For example, 50,000 Btuh is required to raise the temperature 1 gpm (one gallon per minute) of water from 40 degrees to 140 degrees. If you do the math, you’ll see that 150,000 Btuh is required to Figure 2 Figure 3
NORA Advanced Hydronics
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