generate three gallons of domestic hot water per minute. Rather than operating an oversized boiler to deliver adequate domestic hot water, a better alternative is to add a storage tank—often called an “aqua-booster”— in conjunction with the coil. Domestic hot water is generated by the coil and the tank maintains sufficient volume to satisfy larger needs. Figure 4. 38 National Oilheat Research Alliance External tankless coils Figure 5 below, shows an older style external tankless (AKA “sidearm”) coil. It is a small tank with a heat transfer coil inside. As the boiler water circulates by gravity over the coil it heats the domestic water inside. The most popular external coil today is the highly efficient plate heat exchanger (See Figure 6 on following page). It features a series of plates containing two separate water passages sandwiched together. Boiler water flows through every other plate and domestic water flows through the adjoining plates allowing heat transfer while maintaining complete separation of boiler water and potable water. The domestic hot water produced is typically stored in an aqua-booster. Flat plate heat exchangers are also used for radiant heating and snow melting applications. Indirect water heaters An indirect-fired water heater is a storage tank with an internal heat exchanger. See Figure 7 on following page. Compared to a direct-fired water heater, in which the aquastat controls the oil burner operation, in an indirect the aquastat controls a circulator that allows boiler water to flow through the heat exchanger. In contrast to tankless heaters, indirect water heaters typically enable a smaller boiler to be used because of the amount of domestic hot water stored in the tank. Figure 4 Figure 5
NORA Advanced Hydronics
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