Tankless coil water heaters
Tankless water heaters, sometimes called “instantaneous water heaters,” are used in conjunction with steam and hot-water boilers. Most tankless heaters consist of a small-diameter pipe shaped in the form of a coil that is located inside of the boiler (internal generator). (See FIG. 16-7.) The coil might also be located in a casing outside, but connected to, the boiler (external generator). The coil is surrounded by hot boiler water that gives up its heat to the water flowing in the coil. Cold water enters the coil and leaves as hot water with its temperature increased by 100° F. These units are designed for a specific water-flow rate (gallons/minute) through the coil to achieve the desired increase in temperature.
If water flows through the coil at a greater rate, less heat will be transferred to each gallon, resulting in a lower outlet temperature. Consequently, many units have a flow-regulating valve installed in the cold-water supply pipe to the tankless heater to limit the water flow to the capacity of the heater. The design flow through these heaters is usually about 3 to 4 gallons per minute. Assuming a normal water supply to the house, the typical cold-water flow will be between 4 to 8 gallons per minute.
To regulate the temperature of the hot water being distributed throughout the house, there should be a mixing valve between the cold-water inlet and the hot-water outlet pipe. (See FIG. 16-8.) A mixing valve is considered a necessary feature to eliminate the possibility of scalding. Over the years, mineral deposits tend to build up inside the heater coil, especially in hard-water areas. This in turn further reduces the water flow through the heater and results in a higher water temperature at the outlet. The high-temperature water, which is a potential hazard, can be cooled to the design temperature by mixing it with cold water in the mixing valve. This water can then be distributed to the plumbing fixtures throughout the house.
As mineral deposits form inside the heater coil, they restrict the water flow. It is possible for the deposits to build up to such a point that the hot water flow becomes a trickle when two or more faucets are turned on at the same time. The hot-water flow should be checked during your interior inspection, as discussed in chapter 10. A low hot-water flow that is caused by a blockage in the tankless coil can often be corrected by chemically flushing the unit and dissolving the deposits. However, when the flow becomes a trickle, the heater coil should be replaced.
A tankless coil has virtually no storage capacity. Consequently, if there is a hot-water demand that exceeds the design flow (and it often does), there will not be enough hot water. To provide additional hot water, some installations are equipped with a storage tank. Water heated in the tankless coil will be circulated to the tank by gravity flow or a pump. The pipe that distributes hot water to the plumbing fixtures will be connected to the storage tank rather than the tankless heater. When the hot-water flow between the tankless heater and the storage tank is by gravity, the storage tank must be located above the boiler. When the flow is induced by a circulating pump, the tank can be located alongside the boiler. (See FIG. 16-9.)
From an energy-conservation point of view, a tankless water heater is not desirable. It is inefficient and wasteful of energy. To produce domestic hot water, there must be a double heat transfer. The boiler water will first be heated with oil or gas, and it will then transfer its heat to the water in the tankless coil. To produce domestic hot water, the boiler must be heated all year long, winter and summer. As discussed in chapter 14, controls associated with the heating system prevent steam or hot water from being distributed throughout the house when heat is not required. Nevertheless, the heated boiler water during those months does represent wasted energy. It also introduces an additional heat load on the house at a time when heat is not desired. If you should decide to replace a tankless coil with a tank-type water heater, it can be done at a reasonable cost. An oil-fired water heater, however, is more expensive than a gas-fired or an electric unit.