Tankless Water Heaters

Small wall-mounted tankless water heaters (Fig. 9-2) are located next to plumbing fixtures that occasionally need hot water, like isolated bathrooms and laundry rooms. They can be easily installed in cabinets, vanities, or closets near the point of use. Although they use a great amount of heat for a short time to heat a very limited amount of water, these tankless heaters can reduce energy consumption by limiting the heat lost from water storage tanks and long piping runs. Because they may demand a lot of heat at peak times, electric heaters are usually not economical over time where electric utilities charge customers based on demand.

These small tankless water heaters (also called instantaneous or demand heaters) raise the water tem-

Figure 9-2 Point-of-use water heater.

perature very quickly within a heating coil, from which it is immediately sent to the point of use. A gas burner or electrical element heats the water as needed. They have no storage tank, and consequently do not lose heat. With modulating temperature controls, demand water heaters will keep water temperatures the same at different rates of flow.

Without a storage tank, the number of gallons of hot water available per minute is limited. The largest gas-fired demand water heaters can heat only 3 gallons of water per minute (gpm), so they are not very useful for commercial applications, but may be acceptable for a residence with a low-flow shower and limited demand. Gas heaters must be vented.

The largest electric models heat only 2 gpm, and are used as supplementary heaters in home additions or remote locations, or as boosters under sinks. Electric heaters require 240V wiring.

Instant hot water taps use electric resistance heaters to supply hot water up to 88°C (190°F) at kitchen and bar sinks. They are expensive and waste energy. Instant hot water dispensers require a 120V fused, grounded outlet within 102 cm (40 in.) from the hot water dispenser tank, plus a water supply.

Some tankless coil water heaters take their heat from an older oil- or gas-fired boiler used for the home heating system. The hot water circulates through a heat exchanger in the boiler. The boiler must be run for hot water even in the summer when space heating isn't needed, so the boiler cycles on and off frequently just to heat water. These inefficient systems consume 3 Btus of heat energy from fuel for each Btu of hot water they produce.

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