Tankless Water Heaters
Some boilers are manufactured with the option of using a tankless water heater. This device consists of an immersion coil inserted in a steam or hot-water space heating boiler to provide domestic and commercial service hot water. It is called a tankless water heater because no storage tank is used to store the heated water during periods of low demand.
The immersion coil is made of small-diameter copper tubes that are either straight with U-bends at the end or formed in the shape of a spiral (see Figure 15-10). The tankless heater is installed in the nipple port on cast-iron hot-water boilers. On steam boilers, it is installed in the left-hand side of a special back section at a point well below the water line. A uniform water temperature is main- tained by a thermostatic three-way mixing valve installed in the supply line leading from the immersion coil.
Because these immersion coils function as a heat exchanger-type device and operate on an indirect heating principle, they are also var- iously referred to as an indirect water heater, indirect heat exchanger, or heat exchanger coil. In other applications, immersion coils provide hot-water radiant heat from a steam boiler, hot water for snow melt- ing, heated water for pools, or industrial process water.
Additional information about water heaters, including sugges- tions for estimating the hot-water allowance, can be found in Chapter 4, “Water Heaters” of Volume 3.
Leaking Coils
On occasion, the coil of a tankless or indirect water heater installed in a boiler will leak, allowing high-pressure water into the boiler water and resulting in a rise in the boiler pressure. This, in turn, can cause the relief valve to open and fail to close again tightly, or it can cause the boiler pressure to rise above the setting of the pressure- reducing valve. These symptoms are often wrongly attributed to a valve malfunction when the actual cause may be a leaking coil.
You can determine whether the problem is a leaking coil by tak- ing the following steps:
1. Shut off the feed valve and/or break the connection to the coil.
2. Check the pressure reading in the boiler.
3. Wait about 8 hours, and check the boiler pressure reading again.
If the pressure reading has remained approximately the same over the 8-hour period, it is a strong indication that the coil is leaking. If the boiler pressure continues to rise, the problem may lie elsewhere in the system.
Blowing Down a Boiler
Foam sometimes forms on the surface of the boiler water and is usually indicated by drops of water appearing with the steam. This condition is caused by the presence of oil, dissolved salts, or similar organic matter in the water. One method of eliminating this
problem is by draining off part of the water in the boiler and adding an equal amount of fresh, clean water. Another method consists of blowing the foam from the water with a specially con- nected pipe or hose. The boiler should have a blowdown tapping for this purpose. The first method is the easiest. The second one (i.e., blowing down the boiler) requires considerable experience.