HOT WATER BOILERS
HOT WATER BOILERS The function of the boiler is the same as a furnace: to provide heat so that all parts of the home are comfortable. Boilers fulfill this function in a slightly different way than furnaces. Boilers Don’t Really Boil Hot water boilers, or what some people call hydronic heating systems, don’t really boil the water. They typically heat the water to a maximum of 200°F. Normal operating temperatures for many boilers are in the 120°F to 130°F range, depending on a number of factors, including outdoor temperature, design capacity, etc. Steam Boilers There are boilers that do boil water into steam. Steam boilers are not discussed in this book.
1.1.1 Materials Cast Iron The oldest boilers, and some modern boilers, have cast-iron heat exchangers. Cast iron, incidentally, is a very high-quality material that works very well for water heating systems. Steel Many boilers have steel heat exchangers. On some steel boilers, the exterior jacketing is very heavy plate steel. In other systems, the exterior jacket is light gauge sheet steel forming a cabinet that looks very much like a furnace cabinet. In either case, the exchanger steel is heavy gauge. Steel heat exchangers are considered lower quality than cast iron, since they are more susceptible to corrosion. Copper Many modern boilers use copper tubes. Some have aluminum fins on the tubes, and many have cast-iron headers at either end of the copper tubes. Copper is an excellent heat transfer medium, but copper-tube boilers have a shorter life expectancy than steel and considerably less than cast iron. Alloys Some boilers have stainless steel or copper-nickel alloy heat exchangers. These systems are relatively new and their life expectancy and long-term performance are not yet known.
1.1.2 How Boilers Work Several Fuels Boilers work much like furnaces. Heat is generated by burning coal, oil, natural gas, or propane, for example. Heat can also be generated by electricity. There are electric boilers. Distributing the Heat Furnaces use warm air to move heat from the source to various rooms of the house. Boilers use water instead of air to move heat through the house. Furnaces have heat exchangers, and so do boilers. The fire side of the heat exchangers for furnaces and boilers are virtually identical. In a furnace, however, the distribution medium is the house air, and that’s what you find on the other side of the heat exchanger. Boilers have water on the other side of the heat exchanger. Water Is the Heat Transfer Heat is transferred from the fire side of the heat exchanger, through the heat Medium exchanger, into the water. The water is piped to the various rooms of the house, where it is released through radiators, baseboards, or convectors (Figure 1.1). Radiant Heating In some homes, radiant heating is used where the distribution pipes are embedded in floors or ceilings and heat is released along the entire length of these piping grids, buried in the finishes. In radiant systems, there are no radiators or convectors