Domestic water heater
Most boilers used in hot-water systems can be equipped so that they will also heat the domestic water (water used for washing and bathing). This is discussed in detail in chapter 16. When a heating-system boiler produces domestic hot water, the associated burner must fire all year long and not just for the heating season. In this case, the thermostat does not control the burner. It controls only the circulating pump. The burner is activated by an aquastat that controls the boiler water temperature.
Since the boiler will have hot water during those periods when heat is not required, a flow-control valve must be installed on the supply main. This valve prevents the hot water from rising into the distribution piping and heating the house like a gravity hot-water system. When heat is required, the circulating pump produces sufficient force to lift the flow-control valve and circulate the hot water. In a multizoned system, if zone valves are used, flow-control valves are not needed. When the zone valves are closed, water will not circulate in the distribution piping.
Every forced-hot-water heating system must be equipped with an automatic pressure-relief valve as a safety control. To help ensure its effectiveness, the relief valve should be mounted directly on the boiler. Many systems have a relief valve that is mounted in the boiler feed line several feet away from the boiler. This is an improper location because over the years the relief valve can become isolated from the boiler as a result of a lime or scale buildup in the feed line. If this should happen, the valve would be completely ineffective in the event of a pressure buildup. If the heating system does not have a boiler-mounted relief valve, you should have one installed.
Pressure, temperature gauge
Although not safety or operational controls, all forcedhot-water systems should have a pressure and a temperature gauge. Newer systems have a combination gauge that measures both pressure and temperature. In older systems, you probably will find a separate pressure gauge and a pencil-type thermometer attached to the boiler. The combination gauge often has two pressure scales, one in pounds per square inch (psi) and the other in altitude (feet of a column of water). (The altitude scale can basically be ignored, although for your information, one psi is equivalent to a column of water measuring 2.31 feet high.) The normal fill pressure of the boiler is 12 psi. This, then, is equivalent to an altitude of 27.7 feet. If the highest radiator in your house is more than 27.7 feet above the boiler, then a higher boiler-water pressure will be needed. Some pressure gauges have two points-one fixed and one movable. The fixed pointer is usually positioned over the normal altitude setting for that house. The movable pointer shows the actual working pressure of the heating system. If this pressure exceeds 30 psi, the relief valve will discharge.