If a house has a boiler system with radiators that hiss as the heat comes on, chances are that house uses a steam heating boiler. This type of heating system was more common long ago, and homeowners with houses that still use them may need to repair them from time to time. Here are a few trouble shooting tips to diagnose what may be wrong.
Check the Water Level
Steam boilers have small glass windows on them that allow one to check the water level within the steam boiler. Steam heating systems will, by their nature, always lose some of their water with each heating cycle. If there is no water within the boiler, no steam can be produced to heat the house.
Check the Low Water Cut-Off Valve
Continuing to heat an empty boiler may not only damage the boiler itself, but may also build pressure in the system that could lead to an explosion. For this reason, many steam boilers have a low water cut-off valve that senses when water levels are dangerously low and turns off the burner beneath the boiler. If the boiler ran low on water, was refilled, but the burner will not turn back on, it is likely the low water cut-off valve needs to be reset. Models without automatic resets have a reset button on the valve itself. These valves should be flushed weekly to ensure they remain in proper working condition.
Check the Pressure Gauge
Most steam heating boiler systems are calibrated to run at very low pressures—usually ½ psi. Mineral deposits and dirty water can lead to blockages in the piping that will cause pressure to build up within the system, much like heating an empty boiler would. When this happens, a safety valve will shut off the burner of the unit much like the low water cut-off does when the water levels are low. If the pressure in a boiler system is too high, try cleaning the release valves on the radiator itself; these are the valves that produce the hissing noise. Minerals from evaporated water can often stick these valves shut. Always use extreme caution when dealing with boilers or valves that are above their recommended pressure settings.
Listen for Water Blockage
As a house ages it is possible that it will settle or shift a little. This shifting can cause the pipe leading to the radiator to slope at the wrong angle. If this happens, water can no longer drain from the radiator back into the boiler when it condenses. Instead, the water pools in the pipe and blocks the hot steam from reaching the radiator. A clear indicator that water is pooling in the pipes is an excessively loud banging sound coming from the radiator as it turns on. The noise is caused by steam slamming its way through pools of collected water. In some scenarios, simply raising the radiator on blocks will restore the proper slope to the piping. If not, major plumbing may be required.