8.18   Steam Heating and Distribution

Steam heating is less efficient than hot-water heating because steam requires higher temperatures than hot water. For single-family homes, consider replacing a steam heating system with a hot-water or forced-air system. Multifamily buildings, especially multi-story buildings, may have little choice but to continue with steam because of the high cost of switching systems.

Note: You can recognize a steam boiler by its sight glass, which indicates the boiler’s water level. Notice that the water doesn’t completely fill the boiler, but instead allows a space for the steam to form above the boiler’s water level.

8.18.1   Steam Pressure Limits

If the steam-heating system remains, operate it at the lowest steam pressure that heats the building adequately. Two psi on the boiler-pressure gauge is a practical limit for many systems although some systems can operate at pressures down to a few ounces per square inch of pressure. Traps and air vents are cru­cial to operating at a low steam pressure.

8.18.2   Steam System Maintenance

Do these safety and maintenance tasks on steam systems.

✓     Verify that steam boilers have functioning high-pressure limits and low-water cut-off controls.

✓     Verify the function of the low-water cutoff by flushing the low-water cutoff with the burner operating. Combustion should stop when the water level in the boiler drops below the level of the float. If combustion continues, repair or replace the low-water cutoff.

✓     Verify that flush valves on low-water cutoffs are operable and don’t leak water.

✓     Ask owner about instituting a schedule of blow-down and chemical-level checks.

✓     Specify that technicians drain mud legs on return piping.

8.18.3   Steam System Energy Conservation

Specify the following efficiency checks and improvements for steam systems.

Electric vent dampers reduce off-cycle losses for both gas- and oil-fired steam systems.

One-Pipe Steam

✓     Verify that high-pressure limit control is set at or below 1 (one) psi or as low as acceptable in providing heat to the far ends of the building.

✓     Verify that air vents function and that all steam radiators receive steam during every cycle. Air vents release air to make room for steam. Major pipe risers and all radiators should have vents.

✓     Unplug air vents or replace malfunctioning vents as neces­sary. Add vents to steam lines and radiators as needed to get steam to all the registers.

✓     Radiator air vents should be open to release air while the system is filling with steam, then closed when steam reaches the vents. Steam need not fill radiators on every cycle. In mild weather, steam partly fills radiators before the boiler cycles off.

✓     Replace malfunctioning radiator air vents as necessary. However, don’t over-vent radiators because this can cause overheating and water hammer from too much steam con­densing.

Two-Pipe Steam: General Improvements

✓     Consider a modern high-efficiency power burner and electric vent damper as retrofits for steam boilers.

✓     Clean fire side of heat exchanger of noticeable dirt.

✓     Insulate all steam piping that passes through uncondi­tioned areas to at least R-3 with fiberglass or specially designed foam pipe insulation rated for steam piping.

✓     Consider installing remote sensing thermostats that vary cycle length according to outdoor temperature and include night-setback capability.

Two-Pipe Steam: Traps and Orifices

✓     Inspect return lines and condensate receiver for steam coming back to the boiler, which is a sign of leaky traps. Check radiator and main line traps for steam leakage.

✓     Check steam traps with a digital thermometer or listening device to detect any steam escaping from radiators through the condensate return. Replace leaking steam traps or their thermostatic elements.

✓     When you can gain access to all the system’s steam traps, repair leaking steam traps or replace them. Replace all failed traps at the same time to prevent new traps failing because of water hammer from steam leakage through neighboring failed traps.

✓     The only 100% reliable way to test a steam trap is to con­nect it to a test apparatus and see if it allows steam to pass. However if you have an accurate thermometer, the tem­perature on the radiator side of a functioning trap should be more than 215°F and the temperature on the return side of that trap should be less than 205°F when steam occupies the radiator.

✓     When you can’t gain access to all the system’s steam traps at the same time, consider abandoning failed steam traps and installing radiator-inlet orifices in two-pipe steam radiators. The orifices limit steam flow to an amount that can condense within the radiator. Orifices can also reduce steam delivery to oversized radiators by 20% or a little more.

Two-Pipe Steam: Thermostatic Radiator Valves

Consider controlling radiators with thermostatic radiator valves (TRVs) except for radiators in the coolest rooms. TRVs can be used with systems equipped with either traps or orifices.

For effective temperature control, locate the thermostatic ele­ment of the TRVs in the path of cool air moving toward the radiator or convector. TRVs are available with sensors located remotely from the valve, which solves the problem of a valve located where the radiator heats a valve-mounted sensor, fooling it into closing.

8.18.4   Converting Steam Distribution to Hot Water

Quite a few steam systems have been converted to hot water dis­tribution over the years by using the existing distribution piping, radiators, or both. Although some of these systems work well, many have problems. Converting steam to hot water has a num­ber of potential problems, including the following.

•       The conversion increases the operating pressure of the old pipes by a factor of 10 and this could cause leaks.

•       In two-pipe systems, the return may not be large enough and require re-piping.

•       In one-pipe steam the return must be installed to every radiator.

•       The steam radiators may not be large enough to heat ade­quately with hot water.

•       The existing radiators may be the type that won’t work with hot water.

For these reasons, it’s better to fix the steam system than to con­vert the system to hot water.