8.17 Hot-Water Space-Heating Distribution
The most significant energy wasters in hot-water systems are poor steady-state efficiency, off-cycle flue losses stealing heat from the stored water, and boilers operating at a too-high water temperature. For information about boiler installation, see page 281.
8.17.1 Boiler Efficiency and Maintenance
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SWS Details: 5.3104.2 Maintenance: Gas Boiler Service Inspection, 2.0203.3 Combustion Air—Boilers |
Monitor boiler performance and efficiency by inspecting for these problems.
• Corrosion, scaling, and dirt on the water side of the heat exchanger.
• Corrosion, dust, and dirt on the fire side of the heat exchanger.
• Excess air during combustion from air leaks and incorrect fuel-air mixture.
• Off-cycle air circulation through the firebox and heat exchanger, removing heat from stored water.
Boiler Efficiency Improvements
Consider the following maintenance and efficiency improvements for both hot-water and steam boilers based on boiler inspection.
✓ Check for leaks on the boiler, around its fittings, or on any of the distribution piping connected to the boiler.
✓ Clean fire side of heat exchanger of noticeable dirt.
✓ Drain water from the boiler drain until the water flows clean. Then add water to refill the system.
8.17.2 Hydronic Distribution System Improvements
Hydronic distribution systems consist of the supply and return piping, the circulator, expansion tank, air separator, air vents, and heat emitters. A properly designed and installed hydronic distribution system can operate for decades without service. However, many systems have installation flaws or need service.
Note: You can recognize a hot-water boiler by its expansion tank, located somewhere above the boiler. The expansion tank provides an air cushion to allow the system’s water to expand and contract as it is heated and cooled. Without a functioning expansion tank excessive pressure in the boiler discharges water through the pressure-relief valve.
Safety Checks and Improvements
Work with contractors and technicians to specify and verify the following safety and efficiency tests and inspections.
✓ Verify the existence of a 30-psi-rated pressure relief valve. The pressure relief valve should have a drain pipe that terminates 6 inches from the floor. Replace a malfunctioning valve, or install a pressure relief valve if none exists. Look for signs of leakage or discharges from that valve. Find out why the pressure relief valve is discharging.
✓ Verify that the expansion tank isn’t waterlogged or isn’t too small for the system. A waterlogged expansion tank can make the pressure relief valve discharge. Measure the expansion tank’s air pressure. A common pressure for one and two-story buildings is 12 psi. The pressure in taller buildings should be approximately one (1) psi per 2.3 feet of the distribution system’s height.
✓ If you observe rust in venting, verify that the return water temperature is warmer than 130°F for gas and warmer than 140°F for oil. These minimum water temperatures prevent acidic condensation.
✓ Verify that high-limit control disengages the burner at a water temperature of 200°F or less.
✓ Lubricate circulator pump(s) if necessary.
Simple Efficiency Improvements
Do the following energy-efficiency improvements.
✓ Repair water leaks in the system.
✓ Remove corrosion, dust, and dirt on the fire side of the heat exchanger.
✓ Check for excess air during combustion from air leaks and incorrect fuel-air mixture. See “Critical Combustion-Testing Parameters” on page 273.
✓ Bleed air from radiators and piping through air vents on piping or radiators. Most systems fill automatically through a shutoff and pressure-reducing valve connected to the building’s water supply. If there is a shutoff and no pressure-reducing valve, install one and set it to the hydronic-system pressure. Then check the system pressure at the expansion tank, and adjust the pressure as necessary.
✓ Vacuum and clean fins of fin-tube convectors if you notice dust and dirt there.
✓ Insulate all supply and return piping, passing through unheated areas, with foam pipe insulation, at least one-inch thick, rated for temperatures up to 200° F.
Improvements to Boiler Controls
Consider these improvements to control systems for hot-water boilers.
✓ Install outdoor reset controllers to regulate water temperature, depending on outdoor temperature.
✓ If possible, operate the boiler without a low-limit control for maintaining a minimum boiler-water temperature. If the boiler heats domestic water in addition to space heating, the low-limit control may be necessary.
✓ After control improvements like two-stage thermostats or reset controllers, verify that return water temperature is high enough to prevent condensation and corrosion in the chimney as noted previously.
✓ Install electric vent dampers on natural-draft gas- and oil-fired high-mass boilers.