Description:

What Is This Measure?

Condensing boilers capture the heat that is lost through steam in conventional boilers and are therefore more efficient. Under the right conditions, they will outperform conventional boilers by a substantial margin. GSA tested condensing boilers at the Denver Federal Center and the Peachtree Summit Federal Building in Atlanta, Georgia and found energy savings greater than 14%.

Special Considerations

• Conduct a thermal load calculation to select a boiler that meets maximum thermal load without excess capacity. Relying on previous plant sizing is not necessarily reliable.
• Select boilers with a low turndown ratio and low minimum flow requirement.
• Operate multiple smaller boilers in parallel at low loads.
• For “condensing mode” to be achieved, The Return Water Temperature (RWT) must be below 130°F. Ensure a return water temperature below 130°F by implementing some or all of the following strategies: reduce supply water temperature in response to outside air temperature, zone temperatures, or control valve position; reduce the hot water flow rate, particularly when the building is unoccupied; optimize valves with the use of pressure independent control valves, two-way valves, and variable speed drives on booster pumps; install heating coils that provide temperature drops between 40°F and 60°F; use a primary piping system with one water loop that circulates water through both the boilers and the heating coils.
• Use condensing boilers for 75% of a building’s heating load with a conventional boiler as backup during the coldest weather. This strategy has the potential to increase condensing mode operation and lower initial condensing boiler costs.

See GSA’s Green Proving Ground for more details on the field demonstration findings.