Results
This field study demonstrated the energy saving and operational benefits of 100 percent outside air and HTHV direct-fired gas heaters. In a side-by-side comparison of alternating months over the 2013–2014 heating season, the new HTHV direct-fired natural gas heaters consumed 20 percent less natural gas than the existing heaters on a normalized basis over the monitoring period (Figure 2). This 20 percent value exceeds the expected energy savings if simply comparing the nominal equipment efficiencies of 92 percent versus 80 percent for gravity-vent units. This difference was attributed to the overall system efficiency gains through destratification and operational efficiencies.
The HTHV heaters in the noted study operated with a high-pressure supply fan delivering heated air to the floor and creating vertical circulation that reduced temperature stratification between the floor and ceiling and can also deliver outside air requirements for the building. While this strategy reduces thermal energy consumption, the units carry higher fan electricity consumption, which offset a portion of the thermal savings. Nevertheless, over an average heating season for the host site in Bridgeton, Missouri (3705 average HDD, 16 C [60 F]), the new natural gas heaters would save approximately 15 percent on space-heating utility costs and on source energy.
Additionally, the HTHV technologies showed improved temperature control over conventional equipment by reducing thermal stratification between the floor and ceiling. Figure 1 displays the difference in temperature near the floor (i.e. 1.5 m [5 ft] off the floor) and at the ceiling (i.e. 6 m [20 ft] off the floor) during the monitoring period. For buildings with high ceilings, such as the warehouse in this demonstration, warm air naturally rises and can raise the average ceiling air temperature D6 to 11 C (D10 to 20 F) above the thermostat set-point. Due to this temperature gradient, the heating system must run longer and consume more energy to meet the needs of the building’s occupants at or near floor level.
As demonstrated by the temperature differential between floor and ceiling in Figure 3, the HTHV gas heaters eliminate much of the temperature stratification experienced with the existing unit heaters. Generally, the existing gas heaters exhibited vertical temperature differences of more than D3 C (D5 F) higher than those exhibited by the high-efficiency gas heaters. HTHV direct-fired gas heaters and other destratification technologies increase air circulation and provide more uniform temperature distribution throughout the space. Additionally, because the roof is typically the largest area for heat transfer in a warehouse, lowering the temperature of the interior ceiling decreases the heat loss through the roof.
