Evaporator Fan Delay Control

This laboratory assessment investigated the potential energy efficiency benefits when evaporator fan cycle off time was delayed. The objective was achieved by testing a nominal 3-ton split air-conditioner under controlled environment conditions in the laboratory setting. The test unit was equipped with an air-cooled condenser and a single-speed compressor, which is one of the most common air-conditioning (A/C) units found in residential applications. Normal operation of a typical residential A/C unit is such that when the thermostat setpoint temperature is met, both the compressor and evaporator (supply) fan cycle off. When the compressor and evaporator fan are cut off in response to the thermostat control, the evaporator coil is still partially flooded with the liquid refrigerant. This residual liquid refrigerant can be used to provide space cooling. This can occur by running the evaporator fan for a short time after the compressor cycles off. This period is referred to as “fan delay time, or period”. While used for decades in residential space heating, it has not yet been fully evaluated in cooling applications. Fan delay technologies are commercially available for cooling applications either as an on-board by original equipment manufacturers, or as an add-on option. This study involved conducting ten test scenarios at Southern California Edison’s (SCEs) Technology Test Centers (TTC). The duration of each test was one hour. For every test scenario, the thermostat in the indoor test chamber (room) was set to 75 degrees Fahrenheit (°F) while the outdoor test chamber (ambient) was maintained at 115°F. Evaluation of various fan delay periods occurred under part load ratios (PLRs) of 0.34, 0.53, and 0.76. PLR is defined as the ratio of imposed cooling load in the room to cooling capacity of the A/C unit as published by the manufacturer for a particular outdoor/indoor condition. This led to identifying fan delay periods with the highest energy efficiency potential. In addition, it established variations in energy efficiency potentials as a function of PLRs. This project evaluated two types of commercially available add-on delay controllers. One controller was capable of delaying fan cycle off period based on a prescribed time. The other controller had a built-in logic to delay the fan cycle off period based on the compressor’s run time history. The control logic of the latter technology directly correlated fan delay periods with compressor run times. Results obtained from these tests were later used to determine the equivalent electrical energy that was mitigated during fan delay periods. The equivalent electrical energy was determined as a function of the amount of heat extracted from the evaporator coil during fan delay periods. After a close review of findings, three optimum test scenarios were identified. It should be emphasized that project findings and conclusions are specific to the particular 3-ton unit tested. Project findings indicated delaying the evaporator fan cycle off time had no impact on the overall power demand of the tested A/C unit. Electrical energy savings potentials, on the other hand, noticeably varied as a function of delay periods and PLRs. For optimum delay periods of four to five minutes, as the PLR increased from 0.34 to 0.76, the energy savings reduced from 20.6% to 4.5%. Clearly, at 100% PLR the energy savings diminished. The A/C units with single-speed compressor typically operate at full capacity even during periods when cooling load in the conditioned space is less than the A/C unit’s cooling capacity. Under such conditions, while the compressor may operate at full load without substantial variations in power demand, its run time will be decreased. The project findings coupled with building energy simulation results can be used to establish annual energy savings. Subsequently, the eQUEST building energy simulation modeling was performed for a two story residential home. The A/C unit for the model home was a 3.5-ton split system with a seasonal energy efficiency ratio of 13. The conditioned space was 1,768 square feet. The simulation was done for all 16 climate zones. The key variables from the hourly simulation report were extracted and combined with the test findings to calculate the annual energy savings. Results are summarizes in Table 1. To establish the annual energy savings for different building characteristics including vintages and sizes, it is recommended to repeat the same methodology.