A new control algorithm was designed to utilize the full range of variable-speed operation available from our control hardware. With the system controller, there are now 50 possible compressor speeds, 84 indoor fan speeds, and 60 main loop or hot water pump speeds. The object of the new control algorithm was to optimize efficiency while adequately meeting cooling, heating, and hot water demand loads.

The nature of thermal capacity in a large enclosed space is such that a rapid control response is not necessary. Simple proportional control with a slow (30-second) sample rate is sufficient to adjust heating and cooling capacities in the variable-speed range. Initial operating speed is set by the difference between the indoor air temperature and the desired setpoint, with a variable-speed update after each sample period.

As stated in an earlier section, the minimum operating speed for the compressor was set equal to the manufacturer's 1/4 test speed to ensure adequate oil return to the compressor. Maximum compressor speed was limited to 3/4 speed in cooling for increased efficiency but allowed to run at full speed in heating to increase the heating capacity. For initial speed settings, an indoor air setpoint temperature difference of less than 2 F (1 C) will start the system at 1/4 or low speed. In the 2 F to 6 F (1 C to 3 C) range, the operating speed starts as a linear function between low and 3/4 speed for cooling and low to full speed for heating. If the temperature difference is greater...