From the initial VSGSHP testing, it was obvious that considerable gains had been made in comparison to the efficiencies of other existing systems. The OEM controller was inappropriate for this application because of its defrost control logic and lack of water-heating control capability. Therefore, it was necessary to design and build a new system controller with custom hardware and programmable software to implement the desired control logic.

An eight-bit microcontroller-based system was developed to control the variable-speed ground-source heat pump (see Figure 3). The control system determines not only the compressor and indoor fan speeds but the water-source loop pump and hot water pump speeds as well. A custom designed "thermostat" is used for the setpoint input.

Figure 3: Control system schematic.

The microcontroller incorporates many I/O (input/ output) functions and a microprocessor on a single chip, which results in a compact, low-power, and low-cost control system. Several channels of on/off parallel I/O are used to control the motors and relay outputs. On-chip timers are used for generating part of the required pulse-width-modulated (PWM) signals. Serial I/O is used to provide off-chip communication to a microcomputer for debugging and diagnostic needs. The control programs are stored on removable EPROM (erasable programmable read-only memory) chips to allow rapid modifications of control strategies.

The control system currently uses thermistors for temperature sensing. Both the indoor return air and the hot water temperatures are measured. Each thermistor is arranged in series with a fixed resistor that provides the correct range...