High-Pressure Switch
An auto-reset high-pressure switch (single pole, single throw) is located on the liquid line. The switch shuts off the compressor if the discharge pressure rises above the factory setting. The switch is normally closed and is permanently adjusted to trip (open) at a factory-preset maximum high-pressure point. The switch resets (closes) when the pressure drops below a factory-preset minimum low-pressure point.
Low-Pressure Switch
Some heat pumps may have a low-charge switch that functions if there is a potentially damaging loss of refrigerant. The switch is an pressure switch located on the discharge line of the compressor. The switch opens on low-pressure drop in the discharge line to shut off the compressor. The switch opens and closes at preset pressure points.
Other Electric/Electronic Heat Pump Controls and Connections A lockout relay is used on many heat pumps to shut the unit off to protect the compressor from damage if there is a problem in the system. An anti-restart timer is required in scroll and rotary com- pressors to prevent them from running backwards when there is a power failure. Every heat pump system requires a manually operated disconnect switch to turn off power and shut down the system when there is a problem or when repairs have to be made.
A service light thermostat is a service light switch located on the compressor discharge line and directly connected to the service light in the indoor room thermostat. If the compressor stops running, the service line thermostat senses the change in the discharge line and turns on the service light on the room thermostat. The light is turned off when the compressor is restored to operation and the discharge line returns to normal. Some service light thermostats are connected to terminals on the defrost control board.
Accumulator
Some compressors, such as the piston (reciprocating) types, can be damaged by liquid refrigerant entering the compressor. These compressors are designed to compress the gas formed from the refriger ant. They have a problem compressing the refrigerant liquid. An accumulator is installed in the return line to trap and store the refrigerant liquid before it can enter the compressor. The operation of a scroll compressor is such that an accumulator is not required. It can handle small amounts of liquid refrigerant without damage.
Room Thermostat
The heat pump operation is controlled by a room thermostat.
Heat pump thermostats may vary in design among the various manufacturers, but their operation will be essentially the same. Thermostat operation is controlled by the following:
• Temperature selector levers or dial
• Temperature indicator
• Fan switch
• System switch
The temperature selector levers (one for cooling and the other for heating) or the temperature selector dial are used to manually select the desired temperature setpoints for either heating or cooling.
The temperature indicator on the face of the thermostat is used to indicate the actual room temperature. Most will also have an amber (green) light that indicates when the heat pump is operating in the emergency mode.
The fan switch will offer up to four settings. The ON or CONT (continuous) setting provides continuous operation of the indoor blower regardless of whether the compressor or an auxiliary heater is operating. This setting is selected when continuous air circulation or filtering is desired. The AUTO or INT (intermittent) setting restricts blower operation only to those times when the thermostat calls for heating or cooling. It is the recommended setting for humidity control.
The system switch is used to set the heat pump for heating, cool- ing, or auto operation. The heating and cooling mode settings are self-explanatory. The AUTO mode provides the heat pump with the ability to automatically switch back and forth between the heating and cooling modes in order to maintain a predetermined comfort setting.
If the heat pump system is designed to provide supplementary heat when there are excessively cold temperatures, the auxiliary heater is controlled by the thermostat through an emergency heat mode. The emergency heat mode locks out heat pump operation while the auxiliary heater is operating.
If a programmable thermostat is used to control the heat pump, then temperature setpoints can be selected (programmed) for different times of the day. For example, with 7-day programming, the heat pump can be programmed for different temperature and humidity settings at different times every day of the week.
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