Intro to Heat Pumps

In this overview, we'll break down the basic functioning of heat pumps, focusing on their unique ability to switch between heating and cooling modes using a reversing valve. A heat pump essentially operates like a reverse-cycle air conditioner, which allows it to either cool or heat indoor spaces based on the seasonal need. In cooling mode, a heat pump functions much like a standard air conditioning unit. The compressor sends high-temperature discharge gas to the outdoor coil, where the refrigerant desuperheats, condenses, and subcools before moving through the liquid line to the indoor coil. Here, the refrigerant absorbs heat from the indoor air, transferring it outdoors.

Key Components and Function in Cooling Mode

In cooling mode, the outdoor coil acts as a condenser, rejecting heat outside. The refrigerant then travels to the indoor coil, which functions as an evaporator, cooling the indoor air. The system uses metering devices, such as a piston or thermostatic expansion valve (TXV), to regulate refrigerant flow, ensuring efficient temperature and pressure control within the coils. When liquid refrigerant flows from the outdoor to the indoor coil, the metering device allows it to boil and absorb heat efficiently.

Transition to Heating Mode

The heating function of a heat pump involves a four-way reversing valve, which reroutes discharge gas to the indoor coil. Here, the indoor coil acts as a condenser, releasing heat indoors, while the outdoor coil becomes the evaporator, absorbing heat from the outdoor air. The reversing valve has a central discharge port that always receives discharge gas, and its two side ports alter function based on the heating or cooling mode. In cooling, discharge gas moves to the outdoor coil, but in heating mode, it is directed indoors to warm the space.

Thermostat and Reversing Valve Coordination

The thermostat plays a role in directing the system, with specific terminals (O or B) controlling the reversing valve's position to align with either heating or cooling demands. Some reversing valves are designed to fail in heating mode, especially in colder climates, while others fail in cooling mode, a configuration common in warmer areas. This feature helps ensure the system defaults to the most needed mode for the climate.

Metering Devices: Fixed Bore and Thermostatic Expansion Valves

Heat pumps can use different metering devices, such as fixed-bore pistons or TXVs, for controlling refrigerant flow. A fixed-bore metering device, though less common, uses a simple piston that adjusts flow based on directional refrigerant pressure. In contrast, TXVs provide more precise control and allow for efficient temperature and pressure adjustments. Modern TXVs often include built-in check valves, simplifying the system by reducing the need for external bypasses.

In cooling mode, refrigerant flows through the path of least resistance, often bypassing the TXV or piston to prevent over-metering. During heating mode, the check valves allow refrigerant to reverse direction through the system, ensuring efficient flow and heat transfer in both modes.

Additional Components: Biflow Filter Dryer and Suction Line Accumulator

Key additional components in heat pumps include the biflow filter dryer and suction line accumulator. The biflow filter dryer permits liquid refrigerant to flow in either direction without pressure drops, critical for systems that alternate between heating and cooling. The suction line accumulator protects the compressor by storing excess refrigerant that could cause damage if it returned to the compressor in liquid form. This component is especially important during defrost cycles, where condensed refrigerant could otherwise re-enter the compressor.

Defrost Cycle

In colder temperatures, frost may form on the outdoor coil while the heat pump operates in heating mode. To remove this frost, the heat pump switches temporarily to cooling mode via a defrost board or relay, redirecting discharge gas to the outdoor coil to melt frost buildup. The system shuts off outdoor fans during this cycle to accelerate defrosting and activates electric resistance heaters to maintain indoor comfort. This feature, called temper heat, ensures that indoor temperatures stay comfortable even when the system momentarily shifts to cooling mode for defrosting.

This overview covers the fundamental mechanisms that enable a heat pump to efficiently alternate between heating and cooling. The reversing valve, metering devices, filter dryer, and defrost cycles all work in tandem to make heat pumps versatile, efficient heating and cooling solutions.