Unit 62 - Heat Pump System Fundamentals

The [blank_start]high side[blank_end] is created by compressing, or squeezing, the gas. When a gas is compressed, the heat in the gas is concentrated and the temperature rises. This is done by a component called a [blank_start]compressor[blank_end].

Expanding a gas or liquid causes its pressure and temperature to drop. This is done by the [blank_start]expansion valve[blank_end].

The [blank_start]increased[blank_end] pressure on the [blank_start]high side[blank_end] raises the condensing temperature, allowing the condenser to reject large amounts of heat while condensing the refrigerant.

The [blank_start]decreased[blank_end] pressure on the [blank_start]low side[blank_end] lowers the evaporation temperature, allowing the evaporator to absorb large amounts of heat while the refrigerant evaporates.

Electric strip heat: [blank_start]110 - 125ºF[blank_end] Gas heating: [blank_start]120 - 140ºF[blank_end] Heat pumps: [blank_start]95 - 115ºF[blank_end]

The [blank_start]heat source[blank_end] is where the heat comes from when the heat pump is operating in the heating cycle. The [blank_start]sink[blank_end] is where the heat pump puts the heat.

Split systems usually consist of an indoor unit and an outdoor unit connected together through a large [blank_start]gas line[blank_end] and a small [blank_start]liquid line[blank_end].

The large gas line is a [blank_start]low-pressure suction line[blank_end] when operating in cooling and a [blank_start]high-pressure discharge line[blank_end] when operating in heating.

In the summer they move heat from the [blank_start]inside[blank_end] to the [blank_start]outside[blank_end], and in the winter they move heat from the [blank_start]outside[blank_end] to the [blank_start]inside[blank_end].

When a heat pump is in the [blank_start]heating[blank_end] mode, the outdoor coil becomes the evaporator and the indoor coil becomes the condenser. When a heat pump is in the [blank_start]cooling[blank_end] mode, the outdoor coil becomes the condenser and the indoor coil becomes the evaporator.

The outdoor coil is the [blank_start]condenser[blank_end] in the cooling cycle and the [blank_start]evaporator[blank_end] in the heating cycle. The indoor coil is the [blank_start]evaporator[blank_end] in the cooling mode and the [blank_start]condenser[blank_end] in the heating cycle.

During the cooling cycle, the [blank_start]high-pressure[blank_end]/[blank_start]high-temperature[blank_end] discharge gas is directed from the compressor to the [blank_start]outdoor coil[blank_end] (condenser), where it condenses to a [blank_start]high-pressure[blank_end], [blank_start]high-temperature[blank_end] subcooled liquid.

To avoid the restriction of the [blank_start]metering device[blank_end] connected to the outlet of the outdoor coil, a [blank_start]check valve[blank_end] is installed, allowing the liquid to bypass the outdoor [blank_start]metering device[blank_end].

The liquid travels inside through the [blank_start]liquid[blank_end] line, where the indoor [blank_start]check valve[blank_end] closes, forcing the refrigerant through the indoor [blank_start]metering device[blank_end].

The indoor metering device drops the refrigerant [blank_start]pressure[blank_end] and [blank_start]temperature[blank_end] so that heat can be absorbed in the indoor coil.

The refrigerant vapor produced in the indoor coil now travels through the [blank_start]reversing valve[blank_end] to the [blank_start]suction accumulator[blank_end] and on to the [blank_start]compressor[blank_end]

In the [blank_start]heating[blank_end] cycle the reversing valve changes position, changing the direction of gas flow.

The [blank_start]high-pressure[blank_end], [blank_start]high-temperature[blank_end] gas from the compressor flows through the reversing valve to the [blank_start]indoor[blank_end] coil.

The indoor coil now acts as a [blank_start]condenser[blank_end], adding [blank_start]heat[blank_end] into the return air from the conditioned area.

The hot vapor is condensed, and the temperature is reduced to produce a [blank_start]high-pressure[blank_end], [blank_start]high-temperature[blank_end] [blank_start]subcooled[blank_end] liquid.

The indoor [blank_start]check valve[blank_end] opens and allows the liquid refrigerant to flow around the indoor [blank_start]metering device[blank_end]. Continuing through the liquid line, the liquid refrigerant is forced through the outdoor coil [blank_start]metering device[blank_end] by the closing of the [blank_start]check valve[blank_end] connected in parallel with it.

The [blank_start]low-pressure[blank_end], [blank_start]low-temperature[blank_end] mixture of liquid and vapor refrigerant flows into the outdoor coil, which is now the [blank_start]evaporator[blank_end].

The low refrigerant [blank_start]pressure[blank_end] keeps the refrigerant temperature [blank_start]lower[blank_end] than the outdoor temperature. Since the outdoor air is [blank_start]warmer[blank_end] than the refrigerant, heat is picked up from the outdoor air, [blank_start]evaporating[blank_end] the liquid refrigerant.

This refrigerant vapor then flows through the [blank_start]vapor[blank_end] line, [blank_start]reversing valve[blank_end], and [blank_start]accumulator[blank_end] to the compressor.

The reversing valve is composed of three main parts: the [blank_start]pilot[blank_end] valve, the main [blank_start]valve[blank_end], and the [blank_start]solenoid[blank_end] coil.

The solenoid coil controls the [blank_start]pilot[blank_end] valve, which in turn controls the suction [blank_start]bleed[blank_end] to the main valve.

The purpose of the [blank_start]main[blank_end] valve is to [blank_start]reverse[blank_end] the refrigerant route through the indoor and outdoor coils, thereby exchanging the functions of the condenser and evaporator coils.

By imposing the discharge pressure on one end of the [blank_start]reversing[blank_end] valve and the suction pressure on the other end of the valve, the [blank_start]pressure[blank_end] difference will shift the internal cylinder toward the side that is connected to the [blank_start]suction[blank_end] pressure.

The main valve body is constructed with the permanent [blank_start]suction[blank_end] in the middle of the valve on one side and the permanent [blank_start]discharge[blank_end] in the middle of the valve directly opposite the permanent [blank_start]suction[blank_end] port.

When at rest, the sliding cylinder always straddles two tube openings. One of these openings is always the permanent [blank_start]suction[blank_end] port, which is connected to the [blank_start]low[blank_end] side of the system. The other opening will be either the indoor coil port or the outdoor coil port. This makes whichever coil is connected to the permanent [blank_start]suction[blank_end] via the sliding valve the [blank_start]evaporator[blank_end].

The port that is not covered by the sliding valve is open to the permanent [blank_start]discharge[blank_end] port located opposite the permanent [blank_start]suction[blank_end] port. This makes whichever coil is connected to the uncovered port the [blank_start]condenser[blank_end].

What role does the outdoor coil serve as in this illustration (e.g., condenser or evaporator)?

The solenoid valve is de-energized and the control plunger is [blank_start]down[blank_end], closing the bottom port vent line connected to the [blank_start]right[blank_end] end of the piston. The top port vent line connected to the [blank_start]left[blank_end] end of the piston is open to the equalizing line. This allows the pressure to [blank_start]bleed[blank_end] off from the [blank_start]left[blank_end] end of the piston to the permanent [blank_start]suction[blank_end] line.

This creates a pressure difference across the left-side piston (usually 75–100 psig or greater), which causes it to travel to the extreme left. This movement repositions the slide valve to align the control port from the outdoor coil with the control port leading to the compressor [blank_start]suction[blank_end], making the outdoor coil the [blank_start]evaporator[blank_end]. This same movement also aligns the compressor [blank_start]discharge[blank_end] with the indoor coil, making the indoor coil the [blank_start]condenser[blank_end].

What role does the indoor coil serve in this illustration (e.g., evaporator or condenser)?

The solenoid valve is [blank_start]energized[blank_end] and the control plunger lifts, [blank_start]closing[blank_end] the top port vent line connected to the left end of the piston. The bottom port vent line connected to the right end of the piston is [blank_start]open[blank_end] to the equalizing line. This allows the pressure to [blank_start]bleed[blank_end] off from the right end of the piston, which causes it to travel to the extreme right, repositioning the slide valve to align the control port from the [blank_start]indoor[blank_end] coil with the control port leading to the compressor [blank_start]suction[blank_end], making the indoor coil the evaporator. This same movement also aligns the compressor [blank_start]discharge[blank_end] with the outdoor coil, making the outdoor coil the [blank_start]condenser[blank_end].

Since both the indoor and outdoor coils must function as the [blank_start]evaporator[blank_end], both coils need [blank_start]metering[blank_end] devices.

It is possible to have a single expansion valve on packaged [blank_start]water[blank_end]-source units because the two coils are so close together. Using a [blank_start]bi-flow[blank_end] valve that meters in both directions allows manufacturers to build a unit with only one metering device, eliminating [blank_start]check[blank_end] valves completely.

The sensing bulb is not located on the [blank_start]outlet[blank_end] of either coil. Instead, the sensing bulb is located after the [blank_start]reversing[blank_end] valve and before the suction [blank_start]accumulator[blank_end]. In this way the bulb will respond to the refrigerant condition leaving either coil, regardless of mode.

The [blank_start]check[blank_end] valves in the heat pump circuit are vital to ensure that proper pressures are maintained. Today, most heat pump systems use [blank_start]metering[blank_end] devices with built-in [blank_start]check[blank_end] valves, rather than separate [blank_start]check[blank_end] valves. The [blank_start]check[blank_end] valves are still there but are incorporated into the [blank_start]metering[blank_end] devices.