Reversing Valve Symbol
Definition: The Reversing Valve symbol represents the pilot-operated four-way refrigerant valve that switches a heat pump between heating and cooling by swapping which coil receives the compressor's hot discharge gas, drawn as a four-port valve body with crossover flow arrows and an attached solenoid coil whose two leads (Coil A, Coil B) are energized by the thermostat's O or B signal.
Also known as: 4-way valve, four-way reversing valve, heat pump reversing valve, changeover valve, O/B valve, RV solenoid, cycle reversing valve.
What the Reversing Valve symbol means
The Reversing Valve symbol denotes the component that makes a heat pump a heat pump. The valve body has four refrigerant ports: the compressor discharge always enters one port, the compressor suction always leaves another, and the remaining two connect to the indoor and outdoor coils. A sliding shuttle inside the body routes hot discharge gas to EITHER coil: to the outdoor coil for cooling (indoor coil becomes the evaporator) or to the indoor coil for heating (indoor coil becomes the condenser). Electrically, only the small pilot solenoid appears in the wiring diagram — the two coil leads shown as Coil A and Coil B — because the shuttle itself is moved by system refrigerant pressure, steered by the pilot valve.
Which thermostat signal energizes that solenoid is the industry's great convention split: the O convention energizes the valve in COOLING (used by most brands — Carrier, Trane, Goodman, Lennox and others), while the B convention energizes it in HEATING (Rheem and Ruud). Thermostats therefore offer an O/B terminal configurable to match the equipment; wiring the wrong convention makes the system heat when it should cool and vice versa.
How to identify the Reversing Valve symbol
In refrigerant-circuit diagrams the reversing valve appears as a rectangle (the valve body) with four port stubs — one on one side for the discharge line, three on the other for suction and the two coils — containing arrows that show the straight-through and crossover flow paths; a second set of dashed arrows often shows the alternate (de-energized) routing. A small box or coil symbol attached to the body with two leads represents the pilot solenoid, sometimes labeled RVS or RV.
In electrical ladder/unit diagrams only the solenoid coil is drawn — per IEC 60617 as a rectangle coil symbol, or per ANSI/NEMA as a circle or sawtooth coil labeled RVS — fed from the low-voltage terminal strip's O (or B) terminal and common. Distinguish it from an ordinary two-way solenoid valve symbol by the four ports and crossover arrows on the mechanical drawing, or by the O/B terminal reference on the electrical one.
Function in a circuit
The pilot solenoid does not move the main shuttle directly — it is far too small. Instead, the energized pilot valve bleeds high-pressure discharge gas to one end of the main slide chamber and vents the other end to suction; the resulting pressure difference across the slide piston pushes the shuttle over, reconnecting the ports. This is why a reversing valve can only shift while the compressor is running and a meaningful pressure differential exists — energizing the coil with the system off produces only a click from the pilot.
In cooling (O convention, coil energized), discharge gas goes to the outdoor coil to reject heat, and the indoor coil evaporates refrigerant to absorb it. De-energize the coil and the shuttle reverses the roles for heating. Heat pumps also shift the valve during defrost, briefly running in cooling mode to send hot gas through the iced outdoor coil. Failure modes are electrical (open solenoid coil) or mechanical (shuttle stuck mid-travel, causing hot-gas bypass from discharge to suction — symptomatically similar to a weak compressor).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 provides the solenoid-coil symbol used for the valve's electrical representation; fluid-power drawing practice for directional valves follows ISO 1219-1 four-port conventions, adapted in HVAC literature to the crossover-arrow refrigerant schematic. Appliance safety falls under IEC 60335-2-40 for heat pumps. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 coil symbols and NEMA unit-diagram practice show the reversing valve solenoid as a coil labeled RVS on the low-voltage circuit fed from the thermostat's O or B terminal; AHRI 210/240 governs heat-pump performance ratings and UL 60335-2-40 the North American safety standard. |
| Key difference | There is no meaningful IEC-versus-ANSI difference in the symbol itself — both show a solenoid coil plus, on refrigerant schematics, a four-port body with crossover arrows. The consequential 'standard' difference is the O-versus-B energization convention between manufacturers: O = energize in cooling (most brands), B = energize in heating (Rheem/Ruud), selectable in thermostat setup. |
Terminals / pins
| Pin | Name |
|---|---|
| coil_a | Coil A |
| coil_b | Coil B |
Typical values
Pilot solenoid coils are predominantly 24 V AC (from the R–C low-voltage transformer circuit), drawing roughly 6–15 VA with coil resistances of a few ohms to tens of ohms; 120 V and 240 V coils exist on some packaged and refrigeration equipment. Valve bodies are sized by tonnage/flow for systems from about 1 to 20 tons, rated for R-410A/R-32/R-454B working pressures (450–650 psig design). Minimum pressure differential of roughly 75–100 psi across the system is needed for the slide to shift reliably. Coil check: measure resistance across Coil A/Coil B leads — an open reading means a failed solenoid, replaceable without opening the refrigerant circuit.
Where the Reversing Valve symbol is used
- Residential split-system heat pumps, switching between heating and cooling and enabling hot-gas defrost of the outdoor coil
- Package heat pumps and rooftop units with heat-pump cycles
- Ductless mini-split heat pumps (inverter systems), where the control board drives the valve solenoid
- Pool heat pumps and heat-pump water heaters using reversible refrigerant circuits
- Reversible-cycle refrigeration equipment such as hot-gas-defrost commercial systems
- Geothermal (water-source) heat pumps, reversing refrigerant flow between the water coil and air coil
Example
In a heat-pump low-voltage diagram, the reversing valve solenoid's Coil A pin wires to the air handler's O terminal and its Coil B pin to C (common): when the thermostat calls for cooling it energizes O with 24 V AC, the pilot shifts the slide, and discharge gas is routed to the outdoor coil; on a Rheem system the same solenoid would instead be fed from B and energize during heating, which is why the installer must set the thermostat's O/B configuration to match the equipment.
Key facts
- The reversing valve is a four-port, pilot-operated valve: compressor discharge and suction stay fixed, while the indoor and outdoor coil connections are swapped by a sliding shuttle.
- Only the small pilot solenoid is electrical; system pressure differential actually moves the slide — so the valve can only shift while the compressor runs.
- O convention = solenoid energized in COOLING (most brands: Carrier, Trane, Goodman, Lennox); B convention = energized in HEATING (Rheem, Ruud). Thermostats have a configurable O/B terminal for this.
- Coils are typically 24 V AC, 6–15 VA, and can be replaced without opening the sealed refrigerant system; a stuck valve body, however, requires brazing in a new valve.
- Heat pumps shift the reversing valve during defrost, momentarily running the cooling cycle to melt ice off the outdoor coil — the whoosh heard at defrost initiation.
- A valve stuck mid-position bypasses hot gas from discharge to suction, mimicking a weak compressor: low capacity in both modes with abnormal pressures.
- Touch diagnosis: of the three pipes on the body's common side, the center (suction) tube should be cool; a stuck-open pilot or mid-travel slide makes it hot.
- The energized-mode choice is deliberate design: energizing in cooling (O) means a coil failure in winter defaults the system to heating, the safer failure for cold climates.
Frequently asked questions
What is the difference between O and B on a thermostat?
Both drive the reversing valve solenoid; they differ in WHEN. O energizes the valve during cooling — the convention used by most heat-pump brands. B energizes it during heating — the Rheem/Ruud convention. Modern thermostats provide one O/B terminal and a setup option choosing the convention. Setting it wrong reverses the modes: the system heats on a cooling call and cools on a heating call.
Why won't the reversing valve shift when I energize the coil with the system off?
Because the solenoid only moves a tiny pilot valve; the main slide is shoved across by refrigerant pressure differential. With the compressor off there is little or no differential, so energizing the coil produces just a faint click from the pilot. To witness or test a shift, the compressor must be running with adequate pressures — typically at least 75–100 psi across the system.
How do I test a reversing valve solenoid coil?
Kill power, disconnect the two coil leads (Coil A and Coil B), and measure resistance: a reading of a few ohms to a few tens of ohms indicates an intact 24 V coil, while infinite means an open (failed) coil. With the system running, verify 24 V AC arrives at the leads when the thermostat commands the energized mode (O on a cooling call, or B on a heating call for Rheem-convention systems). The coil slides off its stem and replaces without touching the refrigerant circuit.
What are the symptoms of a stuck reversing valve?
A valve stuck fully in one position simply refuses to change mode — cooling works, heating doesn't, or vice versa, despite 24 V reaching the coil. Stuck MID-travel is nastier: hot discharge gas short-circuits straight to suction, so both modes lose capacity, suction pressure runs high, discharge runs low, and the suction line at the valve gets hot — symptoms that closely mimic a failing compressor. The temperature-touch test on the valve's three common-side tubes distinguishes them.
Why do heat pumps energize the reversing valve in cooling rather than heating?
It is a fail-safe choice. With the O convention, a burned-out solenoid coil or lost O signal leaves the valve relaxed in the HEATING position, so a mid-winter failure still leaves the house with heat — the more safety-critical mode in cold climates. Rheem/Ruud chose the opposite (B, energize in heating), which some argue reduces coil duty in heating-dominated climates. Both work; the equipment and thermostat simply must agree.
Does the reversing valve operate during defrost?
Yes. When the defrost control detects an iced outdoor coil (by time, temperature, or demand sensing), it shifts the reversing valve into cooling mode while keeping the compressor running and usually stopping the outdoor fan — sending hot discharge gas through the outdoor coil to melt the frost, while auxiliary heat covers the house. The audible whoosh/swoosh at defrost start and end is the valve shifting under full pressure differential; it is normal.
Related symbols
- Contactor Coil symbol
- Single-Phase Motor symbol
- Pressure Switch symbol
- Relay Coil symbol
- Solenoid Valve symbol
- Thermostat symbol
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