Reversing valve diagram: how a heat pump switches modes
Picture the valve as a railroad switch for refrigerant. Here is what moves inside it and what you can safely check from the outside.
A reversing valve has four ports and a sliding piece inside that decides which way refrigerant flows. Move the slider one way and the system heats your home. Move it the other way and it cools. That single switch is what separates a heat pump from a plain air conditioner.
The four connections on the diagram
Most diagrams show one pipe on one side and three on the other. Each has a fixed job.
| Port | Connects to | Role |
|---|---|---|
| Discharge | Compressor outlet | Brings in hot, high-pressure gas |
| Suction | Compressor inlet | Returns low-pressure gas to the compressor |
| Outdoor coil port | Outdoor coil | Swaps between sending and receiving |
| Indoor coil port | Indoor coil | Swaps between sending and receiving |
The discharge and suction ports never change. The two coil ports trade roles each time the valve shifts.
What happens inside when it shifts
A small electric solenoid sits on the valve body. When the thermostat calls for a mode change, the solenoid energizes and uses refrigerant pressure to slide the inner piece from one end to the other. The slide takes a moment. You may hear a soft whoosh when the unit starts or stops.
In heating mode, hot gas goes to the indoor coil. In cooling mode, it goes to the outdoor coil. Same parts, opposite direction.
Signs the valve may be the problem
A diagram helps you describe what you see. It does not make the valve serviceable by you. Watch for these clues:
- Cool air from the vents when the thermostat asks for heat, or the reverse.
- The unit runs but the house temperature barely changes.
- A constant hissing or a loud clunk each time the mode changes.
- The outdoor coil stays frosted even after a defrost cycle.
These can also come from a dirty filter, a thermostat set to the wrong mode or a low charge. So check the easy items first.
What you can safely check
Confirm the thermostat is in the mode you want and the setpoint is far enough from the room temperature. Replace a dirty air filter. Clear leaves and snow from around the outdoor unit. Then watch what happens for a full cycle.
Why you should not open or repair it
The valve is brazed into the refrigerant line, so any repair means recovering refrigerant. Federal law requires EPA Section 608 certification to do that. The solenoid also runs on line voltage, and the control wiring sits next to parts such as the capacitor, which can hold a dangerous charge with the power off. Leave the panels closed.
Our advice: describe it, then call a pro
Tell a licensed HVAC technician which mode fails, what the air feels like and when it started. That points straight at the valve or rules it out, and it saves a diagnostic hour on the wrong part.
Frequently asked questions
What does a reversing valve diagram show?
It shows four refrigerant ports and an inner slide. The compressor ports stay fixed, and the two coil ports swap roles when the valve shifts between heating and cooling.
How does the reversing valve know when to switch?
The thermostat sends a signal to a small solenoid on the valve. The solenoid uses refrigerant pressure to move the inner slide to the other position.
Can I fix a stuck reversing valve myself?
No. The valve is brazed into the refrigerant line, and handling refrigerant legally requires EPA Section 608 certification. Call a licensed HVAC technician.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Air-Source Heat Pumps
- ENERGY STAR. Energy Savings at Home: Heating & Cooling