What it is

The reversing valve is the four-way valve that swaps a heat pump between heating and cooling by reversing refrigerant flow through the indoor and outdoor coils. But the valve body itself isn't electric — it's shifted by refrigerant pressure. The electric part is a small solenoid coil that operates a tiny pilot valve, and that pilot valve is what actually causes the big slide inside the main body to move.

This article is about the solenoid/pilot as a component: what it does, the O/B control logic, the coil's electrical values, and how to test the coil separately from the valve body — because "reversing valve problem" is often just a dead coil or a control issue, not a failed valve.

How it works

The main reversing valve has a sliding piston inside that directs flow one way or the other. That slide is moved by a pressure difference, not by the coil directly. Here's the chain:

  1. The solenoid coil is an electromagnet wrapped around a small pilot-valve assembly mounted on the side of the main body.
  2. When the coil energizes, it pulls a small plunger that shifts the pilot valve.
  3. The pilot valve routes high-side and low-side pressure to opposite ends of the main slide through small capillary tubes.
  4. That pressure difference pushes the main slide to the other end, reversing the big flow paths.

So the coil only moves a tiny pilot; refrigerant pressure does the heavy lifting on the main slide. That's why a healthy coil and a working pilot can still leave a valve "stuck" if there isn't enough pressure differential (e.g., a very low charge) to shift the slide.

O vs B control logic: Heat pumps energize the reversing-valve coil in one fixed mode, and the thermostat terminal that does it tells you which:

  • O terminal — energizes the reversing valve in COOLING. Most common (most heat pumps are "energized in cool"). The coil is energized when the system is in cooling and de-energized in heating.
  • B terminal — energizes the reversing valve in HEATING. Less common (some brands). The coil is energized in heating.

Either way, the valve has a default (de-energized) position and the coil flips it to the other.

In the field

Confirm the system's O/B convention. Check the thermostat setup and the equipment: is the coil energized in cool (O) or in heat (B)? You need this to know whether the coil SHOULD be energized for the mode you're testing. Energizing in cool is the common default.

Test the coil electrically (the easy part). The coil is usually 24V. With it isolated, ohm across the coil leads — you want a sensible resistance, not open and not shorted. Open coil = the pilot never shifts = the valve can't reverse on demand. You can also confirm 24V is actually reaching the coil when the mode calls for it.

Listen and feel for the pilot shifting. When the coil energizes (or de-energizes) on a mode change, you can often hear a soft click/hiss as the pilot moves and feel the valve "thunk" as the main slide shifts. The temperatures of the four lines swap accordingly.

Separate a coil problem from a valve-body problem. This is the key diagnostic:

  • If the coil is good (correct resistance), getting 24V on the call, and you can slide the coil off and feel the magnetism — but the valve still won't reverse — the problem is the pilot or the valve body (stuck slide, debris, or insufficient pressure differential), NOT the coil.
  • If the coil is open/shorted or not getting voltage, that's your fault — and the coil is field-replaceable by itself (it slides off a retaining clip/nut without opening the refrigerant circuit).

A stuck slide sometimes frees with a light tap. With the system running and the coil energized appropriately, a gentle tap on the valve body (not a hammer-blow) can dislodge a slide stuck mid-travel. It's a diagnostic nudge, not a permanent fix — a valve that hangs up will do it again.

Normal values & targets

  • Coil voltage: 24V AC is standard on residential heat pumps. Read the coil.
  • Coil resistance: a sensible non-zero value (commonly in the range of roughly 10–25 ohms for a 24V coil; varies by part — compare to known-good). Open = OL = bad coil; near-zero/shorted = bad coil that may trip the low-voltage fuse.
  • Energized mode: O-system = coil energized in COOLING; B-system = coil energized in HEATING.
  • Pressure differential to shift: the valve needs adequate high-to-low pressure difference to move the main slide — a severely undercharged or non-running system may fail to shift even with a perfect coil.
  • Coil replacement: the solenoid coil slides off and on without opening the sealed system (no recovery needed) — only the valve body itself requires recovery/brazing.

Common faults & what they mean

  • Heat pump stuck in one mode, coil reads open → dead solenoid coil; replace the coil (no refrigerant work needed).
  • Coil good and getting 24V, valve still won't reverse → pilot valve or main slide stuck, debris, or too little pressure differential. The coil is innocent; look at the valve body and the charge.
  • Valve reverses but partially / leaks internally (hot and cold lines both warm, poor capacity) → internal bypass in the valve body (slide not fully seating); this is a valve-body failure, often needing replacement.
  • Coil energized in the wrong mode for the system's O/B setting → thermostat O/B set wrong, or wired to the wrong terminal; the system heats when it should cool or vice versa.
  • Low-voltage fuse blows when the mode that energizes the coil is selected → shorted coil or shorted wire to it.
  • Valve won't shift on a very low-charge system → not enough pressure differential to move the slide; fix the charge before condemning the valve.

Tech tips & gotchas

The coil and the valve body are two different parts with two different repair paths. The coil is a cheap, slide-off electrical part — no refrigerant recovery. The valve body is a brazed-in component that requires recovery and careful brazing (heat can damage the valve's internals) to replace. Always test the coil FIRST; a dead coil is a quick, cheap fix that's easy to mistake for a failed valve.

Know your O vs B convention before you diagnose "wrong mode" complaints. If a heat pump blows cold in heat or warm in cool, an incorrect O/B thermostat setting is a common, free fix — the coil's being energized in the wrong mode.

A valve that won't shift isn't always broken — it might just lack pressure differential. On a system that's low on charge or barely running, the slide can't move. Verify the system is running with a real pressure split before condemning the valve.

When brazing in a new valve body, keep it cool — wrap it in a wet rag and aim heat carefully. Overheating the body during the braze can warp the slide and ruin the new valve. (Always recover refrigerant first.)

The gentle-tap trick can confirm a sticky slide, but don't sell it as a repair. A valve that hangs up once will hang up again; a chronically sticking valve needs replacement.

Safety / code notes

The solenoid coil can be replaced without opening the refrigerant circuit, but the valve body cannot — recover the refrigerant before any brazing (venting is prohibited under EPA Section 608). When replacing the valve body, protect it from braze heat to avoid damaging the slide, and evacuate to a proper vacuum (500-micron decay test) before recharging. Match the coil's voltage to the control circuit. De-energize before testing the coil for a short, and confirm the O/B convention so the system operates in the correct mode.