What it is

The reversing valve (a.k.a. four-way valve) is what lets one piece of equipment both heat and cool. It reverses the direction refrigerant flows through the system, which swaps the roles of the indoor and outdoor coils. In cooling, the outdoor coil rejects heat (condenser) and the indoor coil absorbs it (evaporator). Flip the valve and it's the opposite: the outdoor coil becomes the evaporator pulling heat out of cold outdoor air, and the indoor coil becomes the condenser dumping that heat into the house.

The compressor always pumps the same direction. The reversing valve is what changes which coil sees the hot discharge gas.

How it works

The valve body has four refrigerant connections: one always-hot line from the compressor discharge, one always-cold line to the compressor suction, and two that swap (one to each coil). Inside is a sliding mechanism that connects discharge to one coil and suction to the other, then slides over to reverse it.

Here's the part techs miss: the valve isn't muscled over by the little coil on the side. That coil operates a tiny pilot solenoid valve, and the pilot uses the system's own pressure difference (high side vs low side) to push the main slide over. No pressure differential, the slide won't move. That's why a reversing valve can refuse to shift on a system that's low on charge or has a weak compressor — there isn't enough ΔP to drive the pilot.

O vs B — when is the coil energized?

  • Most heat pumps energize the reversing-valve coil for cooling via the O terminal. So the coil is hot in cool mode, de-energized in heat. (This is the common residential setup — a "fail to heat" default: lose the signal and you get heat.)
  • Some equipment (notably certain brands) energize for heating via the B terminal instead.
  • Always confirm which one the unit uses at the thermostat/board — getting O vs B backward means the system heats when it should cool.

During a defrost cycle, the board energizes the valve to temporarily run the outdoor unit in cooling mode to melt frost off the outdoor coil — more on that in the defrost article.

In the field

Confirm the valve is shifting at all:

  • Feel/temp the lines. In the mode you're in, the discharge line from the compressor to whichever coil should be hot; the suction side should be cool. When you switch modes (or force defrost), you should feel the roles swap and often hear/feel the valve "thunk" over.
  • Check the coil is getting its signal in the mode that calls for it (24V at the valve coil when O — or B — should be energized). A dead coil or no signal means it never gets commanded to shift.

Diagnose a stuck valve:

  • Valve won't shift even with proper signal and the coil is good (it's a solenoid — ohm it and confirm 24V) → likely stuck slide, or not enough pressure differential. Verify the system has a healthy ΔP (good charge, good compressor). A low/weak system can't drive the pilot.
  • A light tap on the valve body with the handle of a screwdriver while it's trying to shift will sometimes free a slide that's hung up — diagnostic, not a permanent fix. If tapping frees it, the valve is failing.

Diagnose an internally-bleeding valve (the sneaky one):

  • A reversing valve can leak internally — high-pressure discharge gas bleeds past the slide into the suction side without the system even trying to switch. Symptoms: weak capacity, suction line warmer than it should be, and a giveaway temperature signature on the valve itself. Feel the three "tube" connections coming off the body. If the permanent suction line and the line that should be cool are reading much closer together than expected — i.e., the middle suction tube is unexpectedly warm — discharge is bleeding internally.
  • The classic test: with the system running, measure the temperature of the suction (common) tube against the two changeover tubes. A valve bleeding by shows an abnormally small temperature split — hot gas is short-circuiting inside. A good valve shows a clear hot/cold separation across the body.

Normal values & targets

  • Coil: it's a 24V solenoid; ohm it for a normal coil resistance (typically tens to a few hundred ohms — open = replace, and confirm 24V is present when it should be).
  • Pressure differential to shift: the valve needs a meaningful high-to-low ΔP (the pilot uses system pressure to slide it) — a deeply undercharged system or a weak compressor may not build enough.
  • Bleed-by tell: on a healthy valve you expect a large temperature difference between the hot changeover tube and the cool suction tube. A bleeding valve shows them suspiciously close.

Common faults & what they mean

  • No heat or no cool, valve never shifts — no 24V to the coil, open coil, stuck slide, or insufficient ΔP. Check signal and coil first, then charge/compressor.
  • System runs but weak in both modes, suction warm — internally bleeding valve, short-circuiting hot gas to the suction line. Confirm with the tube temperature test.
  • Heats when set to cool / cools when set to heat — O vs B wired or configured wrong, or the valve is stuck in the opposite position. Verify the thermostat's O/B setup matches the equipment.
  • Valve buzzes but won't shift — coil energized, pilot or slide hung up, or not enough pressure differential to move it.
  • Sticks on cold mornings, fine when warm — marginal valve; replace it before it strands the customer.

Tech tips & gotchas

  • The coil doesn't move the valve — pressure does. So a charge or compressor problem can masquerade as a "bad reversing valve." Don't condemn the valve on a low system; correct the charge and retest.
  • A bleeding valve won't throw a hard fault — it just steals capacity. If a heat pump "kind of works but never quite satisfies," put your temp probe on the valve body tubes.
  • Replacing a reversing valve means brazing with the valve body protected from heat (wrap it with a wet rag / heat-sink compound and keep the torch moving) — too much heat warps the internal slide and you'll install a brand-new bleeder.
  • Confirm O vs B on EVERY heat pump; don't assume. Mismatched O/B is one of the most common "it's backwards" service calls.
  • Always recover, pull a proper vacuum, and recharge by the correct method after a valve change.

Safety / code notes

  • Reversing-valve work involves opening the sealed refrigerant system — recover refrigerant per EPA Section 608, never vent it.
  • Use nitrogen purge while brazing and a proper evacuation (deep vacuum with a micron gauge and a decay test) before recharging.
  • Confirm the system charge by weight or the appropriate method after service; a valve diagnosis made on an incorrectly charged system is unreliable.