Reversing Valve Diagnosis and Replacement

Purpose

This standing instruction guarantees that a reversing valve is proved faulty before it is cut out, and that when it is replaced the new one survives the brazing and both modes are verified before the truck leaves.

Reversing valves get condemned far more often than they fail. Three things produce "it will not switch" and none is the valve: no control signal because the thermostat carries the wrong reversing-valve convention, an open or shorted solenoid coil, and a system that cannot build enough pressure differential across the slide to move it, which is what a low charge or a worn compressor looks like from outside. Cut a valve out of that last one and the shop has paid for a valve, a recovery and a charge, and it still will not switch.

Safety actions that gate the work

• Open and lock the disconnect and prove dead before any cover or coil comes off, proved on a known live source before and after per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2), written for qualified persons under 1910.332. Discharge the run capacitor through a resistor first.
• The discharge line burns through a glove faster than you can let go. Read it with a contact probe; never grip it to judge temperature.
• Recover before any cut. Venting is prohibited under 40 CFR Part 82 Subpart F, and refrigerant released in a closet or crawlspace displaces oxygen in the space you are standing in.
• Heating tubing that has held refrigerant releases hydrogen fluoride and carbonyl fluoride, which are inhalation hazards and not contact ones. Flow regulated nitrogen through the joint while brazing, ventilate mechanically, and in a closet or crawlspace work under a written respiratory protection program per 29 CFR 1910.134. Gloves and glasses do not address this route.
• Nitrogen through a regulator with a relief, and never oxygen. Oxygen against refrigerant oil is an explosion, and an unregulated cylinder passes tubing burst pressure instantly.

Scope

Covers reversing valve diagnosis and replacement on residential and light-commercial split and packaged air-source heat pumps with a field-serviceable valve body.

Does not cover ductless inverter equipment whose valve sits inside a sealed chassis, where the manufacturer's fault code and diagnostic sequence govern, or charge correction and compressor condemnation, which have their own SOPs and which this procedure only rules in or out. A valve that fails to shift during a defrost is diagnosed here; whether defrost was commanded at all is diagnosed in the defrost control SOP.

Roles and handoffs

Procedure

1. Test both modes yourself and record which direction fails. Acceptance: a supply and return air pair at the equipment in each mode, plus outdoor ambient. Wrong looks like taking "no heat" off the ticket as the diagnosis; a slide stuck against cooling and one stuck against heating are different searches. Stop rule: if both modes fail this is not a valve, go to the no-heat or no-cool call SOP. Hazard: probing at the indoor unit with the blower running, so keep the probe and your fingers out of the wheel.

2. Measure the control signal at the solenoid coil leads, in both modes, at the coil rather than at the board. Acceptance: roughly 24 V AC across the coil in the mode the equipment diagram says energizes it, near zero in the other. Wrong looks like a reading at the board that never reaches the coil, through a broken conductor or a corroded splice. Stop rule: no signal where there should be one is the thermostat's reversing-valve convention or the wiring, so go to the thermostat SOP and leave the valve alone. Hazard: energized measurement in an open cabinet, one hand in the work area, out of the fan plane.

3. Pull the coil off the pilot stem and test the coil itself. Acceptance: resistance within the manufacturer's published value, and a felt magnetic pull on a screwdriver blade when energized off the stem. Wrong looks like an open coil reading infinite or a shorted one far below published. Stop rule: a failed coil is replaced and the unit retested before anything else is condemned. Hazard: an energized coil off its stem heats fast and is a live source, so hold it by the insulated body and de-energize before it goes back on.

4. Confirm the system can build enough differential to move the slide. Acceptance: discharge and suction pressures recorded running, their difference at or above the valve manufacturer's stated minimum, which is published per valve body and commonly falls on the order of 75 to 100 psi, measured at steady state under a load that lets the system build head. At low outdoor ambient a healthy system legitimately builds low head and may not reach the minimum, so record outdoor ambient beside the reading. Wrong looks like a differential below that minimum, which both a low charge and a worn compressor produce, and no valve shifts against it. Stop rule: below the minimum at steady state and under load, the diagnosis is charge or compressor and the valve is not touched; below it on a cold-morning reading, re-test at load before condemning either. Hazard: hoses onto a running system, so glasses and gloves before the first connection.

5. Read all four ports and decide stuck against bypassing. Take line temperatures at the discharge, outdoor coil, indoor coil and suction ports with the same contact probe at the same distance from the body. Acceptance: the suction port within about 20 F of the saturated suction temperature, and that saturation is read off the pressure-temperature chart for the refrigerant on the equipment data plate rather than from memory - 105 psig is about 33 F on R-410A and about 57 F on R-22, so the wrong chart invents a reading below saturation that nobody can chase. Wrong looks like a suction port tens of degrees above saturation while the discharge port is hot, which is gas crossing the slide internally. Stop rule: infrared reads low on bright copper, so do not make this call from an infrared gun without dulling the surface. Hazard: the discharge port burns on contact, so probe it rather than gripping it.

6. Try to move a stuck slide before condemning it, and cap the attempts. Energize and de-energize the coil while tapping the body lightly with a screwdriver's plastic handle, watching the gauges for the pressures to swap. Acceptance: the slide moves, the pressures swap, the unit runs in the commanded mode. Wrong looks like striking the body with metal, which dents the tube and traps the slide permanently. Stop rule: three attempts with no movement ends it, and a valve that only moved after persuasion is quoted for replacement anyway. Hazard: you are tapping a pressurized line on a running hot system, so keep your face out of the plane of the valve.

7. Recover, cut it out, and braze the new body in cool and purged. Acceptance: recovery documented to the required level, a wet rag or heat-sink compound on the body and rewet between joints, the manufacturer's stated maximum body temperature not exceeded, regulated nitrogen flowing throughout. Wrong looks like brazing without a heat sink, which cooks the internal seals and hands you a valve that bypasses out of the box, then gets blamed on the part rather than the torch. Stop rule: the coil goes on last, after the body has cooled to hand temperature. Hazard: this step generates the decomposition products named above, so the purge, the ventilation and the 1910.134 program apply here specifically.

8. Evacuate, weigh in, and put it back with both modes proved. Acceptance: evacuation to the criterion the evacuation SOP sets with its decay hold met, charge weighed to nameplate plus the line-set correction, and a supply and return pair recorded in cooling AND in heating. Wrong looks like proving heat and going home; a mislanded coil inverts both modes. Stop rule: a mode that will not produce its pair does not close the ticket, so re-check the coil leads against the equipment diagram and re-run both modes before the unit is released. Hazard: this puts refrigerant, pressure and power back with the customer in the house, so stand to the hinge side of the disconnect, keep everyone off the indoor unit, and confirm the high-pressure switch and the defrost sequence still function before you write the ticket.

The record this produces

A valve diagnostic sheet filed to the equipment, which is what justifies the part:

• The step 1 supply and return pairs for both modes, with outdoor ambient
• Coil voltage measured at the coil in both modes, and coil resistance against published
• Discharge pressure, suction pressure, their difference, and the valve's stated minimum
• The four port temperatures, saturated suction from the chart for the plated refrigerant, and the instrument used
• Whether shift attempts were made, how many, and whether the slide ever moved
• Recovery record, new valve part number, and the body temperature control used
• Evacuation result, weighed charge, and the step 8 pairs for both modes

The warranty administrator reads the differential line, because a valve claim on a system whose compressor could not build differential gets denied and should. The next tech reads the shift-attempt line, because a valve tapped over once will come back.

Worked pass: split heat pump, R-410A, cools normally, no heat, 38 F ambient

Step 1: cooling call gives supply 55 F against a 75 F return. Heating call gives supply 62 F against a 68 F return, so supply is 68 minus 62, which leaves 6 F colder than return on a heat call. Failing direction recorded as stuck against the cooling position.

Step 2: this unit energizes its valve for cooling per the equipment diagram. At the coil leads, 24.1 V AC on a cooling call and 0.3 V on a heating call. Correct both ways, so the signal is not the fault. Step 3: coil off the stem, resistance inside the published value, firm magnetic pull on a screwdriver blade when energized.

Step 4: running, discharge 240 psig and suction 105 psig, so the differential is 240 minus 105, which leaves 135 psig against this valve body's published 75 psi minimum, read at steady state with 38 F outdoor ambient recorded beside it. Passes with room to spare, so the low-ambient caveat does not bite here; this is where the visit ends on a weak compressor.

Step 5: contact probe at all four ports. Discharge 168 F, outdoor coil 154 F, indoor coil 44 F, suction 41 F. Saturated suction at 105 psig off the R-410A chart reads 33 F, so the suction port is 41 minus 33, which leaves 8 F above saturation, inside the 20 F gate. Not bypassing, so this is a slide fully seated in the cooling position.

Step 6 FAILS. Three energize and de-energize cycles with light taps from a plastic handle, and the gauges do not move on any of the three. Stop rule taken at three: no fourth attempt, valve condemned, manager notified before recovery starts because recovery is the point of no return.

Step 7: recovery documented, valve cut out, new body brazed with a wet rag rewet between the four joints and regulated nitrogen flowing, body kept below the stated maximum, coil reinstalled after it cooled.

Step 8: evacuation held to the decay criterion, charge weighed to nameplate plus the line-set correction. Cooling proved at supply 54 F against a 75 F return. Heating proved at supply 94 F against a 68 F return, so the rise is 94 minus 68, which leaves 26 F, the direction that was inverted on arrival. High-pressure switch and defrost sequence confirmed.

References

• The valve manufacturer's data sheet for minimum operating pressure differential, coil resistance and maximum body temperature during brazing, which are per valve body and not transferable between models
• Equipment manufacturer's wiring diagram, the only authority on which mode energizes the coil
• 29 CFR 1910.333(b)(2) and NFPA 70E-2021, 120.5, for de-energizing and proving dead; 29 CFR 1910.134 for the respiratory program gating step 7; 40 CFR Part 82 Subpart F for recovery
• See related: brazing with a nitrogen purge standard, evacuation and vacuum decay verification, charge verification by weight and superheat, defrost control diagnosis on a heat pump