HVAC
Heat pump reversing valve and defrost field guide
The four-way valve that flips the cycle, the O versus B control that trips everyone, the defrost that runs cooling on purpose, and how to tell a stuck valve from a low charge.
Direct answer
A heat pump reversing valve is a four-way valve that flips the direction of refrigerant flow, swapping which coil is the condenser and which is the evaporator. It lets one system both heat and cool, and it runs the defrost cycle by switching to cooling to melt frost off the outdoor coil. A small pilot solenoid shifts the main slide.
Key takeaways
- The reversing valve is a four-way valve that reverses refrigerant flow, swapping which coil is condenser and which is evaporator so one system heats and cools.
- Defrost runs the unit in cooling for roughly 2 to 10 minutes to send hot gas to the outdoor coil; the outdoor fan stops and backup heat fires indoors.
- Wire the reversing valve to O or B by the unit's diagram: O energizes the valve in cooling (Carrier, Trane, Lennox, Goodman), B energizes it in heating (Rheem).
- Check refrigerant charge and compressor first on no-heat or lukewarm calls, because low charge mimics a stuck valve and is the most common misdiagnosis.
- A permanent suction line more than roughly 3 to 5°F warmer than the indoor-coil suction line indicates a valve bleeding hot gas across the slide; verify against manufacturer literature.
The reversing valve, and why a heat pump needs one
The reversing valve is the part that makes a heat pump a heat pump. It is a four-way valve in the outdoor unit that reverses the direction refrigerant flows through the system, which swaps the jobs of the two coils. In cooling, the outdoor coil rejects heat and the indoor coil absorbs it, the same as any air conditioner. Shift the valve and the flow reverses, so now the indoor coil rejects heat into the house and the outdoor coil pulls heat out of the cold air. Same compressor, same refrigerant, opposite direction.
Take the valve away and you have a straight-cool air conditioner. Add it and one sealed system heats and cools, which is the whole reason the equipment exists. The heat pump fundamentals guide covers how that heat actually moves and what COP and the balance point mean. This guide is about the valve that does the flipping and the defrost cycle that depends on it.
The valve does double duty. It selects heating or cooling for the thermostat, and it also runs the defrost cycle, where the controls flip the unit into cooling for a few minutes on purpose to melt frost off the outdoor coil. One part, two jobs, and most of the strange heat pump calls trace back to it.
How the four-way valve works
The reversing valve is a slide valve. Inside the main body is a spool, or slide, that sits over a set of ports and routes the gas one of two ways. The body has four tubes: one on top that always carries hot discharge gas from the compressor, and three across the bottom. The center bottom tube is the permanent suction line back to the compressor. The two outer bottom tubes go to the two coils. As the slide moves from one end to the other, it connects discharge to one coil and suction to the other, then trades them.
What moves the slide is not the solenoid directly. It is the pressure difference the compressor makes. A small pilot valve, driven by the solenoid, ports high-side and low-side pressure to the two ends of the main slide. Bleed pressure off one end and the slide gets pushed there by the pressure still on the other end. So the compressor has to be running and making a real pressure split for the valve to shift at all.
That last point is the one that saves you a wasted parts swap. A valve that will not change over is sometimes not a bad valve. It is a system with too little charge or a weak compressor that cannot build the pressure difference the slide needs to move. Confirm the unit is making a normal high-to-low split before you condemn the valve itself.
The pilot solenoid: low power moves big flow
The solenoid coil on the side of the valve is small, and it controls a tiny pilot valve, not the main slide. That is the design idea worth understanding. A 24 V coil moving a small pilot port lets the system's own pressure do the heavy work of sliding the spool across the full refrigerant flow. Low power steers, the compressor pushes.
When the coil is energized, the pilot shifts and the slide moves to one position. De-energize the coil and a spring returns the pilot, the pressures swap back, and the slide moves the other way. The coil itself only ever sits in one of two states, energized or not, and the valve has a default position it rests in with no power.
Because the coil is a small, replaceable part that slides off the valve stem under a single retaining nut or clip, it is the cheapest thing on the valve to replace and the first thing to rule in or out. A dead coil leaves the valve parked in its de-energized default, which on most units is heating. The customer calls because the system will not cool, and the fix is a coil, not the whole valve.
What is the O and B terminal on a heat pump?
O and B are the thermostat terminals that control the reversing valve, and only one of them is ever used on a given unit. The O terminal energizes the valve in cooling. The B terminal energizes it in heating. Most manufacturers, including Carrier, Trane, Lennox, and Goodman, energize the valve in cooling and wire to O, so the valve sits de-energized in heating and the coil is only powered when the thermostat calls cool. Rheem and some others use B and energize the valve in heating instead.
Get this wrong and the system heats when it should cool, or cools when it should heat, which is the number one wiring miss on a heat pump changeout. The reversing-valve wire lands on O or B by the wiring diagram printed on the unit, never by the color you expect and never by habit from the last brand you wired. Two units in the same truck can be opposite.
The thermostat has to match. A heat pump thermostat has an O/B terminal you configure for the equipment, and setting it to the wrong one inverts every call. If a unit runs backward right after a thermostat swap or a board replacement, check the O versus B setting before you touch refrigerant. It is a five-minute check that gets skipped because the wire was on a terminal, just the wrong one.
Which coil is which in heating and cooling
The coils trade jobs when the valve shifts, and keeping that straight is what makes the rest of the diagnosis read clean. In cooling, the indoor coil is the evaporator, boiling refrigerant and pulling heat out of the house, while the outdoor coil is the condenser, dumping that heat outside. The discharge gas off the compressor goes to the outdoor coil, and the suction line comes back from the indoor coil.
In heating, the valve flips both. Now the outdoor coil is the evaporator, boiling cold refrigerant and soaking up heat from the outdoor air, and the indoor coil is the condenser, releasing that heat into the supply air. The discharge gas now goes to the indoor coil, and the cold suction comes back from the outdoor coil. The same compressor discharge tube on the valve always carries hot gas. What changes is which coil it gets routed to.
Hold that picture when you put your hands on the lines. In heating, the large line leaving the valve toward the indoor coil should be hot, and the line back from the outdoor coil should be cold. If those temperatures are not where the mode says they should be, the valve is either in the wrong position or leaking across the slide.
Why the outdoor coil frosts up in heating
In heating, the outdoor coil runs colder than the outdoor air, often well below freezing, because that is how it pulls heat out of cold air. Moisture in the outdoor air hits that cold coil and freezes onto the fins as frost. The colder and more humid the air, the faster the frost builds. Right around freezing with damp air is the worst case, not the deep cold, because very cold air is also very dry and carries less moisture to deposit.
Frost is not just cosmetic. A layer of ice on the coil insulates the fins and blocks airflow through the coil, so the unit loses the heat transfer it depends on. Capacity falls, the suction pressure drops, and the system works harder to deliver less. Left alone, the coil ices over solid and the heat pump nearly stops producing heat.
So the unit has to clear the coil periodically, and it does that with a defrost cycle. Some frost on the outdoor coil in cold, damp weather is normal and expected. The question on a service call is never whether the coil frosts. It is whether the unit is clearing that frost on schedule or icing up because defrost is not working.
How does a heat pump defrost work?
Defrost melts frost off the outdoor coil by running the system in cooling for a few minutes, which sends hot discharge gas out to the outdoor coil and warms it above freezing. The defrost board shifts the reversing valve to the cooling position, so the coil that was cold and frosting becomes the hot condenser. The frost melts and runs off.
Two things happen alongside the valve shift to make it work. The outdoor fan turns off, so the unit is not blowing cold outdoor air across the coil it is trying to warm, which lets the coil heat fast and concentrates the energy on melting ice. And the backup heat comes on indoors, because while the system is in cooling for defrost, the indoor coil is now cold and would otherwise blow cold air into the house. The strip heat or furnace tempers that supply air so the occupant does not get a blast of cold while the unit defrosts.
When the frost is cleared, the board flips the valve back to heating, restarts the outdoor fan, and drops the backup heat. You will often hear a loud whoosh as the valve shifts and the pressures equalize, and you will see steam rising off the outdoor coil. Both are normal. A defrost typically runs a few minutes, commonly in the range of 2 to 10 minutes depending on the control and the frost load.
Demand defrost vs timed defrost
There are two ways a heat pump decides it is time to defrost, and they are not equal. Older and cheaper units use time-temperature defrost. A timer counts run time, commonly 30, 60, or 90 minutes, and when the timer expires and a coil sensor confirms the coil is cold, the unit defrosts whether or not it actually needs to. It is simple and it works, but it defrosts on a clock, so it runs defrost cycles the coil did not need and sometimes skips ones it did.
Demand defrost is the better design and now the common one. It watches the coil with one or more temperature sensors, often comparing coil temperature to outdoor air temperature, and only initiates a defrost when the readings say frost has actually built up enough to matter. It defrosts when needed and ends when the coil is clear, which means fewer cycles and shorter ones.
The difference shows up in efficiency and in comfort. Every defrost costs energy, because the unit is running cooling and backup heat at the same time, so a control that defrosts only when the coil is actually loaded wastes less. The U.S. Department of Energy points to demand defrost as the more efficient approach for that reason. When you replace a failed timed defrost board, check whether a demand-defrost upgrade is available for the unit.
How defrost ends: coil temperature or time
A defrost cycle ends one of two ways, and a healthy unit ends on the first one. The primary termination is the coil temperature sensor. Once the coil warms past a set point, commonly somewhere around 50 to 70°F depending on the manufacturer, the board knows the frost is melted and returns the unit to heating. That is the normal, efficient end: defrost runs exactly as long as the ice takes to clear and no longer.
The backup termination is a time limit, often 10 minutes, that ends the cycle even if the coil never reached the termination temperature. This is a fail-safe, not the intended path. If a unit is regularly riding the time limit out to its maximum, that tells you something is wrong, usually a sensor reading low, a coil that is not warming because of a charge problem, or a fan that is not actually shutting off.
When you watch a defrost and it terminates fast on temperature, the controls are doing their job. When it always runs the full time limit, treat that as a symptom. Confirm the coil sensor reads accurately and is clipped tight to the coil in the right spot, because a loose or mislocated sensor is a common cause of long, wasteful defrosts.
Why backup heat runs during defrost
The backup heat, electric strips or a gas furnace, comes on during defrost so the supply air does not blow cold. While the unit is in cooling to warm the outdoor coil, the indoor coil is acting as an evaporator and would push cold air through the registers. The defrost board energizes the auxiliary heat to temper that air, so the occupant feels neutral or warm air instead of a cold draft for the few minutes defrost runs.
When the backup heat is wired wrong or locked out, the customer complaint writes itself: cold air blows from the vents every so often, then it goes back to normal. That is defrost without aux heat, not a refrigerant fault. Confirm the defrost board is bringing on the backup stage during the cycle and that the strips or furnace actually fire.
This is also why heat pump comfort feels different from a furnace, and worth a word to the customer at install. A heat pump delivers warm air at a lower temperature than a furnace, and during a defrost the system leans on the backup heat. That is the equipment working as designed, not breaking.
Steam off the outdoor coil is normal
Steam rising off the outdoor unit during a winter defrost is normal, not a fire and not a refrigerant leak. The melted frost turns to vapor as the coil heats above freezing, and on a cold day that vapor is visible as a cloud. Customers see it and call it in as smoke. It is water.
Tell the homeowner up front so they do not panic at the first cold snap. The signs of an actual defrost in progress are the whoosh of the valve, the outdoor fan stopping, steam off the coil, and a brief change in the indoor supply air. All four together for a few minutes is a unit defrosting correctly.
How do you tell if a reversing valve is stuck?
A stuck or mid-position reversing valve shows up as no heat or weak, lukewarm heat, and the giveaway is the temperature pattern at the valve body. A valve stuck part way, or one bleeding internally across the slide, lets hot discharge gas mix into the suction side, so the unit cannot build a real temperature split. The supply air runs lukewarm, the suction line is warmer than it should be, and the head and suction pressures sit closer together than the mode calls for.
Start by confirming the basics, because a low charge mimics a stuck valve. The slide needs a real pressure difference to shift, so a system low on refrigerant or with a weak compressor may fail to change over and look exactly like a bad valve. Check the charge and the compressor before you condemn the valve. This is the single most common misdiagnosis on a heat pump, and it sends good valves to the scrap pile and leaves the real leak in the system.
Once the charge checks out, the tap test is the quick field move. With the system running and calling for the mode it will not deliver, tap firmly on the side of the valve body with the plastic handle of a screwdriver. A slide hung up on a burr or a varnish deposit will sometimes break free and slam over with an audible shift. If it does, cycle the valve several times to confirm it moves freely, and know that a valve that needed tapping is living on borrowed time. It will hang again.
Testing the pilot solenoid coil
Before you blame the valve body, prove the coil. On a no-changeover call, confirm the coil is getting the voltage it should for the mode. Put a meter across the coil terminals during a call that should energize the valve, usually a cool call on an O-terminal unit, and verify 24 V is present. If the voltage is there and the valve still will not shift, the problem is downstream of the coil.
Check the coil itself with an ohmmeter. A good coil reads a stable resistance value per its rating. An open coil reads infinite, a shorted one reads near zero, and either one is a dead coil. The coil is cheap and slides off the valve stem under a single nut or clip, so swap it before you cut into refrigerant.
There is a fast magnetic check too. With voltage applied, hold a steel screwdriver shaft near the coil. A live coil pulls on the shaft with a clear magnetic field, and a dead one does nothing. You can also often hear and feel the pilot click when the coil energizes. A coil that energizes, clicks, and reads correct resistance, on a system with a normal pressure split that still will not change over, points you at the valve body.
The temperature differential test for a bleeding valve
A reversing valve that bleeds internally is harder to catch than one stuck flat, and the temperature differential test is how you find it. With the system running, feel and measure the three bottom tubes on the valve. The center tube is the permanent suction line back to the compressor and should run cold and close to true suction temperature. The two outer tubes go to the coils.
The tell is the suction side. Compare the temperature of the suction line coming off the indoor coil to the temperature of that permanent suction line at the valve. They should be nearly the same. If the permanent suction line is noticeably warmer than the line feeding it, hot discharge gas is leaking across the slide and warming the suction stream inside the valve. A common rule of thumb is that a difference of more than roughly 3 to 5°F between those suction tubes points to a leaking valve, but confirm the threshold against the manufacturer's service literature for the equipment.
Take the readings a few inches off the valve body so the hot valve casting does not skew the thermometer, and let the system run ten minutes in each mode before you trust the numbers. A valve with a low temperature split across the ports and a warm permanent suction line is bleeding through. That valve gets replaced. It does not get charge added to it.
Replacing the reversing valve
Replacing a four-way valve is mostly a brazing job done carefully, and the whole risk is heat. The valve has plastic and rubber-like internal seals and a slide that rides a machined seat, and all of that lives inside the body just inches from where you are brazing four tubes. Cook the valve while you braze it and you warp the seat or melt the seals, and the brand-new valve leaks across the slide the first day. The replacement fails the same way the old one did, except now it is your fault.
Recover the charge, then remove the solenoid coil before you bring any flame near the valve. The coil slides off and it has no business being heated. Position the valve so the body is as far from the joints as the piping allows, and plan to flow nitrogen through the system while brazing to keep scale out of the slide. A clean valve interior matters because debris is exactly what hangs a slide up later.
Wrap the valve body in a wet rag and keep it wet, and use a heat sink compound or a second wet rag at the close joints to pull heat away. Braze fast, keep the torch moving, and put the heat into the joint, not the body. The wet rag is not optional on this job. It is the difference between a valve that lasts and a callback.
Do not overheat the valve when brazing
This deserves its own warning because it is the most common way a reversing valve replacement goes bad. The internal seat and seals are temperature sensitive, and the heat from brazing travels up the flow tubes straight into the valve body. If you do not control that heat, you damage the very parts that make the valve seal, and the damage does not show up on your gauges until the system is running and the new valve is already bleeding through.
Keep the body cool, keep the rag wet, and keep the flame moving. Point the torch so the heat goes into the joint and away from the body. If the wet rag dries out and starts steaming hard, stop and re-wet it before you continue. A valve brazed with too much heat fails early, and an early failure on a part you just installed reads as a bad replacement when it was really a bad technique.
How charge affects defrost and frost
A wrong charge changes how the outdoor coil frosts and how defrost behaves, which is why charge gets ruled in before the valve and the board. A low charge in heating runs the outdoor coil colder and starves it, so it frosts faster and heavier and the unit cannot warm the coil quickly during defrost. That can drag defrost out to the time limit instead of ending on coil temperature.
Charge a heat pump by the method the metering device and the data plate call for, and confirm it in the correct mode. The metering devices guide covers superheat and subcooling and which one drives the charge on a given system. The takeaway here is narrow: a defrost or frosting complaint that does not resolve at the board or the sensor often resolves at the charge, so verify the charge is right before chasing the controls deeper.
Water-source and geothermal heat pumps need no air-coil defrost
Water-source and geothermal heat pumps still have a reversing valve, but they do not run an air-coil defrost cycle, because there is no outdoor air coil to frost. Instead of pulling heat from cold outdoor air, they pull it from a water loop, a ground loop, or a well, and that heat exchanger runs in liquid, not freezing air. No frost forms, so there is nothing to defrost.
The reversing valve works the same way in these units, flipping the loop heat exchanger and the indoor coil between condenser and evaporator duty for heating and cooling. So the valve diagnostics in this guide, the O versus B control, the stuck-valve and bleeding-valve tests, all carry over. What does not carry over is the defrost board, the outdoor fan cutoff, and the defrost-triggered backup heat. If you are used to air-source units, do not go looking for a defrost cycle on a water-source machine. It is not supposed to have one.
Common reversing valve and defrost failures
Most calls on this part fall into a short list. The valve itself sticks, either flat in one position or part way, and the unit delivers no heat or lukewarm heat with pressures too close together. The pilot solenoid coil fails open or shorted, and the valve parks in its de-energized default, so the unit will only do one mode. The defrost board or coil sensor fails, and the result splits two ways: no defrost at all, so the outdoor coil ices solid, or constant or excessive defrost, so the unit wastes energy and blows cold supply air more than it should.
The last common failure is self-inflicted: a valve overheated during brazing on a previous repair. It tests fine cold, then bleeds across the slide once it is running, and it looks like a defective new part. It was a good part installed hot.
When the outdoor coil is iced over solid, work the defrost side: sensor, board, and whether defrost is initiating and terminating at all. When the supply air blows cold too often, suspect excessive defrost from a sensor reading wrong or a board cycling on the clock. When the unit will only heat or only cool, suspect the coil or the O/B control. Match the symptom to the failure before you reach for the most expensive part.
| Symptom | Likely cause | First check |
|---|---|---|
| No heat or lukewarm heat, pressures close together | Stuck or bleeding reversing valve, or low charge | Verify charge, then tap test and port temperature differential |
| Will only heat or only cool | Dead solenoid coil or wrong O/B setting | Coil voltage and resistance, thermostat O/B configuration |
| Outdoor coil iced solid | No defrost: failed board or coil sensor | Confirm defrost initiates and terminates, check sensor |
| Cold supply air too often | Excessive defrost or no aux heat in defrost | Coil sensor reading, defrost frequency, backup heat firing |
| New valve bleeds soon after install | Valve overheated during brazing | Replace; protect the body with a wet rag and remove the coil |
Defrost costs energy, so demand control matters
Every defrost is a small efficiency hit, because the unit stops heating, runs cooling, and fires backup heat all at once for a few minutes. You are paying for cooling the house slightly and for the strip heat that covers it, on top of getting no useful heating output during the cycle. A few defrosts in genuinely frosty weather is the cost of keeping the coil clear and is money well spent. A unit defrosting far more than the weather warrants is burning energy for nothing.
This is the case for demand defrost in one line: defrost only when the coil is actually loaded, and end as soon as it is clear. A timed board that defrosts every 30 minutes in mild, dry cold runs cycles the coil never needed, and each one drags the seasonal efficiency down. When efficiency complaints come in on a heat pump in winter, count the defrosts. A unit that defrosts constantly has a control or sensor problem hiding as a high bill.
Defrost on packaged rooftop and data-center heat pumps
The same valve and the same defrost logic scale up to commercial packaged heat pumps and rooftop units, with a few practical differences. Larger units may stage compressors and have more sophisticated demand-defrost controls, and the backup heat carrying the load during defrost can be a sizable bank of strips or a gas section. The reversing valve is bigger and the pressures are higher, but the failure modes and the tests are the same: stuck slide, bleeding valve, dead coil, sensor or board faults.
On equipment serving a space with a tight temperature requirement, such as a data center or a process area on a heat pump rooftop, a defrost that drops supply temperature or leans hard on backup heat matters more than it does in a house. The defrost is a planned interruption, and the controls and the backup capacity have to be sized so a cold-weather defrost does not pull the conditioned space out of range. Confirm the backup heat is sized and sequencing correctly, because on critical spaces the defrost no one tested is the one that trips an alarm at 3 a.m.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
What to document
Write down what the valve was actually doing, in which mode, so the next tech is not starting from zero. A reversing-valve diagnosis is a chain of eliminations, and the value of the record is showing what was already ruled out: the charge, the coil, the pressure split, the port temperatures.
Capture the mode the unit failed in, the valve state you found, the coil voltage and resistance, the pressure split, the port temperature readings, whether defrost initiated and terminated and how, and what you replaced. If you tapped a valve free, note it, because that unit will be back. If you replaced a valve, note that the coil was removed and the body protected during brazing, because the next failure investigation will want to know.
| Field to record | Why it matters |
|---|---|
| Failed mode (heat or cool) | Points at which position the valve would not reach |
| Valve state found | Stuck, bleeding, or shifting normally |
| Coil voltage and resistance | Separates a dead coil from a bad valve body |
| Pressure split, head and suction | A weak split means charge or compressor, not the valve |
| Port temperature readings | Documents the bleed-through test result |
| Defrost initiate and terminate | Shows whether the board and sensor work |
| Part replaced and brazing precautions | Explains a future early failure or confirms good practice |
Common mistakes
- Condemning the reversing valve when the real problem is a low charge or weak compressor that cannot shift the slide.
- Overheating the valve when brazing, which warps the seat or melts the seals so the new valve bleeds through and fails early.
- Landing the control wire on the wrong terminal or setting the thermostat O/B backward, so the unit heats on a cool call.
- Missing a failed defrost board or coil sensor, so the coil ices solid or the unit defrosts far more than the weather warrants.
- Leaving backup heat out of the defrost sequence, which produces a cold-blow complaint that gets chased as a refrigerant fault.
- Calling steam off the outdoor coil during defrost a problem instead of normal melted frost turning to vapor.
Standards and references
The controlling document for any specific valve and unit is the manufacturer's service literature. The O versus B convention, the coil resistance value, the defrost termination temperature, the defrost timing, and the brazing precautions all vary by equipment, and the data plate and installation instructions govern. Component makers such as Ranco, Sporlan, and Emerson publish valve-specific service data, and the unit manufacturer's diagram is what controls the wiring on the job.
Refrigerant handling is regulated. Recovering, evacuating, and charging during a valve replacement falls under EPA Section 608, which requires certification to work with refrigerant and to recover it properly rather than venting. ASHRAE provides the broader design and equipment framework for heat pump performance and ratings, and the U.S. Department of Energy's guidance on heat pump efficiency points to demand defrost as the more efficient control approach.
Cite the manufacturer for the specific numbers and follow EPA 608 for the refrigerant work. The points to carry off this guide are the load-bearing ones: wire O versus B by the unit's diagram, defrost runs the cycle in reverse to cooling to melt the coil, diagnose a stuck or bleeding valve by charge and port temperatures before you condemn it, and never overheat the valve when brazing.
Units and terms
The reversing valve goes by several names on drawings and parts lists, and the defrost controls have their own vocabulary, so the same part can read differently across a manual, a wiring diagram, and a parts counter.
A reversing valve is also called a four-way valve or a changeover valve. The pilot solenoid is the small coil, sometimes called the reversing-valve solenoid. The permanent suction line is the center bottom tube that always returns to the compressor. The terms below cover the rest of the language you will see on the call.
- Reversing valve
- Four-way valve that reverses refrigerant flow to switch a heat pump between heating, cooling, and defrost
- Pilot solenoid
- Small electric coil that shifts a pilot valve, letting system pressure move the main slide
- O terminal
- Thermostat terminal that energizes the reversing valve in cooling, used by most manufacturers
- B terminal
- Thermostat terminal that energizes the reversing valve in heating, used by Rheem and some others
- Demand defrost
- Control that initiates defrost from coil sensor readings only when frost has actually built up
- Time-temperature defrost
- Older control that defrosts on a fixed run-time interval once the coil is confirmed cold
- Permanent suction line
- The center bottom tube on the valve that always returns refrigerant to the compressor
FAQ
What does a reversing valve do on a heat pump?
A reversing valve reverses the direction refrigerant flows, swapping which coil is the condenser and which is the evaporator. That lets one heat pump both heat and cool, and it runs the defrost cycle by switching to cooling to melt frost off the outdoor coil. A small pilot solenoid shifts the main slide.
How does a heat pump defrost work?
Defrost reverses the valve to cooling for a few minutes, sending hot gas to the outdoor coil to melt frost. The outdoor fan shuts off so the coil heats fast, and backup heat comes on indoors so the supply air does not blow cold. When the coil clears, the valve flips back to heating.
How do you tell if a reversing valve is stuck?
A stuck or bleeding valve gives lukewarm or no heat with head and suction pressures too close together. Check charge first, since low charge mimics it. Then compare the suction line to the permanent suction line at the valve; a warm permanent suction line means hot gas is bleeding across the slide. Tap the body to free a stuck slide.
What is the O and B terminal on a heat pump?
O and B are the thermostat terminals that control the reversing valve, and only one is used. O energizes the valve in cooling and is used by most brands like Carrier, Trane, and Goodman. B energizes it in heating and is used by Rheem and some others. Wire it by the unit's diagram, not by habit.
Is steam coming off my heat pump during defrost normal?
Yes. Steam rising off the outdoor unit in winter is melted frost turning to vapor as the coil heats above freezing during defrost. It is not smoke and not a leak. You will also hear a whoosh as the valve shifts and see the outdoor fan stop. All of that together for a few minutes is a normal defrost.
Why does my heat pump blow cold air sometimes in winter?
Brief cold air every so often is usually defrost. The unit runs in cooling to melt the outdoor coil, and if backup heat is not tempering the supply, the air blows cold for a few minutes. Confirm the defrost board fires the auxiliary heat during the cycle. Constant cold air points instead to a stuck valve or low charge.
Can you fix a stuck reversing valve without replacing it?
Sometimes, briefly. Tapping the valve body with a screwdriver handle while it is running can knock a hung slide free, and cycling it several times confirms it moves. But a valve that needed tapping has a worn or fouled slide and will hang again. Treat the tap as a diagnosis, not a repair, and plan to replace it.
Do geothermal heat pumps have a defrost cycle?
No. Water-source and geothermal heat pumps still have a reversing valve, but they pull heat from a water or ground loop, not cold outdoor air, so no coil frosts and there is nothing to defrost. The valve diagnostics and O/B wiring still apply, but there is no defrost board, no fan cutoff, and no defrost-triggered backup heat.
How do you replace a reversing valve without ruining it?
The whole risk is heat damaging the internal seat and seals. Recover the charge, remove the solenoid coil, wrap the valve body in a wet rag and keep it wet, flow nitrogen, and braze fast with the flame on the joint and off the body. A valve cooked during brazing bleeds across the slide and fails early.