HVAC
Refrigerant metering devices field guide: TXV, orifice, and EEV
Know which device is on the system, why a TXV holds superheat and an orifice does not, when you need an external equalizer, and how to read a starved valve from a starved line.
Direct answer
A refrigerant metering device is the restriction between the high-pressure liquid line and the low-pressure evaporator that drops the pressure and meters liquid into the coil at the rate the load needs. A fixed orifice meters a set amount; a TXV or EEV modulates flow to hold evaporator superheat. The data plate and manufacturer set the match.
Key takeaways
- A refrigerant metering device is the restriction between the high-pressure liquid line and low-pressure evaporator, dropping pressure and metering liquid at the load's rate.
- A fixed orifice passes a set amount and floats superheat, so charge it by superheat; a TXV or EEV holds superheat, so charge it by subcooling.
- A TXV balances three forces: bulb pressure opens the valve, while evaporator pressure and the adjustable superheat spring close it.
- Use an externally equalized TXV on any distributor coil or more than about 2 psi coil drop; distributor nozzles drop roughly 15 to 30 psi.
- A lost TXV bulb charge clamps the valve shut, showing very high superheat and low suction; the bulb is not field-rechargeable, so replace the valve.
The metering device, and where it sits in the system
A refrigerant metering device is the restriction between the high-pressure liquid line and the low-pressure evaporator. It does two jobs at once. It drops the pressure on the liquid so the refrigerant can boil at a low temperature in the coil, and it meters that liquid into the evaporator at the rate the load is asking for. That single point is the dividing line between the high side and the low side of the system. Upstream of it is condensing pressure. Downstream of it is evaporating pressure.
Stand at the device and you can read the whole system. Liquid arrives subcooled and under condensing pressure, and it leaves as a low-pressure mix of liquid and flash gas that finishes boiling in the coil. Get the feed right and the evaporator stays full of boiling refrigerant while the compressor gets nothing but dry vapor. Feed too little and the coil starves. Feed too much and liquid carries past the coil toward the compressor.
The device you find depends on the system. A fixed orifice or piston on most residential split systems, a capillary tube on small sealed units, a thermostatic expansion valve on anything with a varying load, and an electronic expansion valve on modern and commercial gear. The charge method follows the device, which the refrigerant charging guide covers in full. This guide is about the devices themselves.
What does the metering device do in the refrigeration cycle?
The metering device is one of the four points in the basic refrigeration cycle, sitting between the condenser and the evaporator. The compressor raises the pressure and temperature of the vapor. The condenser rejects heat and turns that vapor into a high-pressure liquid. The metering device drops that liquid to low pressure. The evaporator boils it and absorbs heat from the air. Then the cycle repeats. Each piece only works if the one before it did its job.
The pressure drop across the device is where the cooling starts. When high-pressure liquid hits the sudden restriction, part of it flashes to vapor instantly. That flash gas is not wasted. The heat it takes to boil comes from the liquid around it, which is what drops the mixture down to evaporator temperature. So the refrigerant entering the coil is already cold, already part vapor, and ready to finish boiling as it picks up heat.
How much flash gas forms at the inlet depends on how much subcooling the liquid had coming in. Liquid that arrives barely subcooled flashes more and carries less capacity into the coil. Liquid that arrives with solid subcooling flashes less and feeds the coil better. This is why subcooling at the device matters, and it comes up again later and in the lineset install guide.
The fixed orifice and piston
A fixed orifice is the simplest metering device made: a fixed-bore restriction, often a piston with a precise hole drilled through it, set in the liquid line at the coil. There are no moving parts and nothing to adjust. The bore size sets the flow, and that is the whole device. It is cheap, it almost never fails mechanically, and it is the standard on a large share of residential air conditioners and heat pumps.
The catch is the thing it cannot do. It does not adjust to the load. The bore is sized for one design condition, so when the load rises or falls, when the outdoor temperature swings, when airflow changes, the orifice keeps passing roughly the same amount and the evaporator superheat moves with the conditions. That is why a fixed-orifice system is charge-critical. The charge is what sets the feed, so it has to be right, and you set it by superheat. The charging guide covers the method.
On a heat pump the piston also does a bi-flow trick. It seats against its bore in one direction of flow and unseats to a larger gap in the other, so the same piston meters in cooling and lets refrigerant blow by in heating, with a second piston metering at the other coil. Some orifices are built to weep or bleed a little after the compressor stops, which equalizes pressures so the compressor restarts against less load.
The capillary tube
A capillary tube is a fixed metering device taken to its simplest form: a long, very small-bore tube, sometimes several feet of it coiled up, that meters by the friction of refrigerant moving through a narrow passage. It is the cheapest device there is and it has no moving parts at all. You find it on refrigerators, freezers, window units, dehumidifiers, and small sealed systems where the load barely changes.
Because the cap tube cannot adjust, the system around it is built to one fixed operating point. These systems are critically charged, usually with no receiver to hold a reserve, so the charge is weighed in to the gram and the tube length is matched to the equipment. Add or lose a little refrigerant and the balance shifts. There is no valve to cover for it.
The cap tube also lets the high and low sides equalize through it during the off cycle, so a small system can use a low-starting-torque compressor. Its weakness is what fits through that tiny bore. A speck of debris or a trace of moisture freezing at the outlet can block it, which is why a clean system and a good drier matter even more here than on larger gear.
How does a TXV control superheat?
A thermostatic expansion valve, the TXV or TEV, modulates refrigerant flow to hold a constant superheat at the evaporator outlet. It is the workhorse metering device on anything with a load that moves. Instead of passing a fixed amount, it opens and closes to keep the coil fed exactly enough that the refrigerant finishes boiling just before the outlet, leaving a steady margin of superheat to protect the compressor.
Three forces balance on the valve diaphragm to do this. A sensing bulb on the suction line is filled with a charge that builds pressure as the suction line warms, and that bulb pressure pushes the valve open. Working against it are two closing forces: the evaporator pressure under the diaphragm and an adjustable superheat spring. When the suction vapor warms, meaning the coil is running short and superheat is climbing, bulb pressure rises and opens the valve to feed more. When the vapor cools toward saturation, bulb pressure falls and the spring and evaporator pressure pinch the valve down.
The result is a valve that hunts toward a target superheat and holds it across a wide range of load. The spring sets the target. It is factory-set for the application, adjustable on many valves but not a knob you turn casually. Because the valve actively holds superheat, superheat stops being your charge indicator on a TXV system, which is why you charge it by subcooling. That is the charging guide's subject. The point here is that the valve, not the charge, sets the feed.
The sensing bulb: mounting and insulation
The bulb is the TXV's only sense of what the coil is doing, so a bad bulb means bad metering no matter how good the valve is. It has to read the temperature of the suction vapor leaving the evaporator, and nothing else. Mount it on a clean, straight, horizontal section of suction line close to the coil outlet, clamped tight to bare metal with the manufacturer's strap so it reads the line, not the air around it.
Clock position matters on larger lines. Common practice is to set the bulb at about the 4 o'clock or 8 o'clock position on the pipe, off the bottom where oil and any liquid runs, and off the top. A bulb reading liquid or oil pooling in the line reads the wrong temperature and the valve meters wrong. After it is strapped, insulate it so it senses the refrigerant inside the pipe instead of the equipment room. An uninsulated bulb in a warm mechanical space reads high, opens the valve, and floods the coil.
This is the spot rookies get wrong more than any other on a TXV. They strap the bulb on loose, leave it on a vertical riser or next to a fitting, skip the insulation, then chase a superheat that will not settle. Before you blame the valve, check the bulb mount. It is the first thing a good tech looks at when a TXV is hunting or feeding wrong.
Bulb charge types and MOP
The fluid sealed inside the bulb and power head is the bulb charge, and it is matched to the refrigerant and the application. A valve built for one refrigerant has a charge tuned to that refrigerant's pressure-temperature curve, which is one reason you cannot freely swap a valve from one refrigerant to another. The charge type also changes how the valve behaves at the edges of its range.
The common types are a liquid charge, a gas or limited-liquid charge, and a cross charge that uses a different fluid than the system refrigerant to shape the response. The gas-charged, or MOP, valve is the one to know. MOP stands for maximum operating pressure. The bulb is charged with a limited amount of fluid that fully vaporizes above a set pressure, so once suction pressure reaches that point the bulb can no longer push the valve open. The valve throttles and caps the suction pressure.
That MOP feature protects the compressor motor from overload during a hot pull-down, when a warm coil would otherwise drive suction pressure and motor current too high. The trade-off is that a MOP valve must keep its bulb colder than the power head so the charge condenses in the bulb, not the head, or it loses control. Match the valve's MOP rating to the equipment. Do not guess it.
When do I need an external equalizer?
You need an externally equalized TXV whenever the evaporator has a meaningful pressure drop between the valve outlet and the bulb location, which in practice means any coil with a refrigerant distributor. An internally equalized valve senses evaporator pressure from its own outlet, right at the valve. That works only when the pressure at the valve outlet is close to the pressure at the coil outlet where the bulb sits.
A distributor changes that. It splits the feed into multiple circuits through a nozzle, and that nozzle is a designed pressure drop, commonly in the range of 15 psi to 30 psi. An internally equalized valve sitting upstream of that drop would sense a higher pressure than the coil outlet actually has. The valve reads that pressure difference as extra superheat it has to overcome, pinches down, and underfeeds the coil. The result is a starved evaporator and high superheat that no spring adjustment fixes.
The external equalizer is a small line tapped from the valve to the suction line near the bulb, downstream of the coil and the distributor. It feeds true coil-outlet pressure to the underside of the diaphragm, so the valve balances against the pressure the coil actually has. The rule a lot of techs carry: distributor or more than about 2 psi of coil pressure drop, use an external equalizer. No distributor and a small low-drop coil, internal is fine. When in doubt, an externally equalized valve works on both, which is why most replacement valves are externally equalized.
The electronic expansion valve
An electronic expansion valve, the EEV or EXV, does the TXV's job under electronic control. Instead of a bulb and a spring balancing pressures, a controller reads a pressure transducer and a temperature sensor at the coil outlet, calculates superheat directly, and drives a stepper motor that positions the valve in small, precise steps. It is the modern metering device, and it is the standard on VRF and VRV systems, most newer commercial equipment, transcritical CO2 systems, and precision cooling.
The advantages over a mechanical TXV are real. The EEV controls to a lower and tighter superheat, often in the low single digits, because the controller measures superheat instead of inferring it through a bulb charge, and it reacts in a fraction of a second instead of waiting for a bulb to change temperature. It holds control across a much wider range of load, evaporating pressure, and condensing pressure, which is what makes inverter-driven and modulating systems work. It can close fully on shutdown, acting as a stop valve, and a single valve can run across the wide pressure range a CO2 system sees.
The cost is complexity. An EEV needs a controller, sensors, and power, so a failure can be electronic instead of mechanical, and diagnosing it means reading the controller and confirming the sensor inputs, not just measuring superheat at the pipe. On the systems that use them, the manufacturer's controller logic and fault codes are part of the device. You do not adjust an EEV with a wrench. You read what the controller is doing and why.
The automatic expansion valve, hot gas bypass, and other valves
An automatic expansion valve, the AXV or AEV, holds a constant evaporator pressure rather than a constant superheat. It opens and closes to keep suction pressure steady against a spring, so as the load rises it actually throttles down and as the load falls it opens up, the opposite of what a TXV does. That gives it one niche strength: it protects against compressor overload and keeps the coil at a fixed temperature on a steady, light load. It feeds poorly on a varying load, so it is uncommon in air conditioning and shows up mostly on small constant-load applications.
Hot gas bypass is not a metering device, but it lives in the same conversation because it manages capacity at low load. A hot gas bypass valve routes a controlled amount of hot discharge gas into the low side, usually at the evaporator inlet or the suction line, to keep suction pressure up when the load drops below what the compressor wants to see. It lets a fixed-capacity compressor run down to a low load without the coil freezing or the compressor short-cycling.
Other valves round out the low side. A distributor splits the metered feed across coil circuits. A solenoid valve gives hard on-off control of liquid flow for pump-down and zoning. A check valve sets flow direction on a heat pump. None of these meter superheat, but they sit next to the device, and a tech reads them as part of the same picture.
Which metering device fits the system?
Match the metering device to how the load behaves and what the system is worth. A fixed orifice fits a simple, cost-sensitive system with a fairly steady design point, which is why it is everywhere in entry-level residential equipment. It is cheap, it is reliable, and it gives up efficiency at off-design conditions in exchange for that simplicity.
A TXV fits anything with a varying load or a need for better part-load efficiency. Because it holds superheat across the range, it keeps the coil fed when conditions move off the design point, which is why higher-efficiency systems and most heat pumps use one. The TXV is the reason a system can hold capacity on a mild day and a design day both. On rated equipment the metering device is part of how the AHRI-matched system earns its efficiency number.
An EEV fits where precision, range, and integration pay for the complexity: VRF, inverter-driven equipment, large commercial, CO2, and precision cooling. If the compressor modulates, the metering usually has to modulate with it, and only an electronic valve keeps up. The selection is not really yours to make on a packaged system, since the manufacturer designed the device in. On a build-up or a replacement it is exactly the call you make, and you make it on the load profile, not on price alone.
How the device sets superheat, and how you charge to it
Evaporator superheat is the number the metering device is really controlling, whether it controls it on purpose or not. On a fixed orifice, superheat is a result. The bore passes what it passes, so superheat floats with charge, load, and airflow, and the charge is what you trim to land it. On a TXV or EEV, superheat is the target. The valve moves to hold it, so the charge no longer sets it.
That split is the reason the charge method changes with the device. You charge a fixed-orifice system by superheat, because superheat is the honest signal of charge there. You charge a TXV or EEV system by subcooling, because the valve has taken superheat off the table and subcooling is what now moves with charge. The full method, the targets, and how to read both numbers together live in the refrigerant charging guide, and that is where to go to actually set a charge.
The reason to understand it here is diagnosis. When you see a superheat reading, the first question is what device is on the system, because the same number means different things on a fixed orifice than on a TXV. High superheat on a fixed orifice points at undercharge. High superheat on a TXV with good subcooling points at the valve or its bulb, not the charge.
Why is my TXV starving or flooding the coil?
A TXV that meters wrong is either starving the coil, feeding too little, or flooding it, feeding too much, and the superheat tells you which. Starving shows up as high superheat with low suction pressure: the coil is short of refrigerant and the vapor leaving it is too warm. Flooding shows up as low superheat, sometimes near zero, with high suction pressure and the risk of liquid reaching the compressor. Read superheat and subcooling together before you condemn the valve.
Starving has a short list of causes. A plugged inlet screen or a clogged drier ahead of the valve, the valve stuck toward closed, a lost bulb charge so the power head has no pressure to open the valve, inadequate subcooling feeding flash gas to the valve, or a missing external equalizer on a distributor coil. A lost bulb charge is the classic dead-TXV symptom: the valve clamps shut, superheat goes very high, and the coil starves no matter what you do at the charge.
Flooding usually traces to the bulb. A bulb mounted loose, uninsulated, or in a warm spot reads high, drives the valve open, and floods the coil. An oversized valve hunts and overfeeds. A valve stuck open feeds wide. Hunting, where superheat swings up and down and the suction pressure cycles, points at an oversized valve, a poorly mounted bulb, or an equalizer problem before it points at charge. Fix the bulb mount and the equalizer first, because they are cheap and they are usually the cause. The charging guide covers how the two readings sort charge from valve.
The filter drier ahead of the metering device
The filter drier sits in the liquid line upstream of the metering device for one reason: to keep moisture and debris out of the smallest, most flow-sensitive passage in the system. The device, especially a cap tube or a TXV inlet screen, is where a trace of moisture or a fleck of solder, filings, or copper oxide ends up, because that is the first tight spot the refrigerant reaches. The drier is the guard in front of it.
Moisture is the quiet killer. Free water in the system freezes at the metering device outlet where the refrigerant is coldest, blocks flow intermittently, and shows up as a coil that ices and clears on a cycle that makes no sense. Water also reacts with the oil and refrigerant to form acid that attacks the compressor. The drier's desiccant holds the moisture and its filter catches the solids. On a TXV there is usually a fine inlet screen in the valve as a last line of defense.
Change the drier any time the system is opened, and size it to the system. A drier loaded with moisture or packed with debris becomes a restriction itself, and a restricted drier starves the metering device exactly like a low charge does, with high superheat and low suction. A temperature split across the drier, where the outlet is noticeably cooler than the inlet, is the field tell that the drier is plugging and pulling a pressure drop it should not.
Flash gas, subcooling, and feeding the device
The metering device needs a solid column of liquid at its inlet to meter correctly. Liquid is what it is built to pass. When vapor reaches the inlet mixed with the liquid, that is flash gas in the wrong place, and it cuts the device's effective capacity because gas takes up volume the liquid should have. The result looks exactly like a starved valve: high superheat, low capacity, a coil that will not get cold.
Subcooling is what keeps the column solid. Liquid leaving the condenser with real subcooling has margin against flashing, so it stays liquid through the line and the drier and arrives at the device as liquid. Lose that margin and the liquid flashes early. The usual causes are undercharge, a long or undersized liquid line, too much vertical lift on the liquid riser, a restriction like a plugging drier, or high line temperature on a hot run. Each one bleeds pressure or adds heat and starts the boil before the device.
This is the tie to the install. A liquid line that is too long, too small, or lifted too high will not deliver liquid to the device under all conditions, and no metering device fixes a feed problem upstream of it. The lineset install guide covers sizing the line and managing lift so the device sees liquid. The lesson here is to confirm subcooling at the device before you blame the device, because a starved valve is often a starved liquid line.
Sizing and replacing a TXV
Size a TXV to the system tonnage and the refrigerant, and do not round up for insurance. The valve is rated in tons of capacity at a set of design conditions, and that rating assumes the refrigerant it was built for. An oversized valve is the more common mistake, and it hunts: it overfeeds, the bulb cools, it slams shut, the coil starves, the bulb warms, and it overfeeds again, swinging superheat instead of holding it. An undersized valve cannot pass enough at full load and the coil starves at the top end.
Match the refrigerant exactly. The valve's bulb charge and port are tuned to one refrigerant's pressure-temperature curve, so an R-410A valve is not an R-22 valve and not an R-32 or R-454B valve. Put the wrong valve on and the superheat target it holds is wrong, even if the valve works mechanically. Confirm the equalizer style too. A distributor coil needs an externally equalized valve, and most replacements are externally equalized so they cover both.
When you replace a valve, protect it during the braze. Wrap the valve body in a wet rag or use a heat sink so you do not cook the internals or the seat, braze with flowing nitrogen to keep oxide off the inside, and install a new drier. Then set or confirm superheat per the manufacturer and charge the system by subcooling. A new valve on a system with an old plugged drier and a moisture problem fails the same way the old one did.
Matching the device to the refrigerant: R-410A, A2L, and CO2
The metering device has to match the refrigerant, because the device is sized and charged around that refrigerant's pressures. The same physical body passing R-22, R-410A, R-32, or CO2 would meter wrong on most of them, and on the high-pressure refrigerants it may not be rated for the pressure at all. A changeout or a refrigerant conversion is a device question, not just a charge question.
R-410A runs at roughly 50 to 70 percent higher pressure than the old R-22, so its orifices, pistons, and valves are built and charged for that. The A2L refrigerants now on new equipment, R-32 and R-454B, are mildly flammable and carry their own component listings, so the metering device, like the rest of the system, is matched and approved for the specific refrigerant rather than swapped from an R-410A system. Use the device the equipment is built and listed for.
CO2, R-744, is its own world. It runs at very high pressure and often transcritical, where there is no clean condensing point on the high side, so the high-side device is a controlled gas cooler valve managing pressure rather than a simple liquid metering valve. That job needs an electronic valve and a controller, which is why CO2 systems are EEV systems. You do not put a mechanical TXV from an HFC system anywhere near CO2 pressures.
Precision cooling and data-center metering
Precision cooling equipment, the CRAC and CRAH units and in-row coolers that hold a data center to a tight temperature and humidity, leans on electronic metering for the same reason the rest of modern commercial gear does. The load is steady but the control has to be exact, and the equipment runs continuously, where a sloppy charge or a hunting valve costs real money in energy and reliability.
Most precision units use EEVs driven by the unit controller, often with multiple coils and circuits each metered independently, so the controller can hold a low, stable superheat and keep every circuit fed evenly across the load. That even feed matters more here than in comfort cooling, because an unevenly fed coil gives up capacity and the room reaches setpoint only by working the compressor harder. On variable-load and free-cooling units, the electronic valve is what lets the system shift modes without losing control of the coil.
The practical note for a tech on this gear: the metering is part of the controls. You confirm superheat through the controller's sensors, you read the valve position the controller is commanding, and you trust the data plate and the manufacturer's commissioning procedure over any rule of thumb carried over from comfort work. ASHRAE's thermal guidelines set the room conditions these units hold. The device just keeps the coil right while they do it.
What to document
A metering-device call that nobody can reconstruct later is a call that gets re-diagnosed from scratch. Whether you installed it, replaced it, or just diagnosed it, write down the device and the readings that back the verdict, so the next tech starts where you finished.
Capture the metering device type and model, the refrigerant it is matched to, the equalizer style if it is a TXV, the superheat and subcooling you measured, the targets from the data plate, whether you replaced the drier, and the reason for the call. If you set a TXV superheat or confirmed an EEV's commanded position, record that too. The table below is the at-a-glance version of which device does what, which is the first thing to confirm before you read a single number.
| Device | What it controls | Best for |
|---|---|---|
| Fixed orifice / piston | Fixed flow; superheat floats with charge and load | Simple, cost-sensitive residential, steady design point |
| Capillary tube | Fixed flow by tube friction | Small sealed units, refrigerators, window units |
| TXV / TEV | Constant evaporator superheat, mechanically | Varying-load comfort cooling, heat pumps, efficiency |
| EEV / EXV | Superheat electronically, low and tight | VRF, inverter, commercial, CO2, precision cooling |
| AXV / AEV | Constant evaporator pressure | Steady, light, constant-load niche applications |
Common mistakes
- Putting a fixed orifice on a system that needs a TXV, then living with poor part-load efficiency and a coil that runs wrong off the design point.
- Mounting the TXV bulb loose, on a vertical riser or a fitting, off the 4 or 8 o'clock position, or leaving it uninsulated so it reads the room.
- Skipping the external equalizer on a distributor coil, which starves the evaporator with high superheat no spring adjustment fixes.
- Oversizing or undersizing the TXV instead of matching it to the tonnage and refrigerant, then chasing a hunting or starved valve.
- Charging a TXV or EEV system by superheat instead of subcooling, because the valve is already holding superheat.
- Ignoring subcooling so flash gas reaches the device, then blaming the valve for what is really a starved liquid line.
- Installing no filter drier, or reusing an old one, and letting moisture freeze at the device outlet or acid attack the compressor.
- Swapping a valve from the wrong refrigerant, so the bulb charge and port hold the wrong superheat target.
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.
Standards and references
The metering device is matched and listed at the equipment level, not chosen in the field on most packaged systems. The manufacturer sets the device, the bore or valve rating, the bulb charge, and the refrigerant it is approved for, and on rated equipment the metering device is part of the AHRI-certified match that earns the published capacity and efficiency. Use the device the equipment is built and listed for, and set superheat or subcooling to the data plate and the manufacturer's charging chart.
For the wider standards, AHRI sets the system rating and matched-component certification, ASHRAE covers refrigeration design and the data-center thermal guidelines that precision units hold to, and the equipment's own listing under UL or the relevant safety standard governs pressure rating and, for the A2L and CO2 refrigerants, the safety requirements. Exact procedures and component listings shift with the refrigerant and the code cycle, so confirm them against the equipment documentation and the adopted edition.
Refrigerant handling itself is regulated. Under EPA Section 608 you recover refrigerant before opening the system to replace a metering device or a drier, and you do not vent. That is law, not a target. Superheat and subcooling numbers, by contrast, are set by the charging method and the manufacturer, so this guide hedges them to the data plate rather than printing a single figure. The charging guide covers how to read them.
Units, terms, and conversions
The metering device goes by a few names and the readings around it show up in a few unit systems, so the same idea can read differently across a data plate, a valve catalog, and a controls screen.
A metering device is also called an expansion device, a flow control, or a restrictor. A TXV is the same as a TEV or thermal expansion valve, an EEV is the same as an EXV or electronic expansion valve, and an AXV is the same as an AEV or automatic expansion valve. Pressure reads in psig in the field and in kPa or bar on metric equipment. Superheat and subcooling read in degrees F here and degrees C on metric gear, and both come off a pressure converted to a saturation temperature, never off pressure alone.
- Metering device
- The restriction between the high-pressure liquid line and the low-pressure evaporator that drops pressure and meters liquid into the coil
- Superheat
- Degrees the suction vapor sits above its saturation temperature at the current pressure; what a TXV or EEV controls
- Subcooling
- Degrees the liquid sits below its saturation temperature at the current pressure; how you charge a TXV or EEV system
- Flash gas
- Vapor formed when liquid drops in pressure or picks up heat before the device, which starves the device of liquid
- External equalizer
- A line feeding coil-outlet pressure to the TXV diaphragm, needed on distributor and high-drop coils
- MOP
- Maximum operating pressure; a gas-charged bulb that caps suction pressure to protect the compressor on pull-down
- Bulb charge
- The fluid sealed in the TXV bulb and power head, matched to the refrigerant, that sets the opening force
FAQ
What is a metering device in HVAC?
A metering device is the restriction between the high-pressure liquid line and the low-pressure evaporator. It drops the liquid to evaporator pressure and meters it into the coil at the rate the load needs. The common types are the fixed orifice, the capillary tube, the thermostatic expansion valve, and the electronic expansion valve.
What is the difference between a TXV and a fixed orifice?
A fixed orifice passes a set amount of refrigerant and cannot adjust to the load, so its superheat floats and you charge it by superheat. A TXV modulates flow to hold a constant evaporator superheat across changing load, so you charge it by subcooling. The TXV costs more and holds the part-load efficiency the orifice gives up.
What is an electronic expansion valve?
An electronic expansion valve, or EEV, meters refrigerant under controller command. A controller reads pressure and temperature at the coil, calculates superheat, and drives a stepper motor that positions the valve in fine steps. It holds lower, tighter superheat than a TXV and reacts faster, which is why VRF, inverter, CO2, and precision cooling use it.
How does a TXV control superheat?
A TXV balances three forces on its diaphragm. A sensing bulb on the suction line builds pressure as the vapor warms and pushes the valve open; evaporator pressure and an adjustable spring push it closed. When superheat climbs, the bulb opens the valve to feed more; when it falls, the spring pinches it down, holding superheat steady.
What superheat should a TXV hold?
A TXV holds the superheat its spring is set for, commonly somewhere around 8°F to 12°F at the evaporator outlet, but the exact target comes from the equipment data plate and manufacturer, not a universal number. Because the valve holds superheat, you verify charge on a TXV system by subcooling, not by superheat.
Do I need an external equalizer on my TXV?
You need an externally equalized TXV on any coil with a refrigerant distributor or more than about 2 psi of pressure drop across the evaporator. The distributor's pressure drop, often 15 psi to 30 psi, would make an internally equalized valve underfeed the coil. Most replacement valves are externally equalized so they cover both cases.
What happens when a TXV bulb loses its charge?
A TXV that loses its bulb charge has no pressure to open the valve, so it clamps shut and starves the evaporator. You see very high superheat, low suction pressure, and a coil that will not get cold no matter what you do at the charge. The fix is a new valve; the bulb and power head are not field-rechargeable.
Why is my TXV hunting?
A hunting TXV swings superheat up and down and cycles the suction pressure because it keeps overfeeding then starving the coil. The usual causes are an oversized valve, a poorly mounted or uninsulated bulb, or an equalizer problem, in that order. Check the bulb mount and the equalizer before you condemn the valve or touch the charge.
What is a capillary tube and where is it used?
A capillary tube is a long, very small-bore fixed metering device that meters by friction, with no moving parts. It is the cheapest device made and is used on refrigerators, freezers, window units, and small sealed systems with a steady load. These systems are critically charged by weight, usually with no receiver, so the charge has to be exact.
Do I charge a fixed orifice and a TXV the same way?
No. Charge a fixed-orifice system by superheat, because superheat moves directly with charge when nothing regulates the flow. Charge a TXV or EEV system by subcooling, because the valve already holds superheat, so superheat no longer reflects charge. Read both numbers, but trim to the one that matches the metering device on the system.