TEV installation, testing, troubleshooting & repair guide:
This article describes how to install, test, or diagnose problems with a thermostatic expansion valve used on air conditioners or heat pumps or similar refrigeration equipment. These include improper thermostatic valve installation, wrong sensor bulb location, sticking TEVs, whistling TEVs, hunting TEVs and other problems.
We define and explain other refrigeration equipment metering devices including AEVs (Automatic Expansion Valves), manually adjusted expansion valves, capillary tubes and Low Side or High Side refrigerant float valves. Page top sketch of a thermostatic expansion valve is provided courtesy of Carson Dunlop Associates.
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Thermostatic expansion valves (TEVs) are designed to meter refrigerant into the cooling coil at the proper rate. This design can keep the proper dose of refrigerant entering the cooling coil for maximum air conditioning or heat pump system operating efficiency.
TEVs are similar to automatic expansion valves (AEVs) discussed below, but incorporate the signal from a temperature sensor mounted at the end of the evaporator coil
If you are diagnosing a problem with an air conditioner or heat pump and the TEV appears to be involved, check the TEV installation details against the information we list below.
The valve fittings and exterior parts must be thoroughly cleaned before soldering or brazing in order to prevent debris from entering the refrigerant system or valve body.
Protect the TEV from heat during soldering or brazing by wrapping the valve body and head with a wet cloth to give 1/4" to 1/2" of thermal protection, and keep the soldering or brazing torch aimed away from the valve itself.
[Click to enlarge any image]
As with any soldering or brazing job, don't use excessive solder or flux or the excess may flow into the valve or tubing to prevent proper operation.
The thermostatic expansion valve sensing bulb that controls the thermostatic expansion valve is clamped to the refrigerant suction line where it monitors the system's temperature.
The TEV should be installed as close as possible to the air conditioning or heat pump evaporator coil inlet. Sketch of CAPILLARY TUBE defects courtesy of Carson Dunlop. Other TEV and cap-tube defects listed below were obtained from an installation guide for TEV's provided by Singer Controls Corp.
Danfoss (2005) points out that the expansion valve itself is installed in the liquid refrigerant line ahead of the evaporator coil (cooling coil) and that its sensor bulb is to be fastened to the suction line as close to the evaporator as possible.
For TEVs that use an external pressure equalization feature, the equalizing pressure line must be connected to the suction line (at the cooling coil outlet end) immediately after the temperature sensing bulb that operates the TEV. (Danfoss 2005).
On a small refrigerant suction line such as on a residential cooling system or heat pump, the sensing bulb is clamped on top of the suction line; If the sensor bulb is located outside of the refrigerated space, additional protection from ambient temperatures is required.
Extend insulation on either side of the suction line at least one foot on either side of where the bulb is located. Our photo of a thermostatic expansion valve above at TEV DETAILS shows that the sensor bulb was outside of the air handler and was taped under insulation covering the suction line.
On a commercial HVACR system that uses a larger diameter line (more than 7/8" in diameter) the thermostatic expansion valve sensor bulb is clamped at the 5-o'clock position or 7-o'clock position on the lower portion of the suction line.
Note: Danfoss gives slightly different instructions:
The [temperature sensing] bulb is best mounted on a horizontal suction line tube and in a position corresponding to betweeen 1 o'clock and 4 o'clock. The location depends on the outside diameter of the tube. - Danfoss (2005)
The intent is to place the TEV's sensing bulb close to but above the level of liquid refrigerant in the line. So for larger diameter refrigerant tubing the 2, 3, or 4 o'clock position may be used while on small diameter tubing the 1 o'clock position is used.
The reason we don't clamp the TEV sensor bulb on the bottom of the cooling coil line is that oil traveling along the bottom of that tubing can act as an insulator to prevent the TEV sensor bulb from accurately sensing the temperature of the evaporator coil.
TEV interference: Most installation guides require that the thermostatic expansion valve has nothing else installed on the refrigerant tubing that extends between the TEV and the evaporator coil, except in commercial installations where a refrigerant distributor may have been installed. In this instance (use of a TEV with a refrigerant distributor), the TEV should be a unit that provides an external equalizer.
TEV location height: Do not install the Thermostatic Expansion Valve higher than the liquid refrigerant receiver. If a TEV is placed too high in a commercial system, refrigerant vapor or flash gas from the receiver may enter the TEV and prevent its proper operation.
WAtch out: a TEV sensor that is too loose or has a poor thermal connection to the suction line can send improper signals to the TEV and can cause liquid refrigerant flood damage to the compressor motor. 
Watch out: the TEV's temperature sensing bulb should never be placed at the bottom or under-side of the suction line. That's becausse there may be refreigerant oil along the bottom of the tubing - a condition that will cause improper reading of the line's temperature, thus causing improper TEV operation.
Watch out: the TEV's temperature sensing bulb must also not be exposed to external sources of heat or cold such as a warm air current. If that exposure is at risk insulation should be added to protect the bulb from that external influence. Otherwise the sensing bulb, intended to sense the temperature of the superheated suction vapor, will instead be influenced by other temperatures nearby.
Watch out: the TEV temperature sensing bulb must not be installed after a heat exchanger - another location giving improper signals to the TEV
Watch out: the TEV temperature sensing bulb must also not be installed close to compoents of large mass (such as a stop valve onthe refrigerant line) because the large mass will influence nearby refrigerant line temperatures, giving the sensor bulb misleading information and causing it to misinform the TEV itself.
Watch out: don't install TEV temperature sensing bulbs on vertical piping and do not install the sensor bulb on a collection tube or riser or even on a horizontal section of tubing that may be located after an oil pocket location such as at a dip or effectively a plumbing trap formed in the refrigerant line.
Watch out: don't install the TEV sensing bulb after a liquid lock. The bulb must be ahead of any liquid lock on the piping system.
On commercial cooling installations it is also important that the TEV be installed in a location leaving enough access to adjust the device. Details about TEV adjustment are below at Adjusting the Thermostatic Expansion Valve
Definition of Hunting at the TEV:Hunting exapansion valves are TEVs that are cycling too frequently between open and closed - a condition that may be detected as rapid changes in suction line pressure by 10-15 psi each time the valve opens or closes. Two causes of hunting TEVs or TXVs (synonyms we include here as readers use both acronyms for thermostatic expansion valves) are improper adjustment of a properly-sized TEV - superheat set too low
If the TEV superheat set too low this can cause rapid-cycling TEVs TXVs and liquid refrigerant flooding the suction line and compressor. Variations in the level of superheat of + or - 0.5°C or less are not considered "hunting" while variations greater than that amount are considered hunting and should be addressedas we explain here.
The superheat must be allowed to change in order for the sensing bulb on the TEV to make corresponding adjustments to the valve opening while it maintains the set superheat. A low superheat setting can allow the superheat to fall to zero. This condition will cause the valve to close and the superheat will soon rise well above its setting.
This in turn can cause the valve to reopen wider than necessary and overshoot the superheat setting. This condition is called hunting. On one of the valve's swings to the wide open position, liquid refrigerant can flood the suction line and allow liquid to enter the compressor, a potentially damaging condition.
In other words, an improper TEV setting can cause rapid opening/closing of the valve resulting in liquid refrigerant flooding the suction line, entering the compressor motor. Because compressor motor moving parts and valves are designed to compress a gas, and because a liquid (refrigerant) is not particularly compressible, the compressor motor, piston, crank, or more immediately its refrigerant valves are likely to be damaged or destroyed.
It is possible that dirt, debris, or even water in the refrigerant piping system and that enters the TEV could cause this malfunction by freezing the valve's moving parts. - Christopherson 
Danfoss (2005) offers this repair procedure to eliminate hunting in the HVACR evaporator: [adpated and paraphrased]
Causes of Refrigerant Floodback in air conditioners, heat pumps, refrigeration equipment. Refrigerant flooding or liquid slugging refers to the return of liquid refrigerant back to an air conditioner or heat pump compressor motor on the low pressure or suction side of the system where only gas is expected.
Watch out: overcharging sufficient to cause liquid slugging can quickly destroy the HVACR compressor motor. The compressor motor's moving parts (piston & cylinder, scroll spirals, or rotary vane pump) expect to be compressing a gas and can't handle liquids. HVAC technicians refer to these conditions as liquid slugging.
Sketch courtesy of Carson Dunlop Associates.
Christopherson and the article cited below listed six causes of refrigerant floodback, and we have added some additional sources of floodback or refrigerant liquid slugging - a quick kill for compressor motors.
Thanks to reader TN Goose for finding the cited reference  explaining the effects of thermostatic expansion valves on A/C or heat pump compressor failures.
After installing a TEV the system must be tested for leaks using dry nitrogen;
The system is then evacuated (a vacuum is drawn on the refrigerant coils and piping to remove gases and contaminants),
The system is charged with proper refrigerant type and amount, the system is turned on and allowed to run until it is in stable operating condition, and then the TEV is checked according to the manufacturer's specifications (such as presence of a liquid seal at the valve and that the suction pressure is in the proper range.)
Technical note about TEVs and refrigerant pressure: TEVs do not maintain pressure on the low side of the refrigeration system. The TEV keeps refrigerant flowing into the cooling coil to keep the superheat level as specified across the cooling coil.
Pressure increase TEV bulb warming test: With the air conditioner or heat pump running and in stable state (it has been on for 5 minutes or longer), remove the sensing bulb from its position on a refrigerant tube or coil and gradually warm the bulb. If system pressure increases the TEV probably needs adjustment.
If the TEV is dispensing too much refrigerant you may see condensation on the suction line or right on the compressor motor case. Details are at TEV STICKING
Watch out: too much refrigerant metered through the TEV can send liquid refrigerant to the compressor motor, causing liquid slugging that can damage or destroy the compressor. If the TEV is stuck "open" it must be replaced. We discuss refrigerant flooding and six common causes of liquid refrigerant slugging the compressor at REFRIGERANT FLOODBACK, LIQUID SLUGGING.
If the TEV is "hunting" or short-cycling you will see pressure vacillating up and down about 10-15 psi on the suction side of the system. This may be caused by a defective TEV if it's original equipment, or by an over-sized TEV installed by the guy who was there before you.
Details are at TEV HUNTING DIAGNOSIS
If the TEV is whistling or shrieking refrigerant is low and there's a leak, as we discuss at TEV WHISTLING
When re-installing the TEV be sure that it is properly placed at the 4 O'Clock position on the piping as we discuss at TEV SENSOR BULB LOCATION.
On refrigeration systems that include a liquid refrigerant receiver and as long as there is adequate level of refrigerant in the system and the TEV is working, the refrigerant charge amount is not absolutely critical.
But on any refrigeration system, receiver or no, if the refrigerant charge gets too low (there is a refrigerant leak somewhere) then refrigerant gas cycles up through the TEV rather than liquid refrigerant. The result is that the TEV will begin to run "wide open" and it will often make a loud whistling sound. A whistling TEV means low refrigerant.
When a TEV is installed on refrigeration equipment we need a heavier (more powerful) compressor motor since the compressor has to start against a head pressure.
Why? In a system using a cap tube there is no device that closes down the complete refrigerant piping between the low and high sides of the system, just an orifice that restricts the flow rate.
But where a TEV is installed, the device can close completely, leaving a pressure difference between the high and low sides even when the system has stopped running. So a hard-starting compressor that is at or near end of life (or has a bad start/run capacitor or is at low voltage) may have a harder time starting against head pressure on a system that uses a thermostatic expansion valve (TEV or AEV) to regulate refrigerant flow.
On systems that use a long evaporator coil, and equalizer may be used to offset the large pressure drop that occurs across the long evaporator coil. This added feature assures that pressure in the valve and at the end of the condenser are equal so that just superheat (and not the pressure drop) operates the TEV.
Perhaps the main problem that occurs with TEVs is that the valve stops working due to loss of refrigerant in its sensing device. the Thermostatic Expansion Valve sensor element along with its small diameter tubing is a closed system that contains a refrigerant too - the same refrigerant as that used in the system for which the TEV is designed.
We use the same refrigerant inside the sensor because we want the sensor to respond to temperature changes at the same rate as the refrigerant in the actual system. That is, we want the pressure exerted by the sensor into the TEV top to be equal to the pressure exerted by the refrigerant inside the valve when both are at the same temperature. Else the system won't operate properly. But on occasion you may get a leak in the TEV sensor bulb or its tubing - the TEV will stop working.
To test a thermostatic expansion valve (TEV) for a leak, that is for loss of refrigerant from its sensor bulb and tubing, take the bulb off of the cooling coil and just hold it in your hand. The warmth of your hand should be more than enough to cause the sensor to open the TEV and begin refrigerant flowing through the cooling coil.
The TEV and also AEVs depend on a little oil or oil mist flowing along with refrigerant to lubricate the interior of the valve. But we suspect that a more common cause of TEV valve sticking is the freezing of a droplet of water at the needle valve, holding it stuck. The valve stops working properly.
We have had field reports of sticking thermostatic expansion valves on some newly-installed residential air conditioners such as the KeepRite® brand discussed at TEV STICKING REPAIR
Sticking TEVs are discussed separately at TEV STICKING REPAIR
To test a thermostatic expansion valve (TEV) for freeze-up, try warming up the valve to see if it starts working again, or let the valve warm up by leaving the system turned off for half an hour or so.
Debris in the refrigeration tubing can also clog and cause sticking of the needle seat in a TEV. If the TEV can be disassembled and cleaned this step may correct that problem but you'll also want to install a new dryer/filter on the system refrigerant piping if there are debris problems in the system.
Capillary Tube problems: see our separate article detailing the inspection, installation, and repair or replacement of CAPILLARY TUBES
Watch out: water or dirt in the refrigerant piping system can cause serious and costly problems in the TEV or cap tube that impact the entire refrigeration system.
Because differences in temperature in and around the cooling equipment can affect the TEV's operation, in commercial cooling installations the Thermostatic Expansion Valve has to be placed where its sensor, usually located in the top or "head" of the TEV, will be warmer than the thermal bulb which controls the device. Otherwise liquid refrigerant may be improperly discharged by the valve.
Thermostatic expansion valves are rated in tons of refrigeration capacity based on the size of the metering orifice in the device. The TEV (or AEV) must be matched to the capacity of the cooling system (actually the coil) and to the specific refrigerant used, since at any given temperature different refrigerants will exert different pressures in the system.
In fact we can sometimes spot the installation of the wrong TEV on an air conditioner or heat pump system by its behavior: if someone installs an R22 valve on an R12 refrigeration system, the higher pressure of the R22 will hold the TEV always open!
at THERMOSTATIC EXPANSION VALVES we discuss identification, label decoding, and selection of TEVs.
HVACR Thermostatic Expansion Valves - TEVs: this air conditioning repair article series explains the function and installation of all types of refrigerant metering devices, beginning with the most-common thermostatic expansion valve or TEV (or thermal expansion valve) that controls release of refrigerant into the evaporator coil of an air conditioning or heat pump system.
Continue reading at LOST COOLING CAPACITY - the air conditioning system seems to be "running" but not enough cool air, or no cool air at all is being delivered to the occupied space. or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see HVACR OPERATING DEFECTS the major air conditioning problem symptoms and how to get the air conditioning system working again.
Or see REFRIGERANT LEAK DETECTION where we describe the effects of dirt and moisture on TEVs and capillary tubes. A quick test for a moisture/ice jammed TEV is to add some heat to see if the device will begin working again.
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