Guide to Thermostatic Expansion Valves & Other Refrigerant Metering Devices
InspectAPedia® -
What is a thermostatic expansion valve or TEV, what is the function of the TEV on air conditioners and heat pumps
Thermostatic expansion valve installation instructions, inspection and testing
How a TEV regulates or meters refrigerant in an air conditioner or heat pump
Guide to all refrigerant metering devices for air conditioners, heat pumps, refrigerators, freezers: TEVs, Automatic Expansion Valves AEVs, Float Valves, Manual Valves, Capillary Tubes
Questions & Answers about refrigeration controls: TEVs, AEVs, & others
This air conditioning repair article 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. 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.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.
All types of refrigerant metering devices are discussed in this article. We also provide a separate article detailing the inspection, installation, and repair or replacement of CAPILLARY TUBES . Also see FROST BUILD-UP on AIR CONDITIONER COILS or start diagnosing air conditioning problems at LOST COOLING CAPACITY. Photograph of the thermostatic expansion valve at page top is courtesy of Alan Carson, Carson Dunlop Associates in Toronto. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Guide to Expansion Valves for Refrigerant Metering on Air Conditioners & Heat Pumps: Definitions of AEVs, TEVs, Manual Valves, Cap Tubes
This article describes how TEVs work, where and how a thermostatic expansion valve is installed on an air conditioner or heat pump, and how the TEV may be adjusted. We also list possible errors in TEV installation such as improper positioning of the TEV sensor bulb.
Most of our discussion below focuses on TEVs (Thermostatic Expansion Valves) as these refrigerant metering devices are most widely used on residential air conditioners and heat pump systems.
Definition of TEV - Thermostatic expansion valve: An air conditioner thermal expansion valve or "TEV" or just "expansion valve" (tan colored device in the page top photo) is a device located at the cooling coil and connected between the incoming liquid refrigerant line and the refrigerant
inlet to the cooling coil in the air handler. (Schematic of a thermostatic expansion valve courtesy of Carson Dunlop.)
A temperature sensor mounted at the end of the cooling coil controls the rate at which the TEV releases refrigerant into the coil - hence the term "thermostatic" expansion valve.
It's an "expansion" valve because by controlling the release of refrigerant into the coil the expansion valve releases high pressure refrigerant into the low pressure environment of the cooling coil, causing the refrigerant to expand and evaporate - cooling the coil. We explain this concept in more detail at Refrigeration Basics.
Other types of refrigerant expansion valves control refrigerant by various means other than sensing coil temperature. For completeness in this article we also define and explain other refrigerant metering devices including AEVs (Automatic Expansion Valves), "non-adustable" expansion valves (actually they can be set), and manually adjusted expansion valves, CAPILLARY TUBES and Low Side or High Side refrigerant float valves.
Refrigerant expansion valves or metering components are used on both air conditioning systems and on heat pumps as well as on dehumidifiers; heat pumps are essentially the same in components as air conditioners except for additional control features to permit refrigerant to circulate in either direction in the system, moving heat outside (air conditioning) or moving heat inside (heat pumps).
On residential refrigerators and freezers, room and portable air conditioners, and dehumidifiers a simpler (and not adjustable) CAPILLARY TUBES may be used for refrigerant metering.
Refrigeration Basics: Why we need a refrigerant metering device like a capillary tube or thermostatic expansion valve
Before we explain how refrigerant metering devices work in detail it's useful to get a most basic view: the refrigerant metering device provides a restriction in the flow of liquid refrigerant from the compressor/condenser into the evaporator coil.
It is this restriction that, by limiting the flow rate of refrigerant into the evaporator, allows the compressor (a pump) to raise the refrigerant pressure on the high side (condensing it into a liquid) and drop the refrigerant pressure on the low side (evaporating the liquid back into a gas in the cooling coil).
The state change (vaporization from liquid to gas in the cooling coil) is what cools the coil and thus cools indoor air air blown across the cooling coil. But that state change is not enough. To get our refrigerant gas back to a liquid state (to continue the cycle) we need to be able to raise the temperature at which the refrigerant gas will change back to a liquid. It is the high pressure provided by the compressor that accomplishes this step.
It is the flow restriction provided by a cap tube or by an expansion valve such as a TEV in the refrigerant piping system that allows the compressor pump to raise the system pressure and thus increase the temperature at which the coolant changes state. Raising the coolant temperature above outdoor ambient temperature causes heat to flow from the coolant into outdoor air. So in sum the TEV or cap tube allows the compressor to reduce pressure on the LOW side of the metering device and raise pressure on the HIGH side of the metering device.
Expansion Valves & Thermostatic Expansion Valves for Refrigerant Metering Actually Work
All cooling and refrigeration systems and heat pumps using refrigerant gases make use of some type of expansion valve or refrigerant metering device, of varying complexity.
Even a simple
window air conditioner or a refrigerator make use of an expansion valve [shown at left] or a small-diameter CAPILLARY TUBE or "cap tube" which meters
refrigerant into the cooling coil. How do these refrigerant metering devices actually work?
Step 1: Expansion valve meters liquid refrigerant into the cooling coil: Inside of the thermostatic expansion valve (TEV) or other metering device the refrigerant passing through is mostly liquid. The refrigerant metering device is the "doorway" between the refrigeration system high side (compressor output) and low side (cooling coil interior) as it releases liquid refrigerant into the cooling coil at a controlled rate.
Step 2: Liquid refrigerant boils to a gas in the cooling coil: Inside the cooling coil, the liquid refrigerant being metered in through the TEV (or equivalent metering device) converts increasingly to a gas (it "boils" and changes state from liquid to gas) as it enters and then flows down through the evaporator coil, until the refrigerant is totally in a low-pressure, low temperature gaseous state by the time it reaches the end of the evaporator coil. The energy absorbed by the change in refrigerant from a liquid to a gas inside the cooling coil is what absorbs sensible heat and chills the indoor air handler cooling coil so that the coil cools air blown across it.
Step 3: Low pressure refrigerant gas is drawn back into the compressor/condenser unit: This low pressure, low temperature refrigerant gas present at the end of the cooling coil or "evaporator coil" is then drawn back into the compressor via the suction line connecting the evaporator coil outlet to the compressor inlet port.
Step 4: Low pressure refrigerant gas is compressed to high temperature/high pressure gas and then condensed back to liquid refrigerant out in the compressor/condenser unit (typically located outdoors). It's the high temperature of the gas entering the outdoor condensing coil that allows heat to ultimately be transferred into outdoor air (or into water if we're using a water-based air refrigeration system).
Our photo (above left shows another tan TEV. The very thin coil of copper tubing connects the TEV to its sensor bulb that appears to be taped to the refrigerant suction line just outside of this air conditioning air handler. The TEV shown in this photo is used on a heat pump system so it includes extra tubing so that it can permit the refrigerant to reverse its flow
of direction when changing from cooling mode (move indoor heat to outdoors) to heating mode (collect and move outdoor heat to indoors).
The larger diameter copper tubing feeds liquid refrigerant into the TEV from the compressor/condenser unit (the left side of this valve) and the copper tubing on the right side of the valve loops up and into the air handler where inside the unit you'd see it entering the top of the cooling coil. In this photo you cannot see the adjustment point on the bottom of this valve. We say more about the proper position and location of this valve below.
The TEV valve maintains the pressure difference (high and low) at the entry point to the cooling coil, thus assuring that as the high-pressure refrigerant enters
the low pressure space of the cooling coil, it can "evaporate" from a refrigerant liquid to a gaseous form, thus producing the temperature drop
that cools the cooling coil itself.
The thermostatic expansion valve is a refrigerant metering control device, and it is not a control or switch which can be directly operated by the user when using an
air conditioning system, but it is a critical control needed for metering refrigerant into the cooling coil.
What's the difference between a TEV (Thermostatic Expansion Valve) and an AEV (Automatic Expansion Valve) ?
A Thermostatic Expansion Valve or TEV (circled in red) is similar to an Automatic Expansion Valve (AEV) but unlike the single-bellows internal design of the AEV, a TEV has a second bellows on the top of the valve along with a tube (purple) that attaches the TEV valve top bellows to a temperature sensing bulb mounted at the end of the cooling coil.
The sensing bulb of a TEV (purple rectangle above the "In" of "InspectAPedia) measures the superheat across the cooling coil (light blue) regardless of the actual refrigerant pressure inside the coil and uses that information to maintain the frost line at the end of the evaporator coil.
In contrast, the AEV regulates refrigerant just by pressure - it does not include the sensing bulb control.
Our TEV operation sketch (above left) illustrates the placement of the sensing bulb and its small diameter tube attached to the TEV top (purple) that controls the TEV release of refrigerant into the cooling or evaporator coil. You'll also see the temperature numbers written along the evaporator coil (blue coil at left side of sketch) showing the likely increase in temperature as we get close to the end of the evaporator coil.
Here is Exactly how the Thermostatic Expansion Valve Works to Control Refrigerant Release into the Cooling Coil (Evaporator Coil):
When the pressure sensed by the TEV sensor bulb (P2) is less than or equal to the pressure in the TEV bottom (P1) then this condition allows the spring inside the TEV to close the valve.
When the pressure sensed by the TEV sensor bulb (P2) [and transmitted to the TEV valve top by the sensor tubing] is greater than the pressure in the TEV bottom (P1) then this condition allows the valve to open [the cooling coil temperature is up] thus allowing more refrigerant to enter the cooling coil, thus boiling more liquid refrigerant, thus dropping the cooling coil temperature back down.
Our sketch also show the two bellows on the typical TEV control (circled in red). The bottom bellows are adjusted by the TEV adjustment screw, and the top bellows are adjusted by the temperature sensing bulb and copper tubing (purple) that connect that bulb to the TEV top cap. The high pressure line is shown in red. A dryer is shown in green. [Click any image to see an enlarged, detailed version.]
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.
Thermostatic Expansion Valve Location: 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.
TEV Installation: 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. 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.
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.
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.
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 Details about TEVs shows that the sensor bulb was outside of the air handler and was taped under insulation covering the suction line.
On a commercial 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.
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 sensor bulb clamp on locations: As the Singer Corporation illustration at left shows, the sensing bulb that controls the thermostatic valve can be clamped on a vertical or on a horizontal section of the refrigerant suction line, but never on a trap or elbow in the suction line where oil and liquid refrigerant can interfere with proper bulb and temperature sensing operation.
And the sensor should not be clamped to the under-side of the suction line but rather on its top (small diameter lines) or at the 5 or 7 o-clock positions (larger suction lines).
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. [2]
If multiple TEV's are installed (on a multi-evaporator cooling or heat pump system), each thermostatic expansion valve has to be installed at a refrigerant suction line that will indicate conditions in each individual evaporator, never on a common suction line fed by multiple evaporators.
Access to TEV adjustment: 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
Hunting Theromstatic Expansion Valves can damage the air conditioning or heat pump compressor: If the TEV superheat set too low this can cause rapid-cycling TEVs TXVs and liqid refrigerant flooding the suction line and compressor.
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 [2]
Christopherson lists and the article cited below[2] elaborates six causes of refrigerant floodback - a quick kill for compressor motors, including these:
Oversized metering device (discussed below)
Thermostatic expansion valve (TXV) superheat set too low (discussed above)
TXV thermal bulb loose or not thermally tight (discussed above)
Head pressure too high and/or low pressure too low. (perhaps caused by other conditions suchas a blocked condensing or evaporator coil or sticking TEV (TXV))
Thanks to reader TN Goose for finding the cited reference [2] explaining the effects of thermostatic expansion valves on A/C or heat pump compressor failures.
TEV head temperature: 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.
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.
TEV size selection: 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!
Testing Thermostatic Expansion Valves after installation: 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.)
Whistling Thermostatic Expansion Valves (TEVs): 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.
Relationship between TEVs and Compressor/Condenser motors: 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.
TEVs and long evaporator coils: 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.
TEV Sensor element leak problems: 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.
Sticking TEV valves: 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.
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.
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.
Example Refrigeration Equipment Field Diagnosis & Repair: Thermostatic Expansion Valve Inspection, Testing, Experiments with TEV and Pressure Control Switch
The following are from my [DF] notes from a refrigeration service call [1982] VERY early in my [DF] refrigeration training:.
Case outline & initial observations: Commercial cooler running too warm - (WACOOP), hermetically sealed compressor, Kramer W14, unknown refrigerant (thought from a label maybe it should be R22 but someone may have charged with R12), cooler running too warm, need to diagnose cooling coil and fan operation and control settings on an old, used cooler just brought in. Very common on old equipment like this: no labels, no data tags, not much information at all.
Fanco refrigerant pressure switch: found set at 35# and 10# differential, connected improperly to the low side service port, cannot fully shut off the service port as a result - maybe leaking?
Compressor pump: running continuously.
Ambient temperature about 90 DegF; R12 in my service canister is at 100 psi static.
Low side refrigerant pressure: measured 40 psi. If there is R12 in the system I'd expect about 45 deg. temp at proper charge, and if R22 in the system I'd expect about 20 degF temp at proper charge and operation. But there were NO frost lines on the equipment, so I know that there is little or no liquid refrigerant and the system is operating at about 45 degF so must be filled with R12.
Actions and Tests:
Set the TEV 8 quarter turns more open - out and down, to see what happens.
With the fan off the Low Side goes to 25 psi and 25 degF.
The frost line moved at least to the se3nsor bulb and the pump (compressor) shut off. The low side pressure went up to 36 psi and then the pump restarted. This is telling me what the pressure control switch is doing.
Further actions and observations:
Opened the TEV 1/4 turn more to see the effect.
System shut off at 23# and came back on at 37#
Turned the blower cooling fan back on since the Dx was iced coil. Low side went up to 50, then dropped to 39# and stabilized.
Kept a series of observations from 10:55 PM to 12:42 AM (service call made during hours the business was closed to avoid disruption)
5 then 9 more turns opening the TEV, low side up to 75# & can see gas in the sight glass in the refrigerant line - this is "wide open" TEV setting
9 turns closing down the TEV to almost shut - so there are about 10 turns from wide open to fully shut on this TEV. At 9 turns towards shut from wide open, the low side pressure falls FAST!
9 turns back open at the TEV confirms gas bubbles again in the sight glass and 70# pressure.
Closed the TEV completely (about 9+ turns to the right or "up" or "in"). Suction lines closed, no gas in the sight glass, rapid low side pressure drop to 26#, compressor turns off at 20#.
I am convinced the pressure control switch is working properly, that is it does what it's pressure settings say it should be doing.
Set the TEV to 12 1/4-turns (in other words 3 full turns) open from fully shut. Suction line very cold, low side goes to 30#. 2 more turns open, low side goes up to 34#.
Finally decide to run the system with the TEV open 3 quarter-turns (about 3/4 of one turn) from fully shut. The system stabilizes with the cooler (a refrigerator) in the mid to upper 40's, no more oscillating, no coil frosting.
If I set the cut-in pressure way down the cooling coil ices over and the compressor will run continuously without cooling anything. See frost moving down the low side line. So that's not the right "fix".
I could set the pressure switch to 12.5 psi, left the TEV alone, and got the cooler down to our target of 32 degF.
The HI event sets the defrost cycle by setting the cut-in. The low event sets the cutout and therefore the lowest temp we will reach. A bigger low event means a lower target temperature, but the risk is that if you set it too low the compressor will run continuously and ice up the coil without ever running a defrost cycle.
The TEV seemed to be sometimes sticking. The low side pressure would hang at 28# or rise only very slowly as if the TEV was not opening when I expected it to. Have to be sure the blower fan is also running when checking this performance.
Final resolution of the cooler operation troubles:
Ultimately I replaced the TEV with a Singer TXV223FA 1/2 with a TE value of 9 (heat delta), installed a filter dryer (#082 PN 2003), set the pressure control to 35# on and 20# off. adjusted the system to get NO frost on the suction line near the compressor. Final pressure switch settings were 33# and 18# hi and low. We were able to get the cooler, charged with R12, to hold a stable 34 degF at cutoff, rising to 39 degF at which point the compressor would cut back on.
Guide to Other Types of Refrigerant Metering Devices besides TEVs: AEVs, Cap Tubes, Float Switches, Manually Adjustable Refrigerant Metering Valves
Capillary Tubes - a simpler method for metering refrigerant
Separately at CAPILLARY TUBES . we explain how capillary tubes are used to meter refrigerant in air conditioners, dehumidifiers, refrigerators, & freezers. We include a description of the operating properties of cap tubes, we contrast their use and function with thermostatic expansion valves or similar devices, and we include cap tube problem diagnostic tips for air conditioning service and repair purposes.
What's the Difference Between a Refrigerant Capillary Tube or "Cap Tube" and a Refrigerant Expansion Valve or TEV / AEV?
As we detail at CAPILLARY TUBES , compared with a capillary tube, the TEV or AEV adds a level of control - the TEV / AEV or even float valves and manually and adjustable refrigerant metering valves can open or shut in response to an attached bulb or pressure sensor (AEVs) which actually monitors temperatures in the refrigerant tubing. Capillary tubes are found on residential refrigerators, dehumidifiers, and many window air conditioners. TEVs are found on larger air conditioners and central air conditioning systems where more control is needed.
In our TEV sketch (left) the small diameter tube at the top of the thermostatic expansion valve is connected to a temperature sensing bulb (not shown) that is located at the outlet end of the cooling or evaporating coil in the air handler. The tubing at the left and right permit liquid refrigerant to flow into the valve from the compressor/condenser and, metered by the TEV, onwards into the evaporator coil. The large nut on the bottom of this TEV covers an adjustment screw that can change the latent heat settings and thus the behavior of the valve once it is installed. (Normally you should leave the valve at its factory setting.)
"Non-Adjustable" & Manually Adjustable Expansion Valves: How they are Set
Singer and other manufacturers point out that TEVs are adjusted at the factory before shipment. The factory setting of a thermostatic expansion valve is printed on a label found on the head of the valve and for most installations the factory superheat settings should be left alone.
How & When to set "Non-Adjustable" Thermostatic Expansion Valves
Non-adjustable TEVs (such as Singer TEV models 226, 326, 426) can actually be adjusted before the valve is installed, by turning an adjustment screw through the valve outlet opening. Once these valves have been installed, however, adjusting the valve would require removing it from the system, thus also requiring an evacuation and recharge of system refrigerant - not something to do casually.
How to Set Manually Adjustable Thermostatic Expansion Valves
Manually adjustable TEVs permit the device to be set to continuously maintain the proper refrigerant level entering the evaporator coil or cooling coil. Automatic expansion valves are discussed below.
Adjustable TEVs include an adjusting stem that can be turned with a screwdriver. Some valves may require that a covering cap be first removed to provide access to the adjusting screw or stem. On Singer adjustable TEVs (other controls will be similar),
Turn the adjusting screw only one turn at time to prevent over adjustment. We make a tiny scratch on the valve bottom at the start end of the screw slot so that we can keep careful track of how far a screw has been turned.
To increase the superheat setting, turn the TEV valve stem clockwise. When you turn the valve stem "in" or "clockwise" you are increasing pressure on the spring - the bellows will need more pressure to force the needle off of the seat, so the frost line will recede (less refrigerant is passing through) - and vice versa.
To decrease the superheat setting, turn the TEV valve stem counter-clockwise. Normally the TEV is set for 7 to 12 degrees of superheat across the cooling coil (evaporator coil).
Allow the air conditioning or heat pump system to stabilize for half an hour after each adjustment turn before trying to adjust the valve further.
Automatic Expansion Valves for Refrigerant Metering - AEVs
Automatic expansion valves used to meter refrigerant into a cooling system are similar to Thermostatic Expansion Valves (discussed above) but AEVs do not use a temperature sensing device mounted on the cooling coil. Rather the automatic expansion valve is mechanically set by adjusting a screw that presses a spring that presses a diaphragm that regulates refrigerant release by constant pressure.
Automatic Expansion Valve (AEVs) (center of sketch at left)
In normal operation the adjusting screw (see our sketch) is set to maintain a given pressure on the low side of the refrigeration system. During system operation the Automatic Expansion Valve pulsates - it opens and closes constantly.
Diagnosing AEV trouble: AEVs can close up (stop working) if you add extra load to the refrigeration system - because of the extra heat load - the system will appear to run as if it were short-charged. So automatic valves are used on systems where the load is more consistent.
[Click any image to see an enlarged, detailed version]
Automatic Expansion Valves (AEVs) are repairable. These devices are most often used on constant-load refrigeration systems such as restaurant coolers.
Avoiding Trouble with Adjustable Expansion Valves
Watch out: on AEVs that have an accessible adjusting screw, it's important to keep the protective rubber cap on the device. The cap provides insulation to prevent the water produced during TEV defrost cycle from seeping into the bellows of the device where it can freeze and rupture the bellows.
Also review our advice on Adjusting the Thermostatic Expansion Valve - if you don't allow sufficient time for the system to stabilize after each turn of the TEV adjusting screw you can easily over adjust and lose control of the system.
Commercial Refrigeration Equipment Devices for Metering Refrigerant: Low Side & High Side Floats
On many commercial air conditioning and larger refrigeration systems you may see a large canister on top of the equipment (sketch at left) that acts as a refrigerant reservoir or receiver. Vapor exits at the top of the reservoir and liquid refrigerant enters lower in that canister, typically from a side port.
A float with a needle valve allows liquid refrigerant to enter the canister to replace refrigerant that has boiled off and exited out of the upper valve as a gas. Float type refrigerant metering systems are used on refrigeration systems in which the evaporator is the "flooded type" - always kept full of refrigerant down to the end of the coil.
Low-Side Float Refrigerant Metering Devices
Low-side floats are used mostly on So2 refrigerant systems.
And if the refrigeration system is indeed using SO2 experts recommend that you do not attempt to repair this device.
[Click any image to see an enlarged, detailed version]
High-Side Float Refrigerant Metering Devices
A high side float refrigerant metering device is illustrated in our sketch. As our drawing notes explain, the float is located on the HIGH side of the system.
A pressure-regulating valve prevents frosting at the receiver outlet (PRV in in the sketch at top center).
Diagnosing Problems with Thermostatic Expansion Valves for Refrigerant Metering
Capillary Tube problems: see our separate article detailing the inspection, installation, and repair or replacement of CAPILLARY TUBES
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.
Watch out: as we discuss at REFRIGERANT LEAK DETECTION, water or dirt in the system can cause serious and costly problems.
See THERMOSTATS for other details of the operation of primary air conditioning thermostats and switches.
At A/C - HEAT PUMP CONTROLS & SWITCHES we explain the many electrical switches and controls that control an air conditioner or heat pump system. You'll need to check these if your air conditioner won't start.
At OPERATING DEFECTS we take you through the major air conditioning problem symptoms and how to get the air conditioning system working again.
At a companion article, LOST COOLING CAPACITY, our focus is on the case in which 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.
Questions & Answers regarding this article
Questions & Answers about refrigeration controls: TEVs, AEVs, Capillary Tubes, Float Valves, & Other Refrigerant Controls
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
[1] "Thermostatic Expansion Valve Installation Instructions", Singer Controls Division, Schiller Park IL, 1979 parts brochure
[2] Christopherson, Norm, "Preventing Premature Compressor Failures, An Ounce of Prevention for Years of Service", web search & reader TN Goose referral, 08/12/2011, original source: http://www.hvacfun.com/a-once-prevent-years-service-comps.htm
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Mobile View
