Before the 13 SEER mini-mum efficiency standard,thermostatic expansionvalves (TXVs) were rarelyseen in residential air-con-ditioning systems. They

were used on some high SEER sys-tems and on heat pumps (usually onthe outdoor coil), but these repre-sented a relatively small portion ofthe overall market.

However, that has changed dra-matically now that virtually all new13 SEER equipment will be manu-factured with TXVs. As a result, thereis a growing need for many servicetechnicians to reacquaint themselveswith TXVs in terms of operation,troubleshooting and replacement.

Refrigerant flowMetering the flow of refrigerant to theevaporator is the sole function of aTXV. It must meter this flow at pre-cisely the same rate the refrigerant isbeing vaporized by the heat load. The

} TXVdoes this by keeping the coil sup-plied with enough refrigerant to main-tain the right superheat of the suctiongas leaving the evaporator coil.

There are three forces that governthe TXV operation. Refer to Figure 1,which shows the basic force balancediagram of a TXV. In the chart:

P1: Power element and remotebulb pressure.

P2: Evaporator pressure.P3: Superheat spring equivalent

pressure.For the valve to be stable, the

forces need to be balanced, or P1 =P2 + P3.

needed to initiate movement of thevalve pin to just start to move. This isdefined as .002-inch of stroke.

� Gradient. The amount of super-heat required to move the pin fromthe static set point to the rated strokeis called gradient. Figure 2 depictshow a TXV regulates flow inresponse to changing superheat.

Starting from the origin, nochange in valve stroke occurs as thesuperheat slowly increases. It is notuntil the static setpoint is reachedthat the valve begins to open. Fromthis point forward, further increasein superheat results in a proportional

Figure 1In a TXV, the forcesneed to be balancedfor it to be stable.

As the evaporator outlet temperaturebecomes warmer, the pressure (P1)increases, causing the diaphragm to flexin a downward direction. This forcesthe valve pin in an open position, result-ing in increased refrigerant flow.

The underside of the diaphragmalways senses the evaporator pressure(P2). As this pressure increases, itforces the diaphragm in an upward, orclosing position, decreasing refrigerantflow.

The spring pressure (P3) also acts onthe underside of the diaphragm. Thisspring is adjusted to provide staticsuperheat for the valve. The staticsuperheat is the amount of superheat

� Everything YouNEED to KNOW

About TXVs

P1 = P2 + P3

With the higher

SEER air conditioners,

technicians need to

reacquaint themselves

with thermostatic

expansion valves

B Y A L M A I E R

duced in this manner are termedinternally equalized valves.

In most a/c systems, the evapora-tors are quite large and, therefore,have significant pressure drop acrossthem. For the TXV to sense the evap-orator outlet pressure, a separateline is needed from the suction line(near the TXV bulb location) to theexternal equalizer connection on thevalve body.

If a distributor is used to supplyrefrigerant to various evaporator cir-cuits, an externally equalized valvemust be used. Distributors typicallyhave between 15 psi and 30 psi pres-sure drop across them. Hence, usethem only with externally equalizedvalves.

� Balanced-ported valves. Witha conventional TXV, the pressure dif-ferential across the valve results in aforce that tends to “open” the valve.As operating conditions vary, thispressure differential changes andresults in a variation of the originalsuperheat. Engineers have developedthe balance-ported TXV to compen-sate for this (Figure 4).

In this design, the inlet pressure isapplied across the valve pin as wellas an undercut on the push-rod.Since these forces are in oppositedirections, they cancel or balanceone another resulting in no changein superheat, regardless of operatingconditions.

Balanced-ported valves are idealfor use in systems that operate overlarge changes in operating condi-tions. An example of this is a com-mercial a/c system that must operateboth winter and summer, resultingin system operation under widelyvarying head pressures.

Troubleshooting TXVsThere are really only three failuremodes that a TXV can experience:

1. Starving. This is defined asinsufficient refrigerant flow causinghigh superheat at the evaporator out-let. Symptoms include high super-heat at the compressor inlet, highdischarge temperature and possiblycompressor overheating (the protec-tor trips).

Thermostatic expansion valves

Figure 2A TXV regulates flow in response to charging superheat.

increase in valve stroke until themaximum stroke position is attained.

Gradient is an important aspect toTXV performance in a system. Toolow a gradient and the valve will beunstable and tend to “hunt” (moreon this later). If the gradient is toohigh, more superheat will be neededfor the valve to open, resulting inhigh operating superheat and poorevaporator efficiency.

� Measuring superheat. Sincegood superheat control is the criteri-on of TXV performance, accuratemeasurement of the superheat isvital. This involves four steps, asshown in Figure 3. They are:

1. Measure the suction pressure atthe evaporator outlet with an accu-rate gauge. If a gauge connection isnot available, a tee can be installedin the equalizer line.

2. Referring to a P/T chart for therefrigerant used in the system, findthe saturation temperature that cor-responds to the pressure observed instep 1.

3. Measure the temperature of thesuction line at the remote sensingbulb. This can be done with a strap-on type thermometer or an electron-ic device.

4. Subtract the saturation temper-ature determined in step 2 from the

suction gas temperature measured instep 3. The difference is the operat-ing superheat.

� Internally and externally equal-ized TXVs. In a system with a rela-tively small evaporator, the pressuredrop across that evaporator is sosmall you can assume it is zero.Therefore, the TXV outlet pressure

and evaporator outlet pressure areequal.

By drilling a small bleed passagebetween the underside of thediaphragm and the outlet of thevalve, you can sense the pressureinternally and eliminate the need foran external connection. Valves pro-

In most a/c systems,the evaporatorsare quite large and,therefore, havesignificant pressuredrop across them

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for that particular model. As a rule ofthumb, operating superheats between8º F to 12º F is considered normal.

Some “tips” to help in troubleshoot-ing TXV performance follow:

• Check the bulb to assure it isproperly connected to the suction line.If you can move the bulb by hand,then it is not secured adequately.

• Some manufacturers insulatethe bulb to protect it from the effectsof an airstream. If this was done bythe oem, make sure the insulation isstill intact.

• Check the equalizer line forrestrictions (kinks) or signs of frost.A frosted equalizer line indicatesinternal leakage and will requirereplacement of the valve. You willneed to repair or replace a kinkedequalizer as well for the valve tooperate properly.

A TXV is designed to meter theflow of liquid refrigerant. If the refrig-erant at the valve inlet contains flashgas the capacity of the valve will bereduced. Make certain the system isproperly charged and that some sub-cooling exists at the inlet of the valvebefore condemning the TXV.

With the use of R-410A and POEoils, there is a greater risk of dirt andcontaminants being circulated with-in the system. Some manufacturersuse inlet strainers or screens to pre-vent debris from clogging the valve.

If such a condition is found, cleanand replace the strainer. It wouldalso be wise to install a large fil-ter/drier at the inlet of the TXV toprevent a recall.

Replacing a TXVIf you determine that you need toreplace the valve after checking thesuperheat, here are some tips toassure proper replacement:

1. Whenever possible, use thevalve recommended by the manufac-turer of the equipment. If this is notpossible, be sure the replacementhas the same:

• Rated capacity.• Refrigerant designation.• Charge type.• Internal/external equalizer style.

Figure 3Four steps are required to accurately measure superheat.

Figure 5Wrap the valve with wet rags toavoid overheating.

Figure 4The balanced port TXV compen-sates for varying operatingconditions.

2. Flooding. This occurs when therefrigerant flow to the evaporator isso high that all of it can’t evaporatewithin the coil. The result is liquidrefrigerant getting back to thecompressor. Symptoms include lowevaporator superheat, diluted oil andnoisy compressors. If not corrected,this can lead to permanent compres-sor damage.

3. Hunting. When the superheat inan operating system is constantlychanging from little or no superheatto very high superheat, it is calledhunting. You can easily recognizethis by noting extreme cyclic changesin the evaporator or suction pressure.

Hunting is caused by many fac-tors, but usually occurs when thevalve is oversized for the load. Beforecondemning a valve for this symp-tom, make certain the evaporator isclear of frost and has proper airflowsince these conditions will result invery low loads potentially resultingin a “good” valve hunt.

Checking TXV operationIf a TXV is suspected of workingproperly, checking the superheat isthe only way to know for sure. Dothis with accurate instrumentationto get meaningful results.

Check the a/c equipment manufac-turer’s installation and service manu-al to verify the acceptable superheat

Reprinted from August 2006 - RSES Journal

• Internal check valve (if suppliedon original valve).

• Inlet/outlet connection size andtype.

2. To maintain system cleanliness,replace the filter-drier wheneveropening the sealed system. While thisalways has been a recommendedservice procedure, it is of even greaterimportance with HFC/POE systemsdue to the hygroscopic nature of thePOE oils and their greater solvency.

3. Do not overheat the valve dur-ing the brazing process. Overheatingcan cause deterioration of the inter-nal seals, which could lead to frostedequalizer lines. To avoid this:

• Wrap the valve with wet rags asshown in Figure 5.

• Keep the torch flame pointedaway from the valve body.

• Never allow the torch to comein contact with the bulb.

TXVs in heat pumpsIn heat pump applications, the liquidrefrigerant must flow through oraround the TXV when operating inthe reverse direction. Historically,this was accomplished by installing acheck valve around the valve.

However, in recent years TXVmanufacturers have modified theirproducts with internal check valves.Many oems have adopted these sincethey eliminate joints and the poten-tial for leaks.

Figure 6 is a cross-sectional draw-ing of one such valve. In the forwardflow direction, inlet pressure pushes

Systems using TXVs are quicklybecoming the norm in this post 13-SEER world. Understanding theirfunction and operation will enableyou to properly service systemsusing these devices. Follow thebasic troubleshooting and replace-ment guidelines discussed here, toensure optimal system performanceand prevent permanent compressordamage.�

Al Maier has over 30 years of refriger-ation system and component designexperience. He is currently vice presi-dent of application engineering forEmerson Climate Technologies, FlowControls Division.

the ball against the seat, forcing itclosed. All the flow must then passthrough the main valve port and thevalve operates as a normal valve.

When the flow is reversed the inletpressure pushes the ball up, allowingflow through the check port. In thismode, the flow bypasses themain portand liquid will flow through the valvewith only a small pressure drop.

When replacing a valve in a heatpump verify if the original valve hadan internal check. If it did, be surethe replacement has one too. If noneis available, use a standard valve.You must install a check valve tobypass the TXV when reverse flow isencountered.

Thermostatic expansion valves

�

Figure 6Here is a diagram of an internal check valve.

Emerson Climate TechnologiesFlow Controls Division

11911 Adie Rd.,St. Louis, MO 63043

314.569.4500; fax: 314.569.4593www.emersonclimate.com/flowcontrols

Form No. 2006FC-174R1

18 RSES Journal ~ January 2008

With the advent of the 13 SEER regulations inJanuary 2007, nearly every manufacturer ofsplit residential air-conditioning equipmentchanged their designs from using flow raters tousing thermal expansion valves (TXVs) for con-

trolling the rate at which refrigerant is fed to the evaporator.The TXV enables a more energy-efficient system because itmaintains the evaporator closer to the optimum superheat atall operating conditions, and it enables the system to pull-down to optimum conditions faster after an off cycle.

TXV functionand operationAlthough TXVs offer many oper-ating advantages, they are viewedas a somewhatmysterious deviceby those not familiar with themand as a result are often replacedneedlessly. The TXV has only onefunction. Itmeters the flowof liquid refrigerant into the evap-orator in exact proportion to the evaporation rate of therefrigerant in the evaporator. By responding to the tempera-ture of the refrigerant gas leaving the evaporator and thepressure of the evaporator, the TXV can control the gas leav-ing the evaporator at a predetermined superheat.

Superheat is the temperature of a vapor above its satura-tion temperature. It is found bymeasuring the actual temper-ature at the outlet of the evaporator and subtracting the tem-perature corresponding to the evaporating pressure from it.

In order to correctly troubleshoot a TXV, first under-stand how the valve works. There are three forces shownthat govern a TXV’s operation. These are P1, the pressurecreated by the remote bulb and power assembly; P2, theevaporator or equalizer pressure; and P3, the equivalentspring pressure of the superheat spring.

The pressure in the remote bulb assembly (P1) corre-sponds to the refrigerant temperature leaving the evaporator.As this pressure increases, it tends to move the valve in an“opening” direction. Opposed to this force on the diaphragm’sunderside and acting in a closing direction is the force exert-ed by the evaporator pressure and the pressure exerted by thesuperheat spring. The valve will assume a stable positionwhen these forces are in equilibrium (P1=P2+P3).

As the temperature of the refrigerant leaving the evapora-tor increases above its saturation temperature—correspon-ding to the evaporator pressure—the pressure in the remotebulb in-creases, causing the valve pin to move in an “open-ing” direction. Conversely, as the refrigerant gas temperatureleaving the evaporator decreases, the pressure in the remote

bulb decreases and the valve pinthen moves in a “closing” direction.

A TXV’s factory superheating set-ting is made with the valve pin juststarting to move away from the seat.A further increase in superheat isneeded for the valve to open to itsrated position. This is an importantconcept tounderstand since itmeansthat a valvewith a factory settingof8° will not maintain an 8° super-heat at a rated load. Additional

superheat is needed to get the valveto “stroke” to its rated capacity.

Troubleshooting TXVsThere are three principle modes of failure of a TXV. Thevalve may feed too much refrigerant (flooding); it may notfeed enough refrigerant (starving); or can alternately feedtoo much and then too little (hunting). Each of these willbe discussed in greater detail:

FloodingFlooding occurs when the refrigerant amount being fed tothe evaporator is more than can be evaporated, resultingin liquid refrigerant going back to the compressor.Symptoms of flooding include frosting of the compressor

Thermal ExpansionValves

TROUBLESHOOTING

Here is the correct way to diagnoseand troubleshoot a system with a

TXV as its flow control device.

BY AL MAIER➦

➦

➡

P1

P2

P3

valve is inbalance whenP1=P2+P3

January 2008 ~ RSES Journal 19

To measure superheat:1. Determine suction pressure at the evaporator outlet;2. Use a pressure/temperature chart for the appropriate refrigerant,

and determine the saturation temperature corresponding to the pressuremeasured in step 1;

3. Measure the temperature of the suction line at the remote bulblocation; and

4. Subtract the saturation temperature determined in step 2 from thetemperature measured in step 3. The difference is the superheat.

Adjusting superheat for externally adjustable TXVs only:1. Remove the seal cap from the bottom of the valve, exposing the

adjusting stem;2. Rotate the stem clockwise to increase the superheat—decreases

flow of refrigerant; and3. Rotate the stem counterclockwise to decrease the superheat—

increases flow of refrigerant.

Measuring/adjusting superheat stepsshell, noisy compressors, low super-heat at the evaporator, and normal- orhigher-than-normal suction pressures.Causes of flooding include:

• Undersized or inefficient com-pressor. If the compressor capacity islow, the suction pressure will be higherthan normal and the superheat will below. If this condition is suspected, con-sult with the compressormanufacturer;

• Low superheat setting. Onexternally adjustable valves, turn theadjusting stem clockwise to increasethe superheat;

• Moisture. Any moisture in thesystem can freeze in the TXV, prevent-ing the valve from functioning asintended. If this is suspected, install ahigh-quality liquid-line filter drier. Italso is advisable to install a liquid-linemoisture indicator to enable the tech-nician to monitor the moisture levelwithin the operating system;

• Dirt or debris. Any foreignmaterial that gets past the inlet strain-er can become lodged between the pinand the port of the TXV, preventing itfrom properly closing. Here again, ahigh-quality filter drier should beinstalled in the system to prevent thecirculation of dirt and debris andcause system malfunctions;

• TXV seat leak. If the pin andport do not seat properly, liquid refrig-erant will flow through the valve whenthere should be none. Inspecting thevalve may reveal dirt and debris, inwhich case the TXV can be cleanedand put back into service. Inspectionmay also reveal damage to the pin orport due to wire drawing or erosion ofthe pin. In such a case, the valveshould be replaced;

• Oversized valve. Make certainthe correct valve specified by theequipment manufacturer is installedin the system. A valve too large for thecapacity of the system will tend tooverfeed; and

• Incorrect bulb position. Ideally,the power element bulb should beattached to a horizontal run of suctionline—immediately after the evapora-tor outlet. It should be in close proxim-ity to the equalizer connection, butupstream of it. The bulb needs to befirmly attached to the suction line tomaintain good thermal contact.Additionally, the bulb must not beinfluenced by external sources of heat.

• Undercharged system. If a sys-tem is low on charge then the super-heat will be high, the suction pressurelow, but the system still will not providesufficient cooling. A sight glassinstalled in the liquid line immediatelybefore the TXV is the best way to deter-mine this. “Bubbles” in the sight glassare a clear indication that the system islow on charge. On residential A/C sys-tems, the manufacturers include achart indicating what the correct pres-sures are in different operating condi-tions. These tables can be used todetermine if the system charge is cor-rect. Additionally, measure the sub-cooling at the inlet of the TXV. If thereis no subcooling, that too is an indica-tion that the system charge is low;

• Flash gas at inlet to TXV. Anyrestrictions in the liquid line leadingto the TXV inlet will cause a pressuredrop and result in flash-gas forma-tion. Because the gas density is muchless than that of liquid refrigerant,having flash gas feeding the TXV willreduce the valve’s capacity and causehigh superheat and starving. Flash gascan be found in the following ways:

1. Look for frost or moisture onthe liquid line. As the flash gas isformed, it causes a refrigeration effectmaking the area around it cold. Sincethe liquid line is normally warm to thetouch, a cold spot is usually a sure signthat a restriction exists at that point;

2. If a sight glass is installed,

StarvingStarving occurs when the refrigerantamount feeding the evaporator is fullyevaporated long before the outlet.Symptoms of starving include insuffi-cient refrigeration effect—warmerthan desired load temperature—highsuperheat at the evaporator outlet andlow-suction pressure. Causes of starv-ing include:

• Moisture. Any moisture in thesystem can freeze in the TXV, pre-venting the valve from functioning asintended. If this is suspected, install ahigh-quality liquid-line filter drier. Itis also advisable to install a liquid-linemoisture indicator so the techniciancan monitor the moisture level withinthe operating system;

• Dirt or debris. Any foreignmaterial that gets past the inlet strain-er can become lodged between thepin and the TXV’s port, preventing itfrom correctly closing. Here again, ahigh-quality filter drier should beinstalled in the system to prevent thecirculation of dirt and debris thatcause system malfunctions;

• Insufficient Delta P acrossvalve. If the pressure drop across thevalve is too low the valve’s capacity isreduced. This sometimes happens dur-ing low ambient operation when thehead pressure is allowed to “float” withthe ambient. To correct for this, raisethe head pressure or replace the valvewith one that has a larger capacity;

look for bubbles. A steady stream of bubbles indicateseither a low charge or vapor in the liquid line; and

3. Check for subcooling. If subcooled liquid is pres-ent at the outlet of the condenser, but not at the inlet ofthe evaporator, then determine where you have lost thesubcooling. In some cases, the liquid line may be under-sized, or liquid-to-suction heat exchange may have beenlost, resulting in a loss of subcooling.

• Plugged equalizer line. If the equalizer line is plugged,the pressure underneath the diaphragm can be higher thanthe actual evaporator pressure, resulting in a valve that tendsto be in a more “closed” position. To correct this, replace orrepair the equalizer line;

• Valve too small. Check that the valve is correctlysized for the system. Replace with the proper sized valve;

• Superheat adjusted too high. On externallyadjustable valves, turn the adjusting stem counterclock-wise to decrease the superheat;

• Power assembly failure or partial loss of charge. Ifthe power assembly has lost all or part of its charge, it willnot generate sufficient pressure to cause the valve to “open.”A tech can verify this in the field by holding the bulb in hishand while monitoring the system operating conditions.Hand warmth should cause the pressure to increase, result-ing in increased refrigerant flow and an increase in suctionpressure. If no change is noted, the valve may have lost itscharge. This technique is only valid if the system is correct-ly charged and the valve has high-quality liquid refrigerantat its inlet. If it is found that the valve has lost its charge, thepower assembly must be replaced. In some cases, the powerassembly is an integral part of the TXV, in which case theentire valve must be replaced.

• Cross ambient effect. Some TXVs have specialcharges designed to limit the maximum operating pres-sure of the system. These are designated with a “W” inthe charge code followed by a number—for exampleHW100. With charges of this type, the bulb must alwaysbe colder than the power element or the valve’s body. Ifnot, the charge can migrate from the bulb to the powerelement with a resultant loss of control. If this is suspect-ed, replace the valve with a non-maximum evaporatorpressure type charge—in this example, an R-22 refriger-ant medium temperature (HC) or R-22 heat pump(HCA); and

• Strainer clogged. Dirt or debris in the refrigerantleading to the valve can result in the strainer becomingclogged. Remove and clean the strainers and add a high-capacity liquid-line filter drier to prevent recurrence.

HuntingHunting occurs when the TXV alternately opens and clos-es, causing large fluctuations in superheat. This can becaused by the difference in the time response of the systemversus the TXV. Different type charges have been designedwith various time constants to minimize this effect, butother factors—such as sudden changes in load or operationat low-load conditions—also can impact this. Causes ofhunting include:

• Valve oversized for application. A valve that is toolarge for the application will tend to hunt. If this is suspect-ed, replace the valve with one that is correctly sized for theapplication;

• Bulb location. Verify that the bulb is not located down-streamof a P trap on the suction line. Relocate the bulb if thisis found. It may also be helpful to insulate the bulb to makecertain it is not affected by an air stream;

• Refrigerant distribution. On systems with distribu-tors, it is not uncommon to have different circuits withlargely differing loads. This can result in some circuits occa-sionally overfeeding sufficiently to allow liquid to reach thebulb. This, of course, will force the TXV closed until super-heat is again achieved; at which time it will open. To correctthis requires elimination of the distribution problem;

• Superheat adjustment. TXVs have their superheatset at the factory to operate correctly on most systems. Attimes, however, the factory setting may need to be adjust-ed. In most cases, increasing the superheat will minimizehunting. Turning the adjustment screw clockwise willincrease the superheat; and

• Moisture. Any moisture in the system can freeze inthe TXV, preventing the valve from functioning as intend-ed. As the moisture freezes and then thaws, the superheatwill vary erratically. The use of a high-quality liquid-linefilter drier is recommended to prevent this.�

Al Maier is vice president of application engineering at EmersonClimate Technologies Flow Controls Div. For more informationcall 314-569-4680 or e-mail al.maier@emersonclimate.com.

Reprinted from January 2008 - RSES Journal

Emerson Climate TechnologiesFlow Controls Division

11911 Adie Rd.,St. Louis, MO 63043

314.569.4500; fax: 314.569.4593www.emersonclimate.com/flowcontrols

Form No. 2008FC-21