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