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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDE, AIR CONDITIONERS / HEAT PUMPS
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
HVACR Thermostatic Expansion Valves - TEVs: 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Guide to Expansion Valves for Refrigerant Metering on Air Conditioners & Heat Pumps: Definitions of AEVs, TEVs, Manual Valves, Cap Tubes
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.
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  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.
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.
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.)
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.
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.
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
High-Side Float Refrigerant Metering Devices
TEV Thermostatic Expansion Valve problems: see Thermostatic Expansion Valve (TEV) Installation, Inspection, Diagnosis & Testing Guide where we also address AEVs, float valves, manual and automatic expansion valves, etc.
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.
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