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TROUBLESHOOTING WITH SUPERHEAT
This troubleshooting routine was developed to diagnose cooling
problems using superheat in TXV systems. It is effective on heat
pumps in cooling mode as well as air conditioners. The system
must utilize a TXV as the expansion device in cooling mode.
Basic
Diagnostics
NOTE: When checking refrigerant charge and troubleshooting
operating systems, the indoor airflow has significant effect on the
determination. If you are at this stage, it is assumed you have
already checked the subcooling once and believe the charge is
correct. From this point, the airflow must be verified prior to
proceeding, hence step 1 below.
1. Check or verify proper indoor airflow
S Indoor air filter
S Duct dampers and supply registers are open
S Indoor coil for debris
2. Check subcooling at OD unit liquid service valve
S Outdoor airflow (debris on coil, etc.)
S Set the subcooling at value listed on rating plate if
standard lineset
S Set the subcooling at the maximum of 10°Forvalue
listed on rating plate if a long line application
3. Check superheat at OD unit vapor service valve.
S If low (< 2°F), proceed to Low SuperHeat section.
S If between 2 and 20°F/11_C valve is probably operating
properly.
S If greater than 20°F/11_C, perform Pseudo Evaporator
SuperHeat Instructions check as follows:
⎯ Check refrigerant pressure at vapor servicevalve and
refrigerant temperatureat outlet of evaporator.
⎯ Use suction line geometry (diameter and equivalent
length), unit capacity and Tables 7 and 8 to determine
suction pressure drop.
S For standard lineset diameters (vapor service
valve diameters and larger) and lengths (less than
80 ft), generally no pressure adjustment (per
Table 6 or Table 7) is required.
S For longer (greater than 80 ft) and small diameter .
linesets (less than service valve size), correct
pressure (add to gaugepressure reading) per
Table 6 and Table 7.
S If Pseudo Superheat is greater than 15, proceed to High
SuperHeat section.
S If Pseudo Evaporator Superheat is between 2 and 15,
TXV appears to be functioning properly.
S If operation erratic (hunting), proceed to Hunting
Superheat
°F Superheat in repetition section.
NOTE: Hunting is when the valve superheat swings more than
10_.
Low Superheat with Normal or Low Suction
Pressure
NOTE: Normal or low suction pressure is considered for
R--22: < 80 psig, Puron: < 135 psig
1. Re--check airflow and then check sensing bulb tightness,
orientation on vapor tube and is properly wrapped.
S Low Superheat with Normal or Low Suction Pressure
If OK proceed to Step 2
2. Check superheat at Vapor Service Valve and Pseudo
Evaporator Superheat.
S If both are less than 2°F, TXV likely not controlling
properly, i.e. stuck open --> REPLACE VALVE
S If superheat is higher than 15°F, proceed to Step 3
3. Perform TXV function check.
S With system running, place sensing bulb in ice bath for
∼1 minute --> superheat should increase.
⎯ If no response, Replace Valve
⎯ If OK proceed to Step 4
4. Check for even temperature distribution at outlet of each
circuit of evaporator
S If greater than 15°F between circuits, distributor or coil
has a restriction.
S If OK proceed to Step 5
Low Superheat with High Suction
Pressure
NOTE: High suction pressure is considered for R--22: > ∼80 psig,
Puron: > ∼135 psig. An application issue or other system
component failure typically causes this condition.
5. R--22 Systems: Check that proper valve used (not an
R--410A valve)
S If OK proceed to Step 6
6. Check airflow, sensing bulb tightness, orientation on vapor
tube and ensure bulb is properly wrapped.
S If OK proceed to Step 7
7. Check that compressor is pumping properly
NOTE: Loose Rules of Thumb: Is discharge saturated ∼20°F
higher than ambient temperature? Is discharge superheat between
15 and 50?
S If OK proceed to Step 8
8. Recheck Airflow and Subcooling.
S If OK proceed to Replace Valve
High Superheat with Normal or Low
Pressure
NOTE: Normal or low suction pressure is considered:
R--22 < 80 psig, Puron < 135 psig.
9. Check for restriction in liquid line (kinked line, filter drier
restricted, etc.)
S If OK proceed to Step 10
10. Check for restriction in suction line (kink, restricted suction
filter drier etc.))
S If OK proceed to Step 11
11. Check power element cap tube is not broken
S If OK proceed to Step 12
12. Check that equalizer tube is not kinked or plugged
S If OK proceed to Step 13
13. Check that inlet screen (R--22 systems) is not restricted
S If OK proceed to Step 14
14. Replace Valve