Carrier Residential Air Conditioners and Heat Pumps Application Manualline page 34

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°F or value
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 service valve and
refrigerant temperature at 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
5 or 6) is required.
S For longer (greater than 80 ft) and small diameter .
linesets (less than service valve size), correct
pressure (add to gauge pressure reading) per
Tables 5 and 6.
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
°F Superheat in repetition
Superheat
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
2. Check superheat at Vapor Service Valve
Evaporator Superheat.
S
S
3. Perform TXV function check.
S
4. Check for even temperature distribution at outlet of each
circuit of evaporator
S
S
Low Superheat with High Suction Pressure
NOTE: High suction pressure is considered for R- -22: > ∼80 psig,
Puron: > ∼135 psig.
component failure typically causes this condition.
5. R- -22 Systems: Check that proper valve used (not an
R- -410A valve)
S
6. Check airflow, sensing bulb tightness, orientation on vapor
tube and ensure bulb is properly wrapped.
S
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
8. Recheck Airflow and Subcooling.
S
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
section.
10. Check for restriction in suction line (kink, restricted suction
filter drier etc.))
S
11. Check power element cap tube is not broken
S
12. Check that equalizer tube is not kinked or plugged
S
13. Check that inlet screen (R- -22 systems) is not restricted
S
14. Replace Valve
34
Low Superheat with Normal or Low Suction Pressure
If OK proceed to Step 2
If both are less than 2°F, TXV likely not controlling
properly, i.e. stuck open - -> REPLACE VALVE
If superheat is higher than 15°F, proceed to Step 3
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
If greater than 15°F between circuits, distributor or coil
has a restriction.
If OK proceed to Step 5
An application issue or other system
If OK proceed to Step 6
If OK proceed to Step 7
If OK proceed to Step 8
If OK proceed to Replace Valve
If OK proceed to Step 10
If OK proceed to Step 11
If OK proceed to Step 12
If OK proceed to Step 13
If OK proceed to Step 14
and Pseudo