Service Manual
31
S If OK proceed to Step 2
2. Check superheat at vapor service valve
and Pseudo Evaporator Superheat.
S If both are less than 2F, TXV likely not controlling
properly, i.e. stuck open - > REPLACE VALVE
S If superheat is higher than 15F, 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 15F 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, R- 410A: > 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
20F 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, R- 410A < 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
High Superheat with Normal or High Suction
Pressure
NOTE: Normal to High suction pressure is considered
for R- 22: > 65 psig, R- 410A: > 110 psig. An application
issue or other system component failure typically causes
this condition.
15. Check airflow, sensing bulb tightness, orientation on
vapor tube and ensure bulb is properly wrapped.
S If OK proceed to Step 16
16. R- 410A Systems: Make sure proper valve is used
(Not R- 22)
S If OK proceed to Step 17
17. Check for even temperature distribution at outlet of
each circuit of evaporator
S If OK proceed to Step 18
18. Check for high evaporator load: Return Air Leaks,
high indoor wet bulb and/or dry bulb temp, undersized
system, etc.
S If OK proceed to Step 19
19. Check that compressor is pumping properly
S Loose Rule of Thumb: Is discharge saturated
20F higher than ambient temperature? Is
discharge superheat between 15_F and 50_F?
Hunting
Superheat
NOTE: Hunting is when the valve superheat swings more
than 10F Superheat in repetition. This is typically an
application issue.
20. Check for obvious kinked or pinched distributor
(capillary) tubes causing imbalance to the circuiting.
S If OK proceed to Step 21
21. Check that proper size valve is used per Product
Literature.
S If OK proceed to Step 22
22. Check airflow, sensing bulb tightness, orientation on
vapor tube and ensure bulb is properly wrapped.
S If OK proceed to Step 23
23. Check for even temperature distribution (5
difference) at outlet of each circuit of evaporator and
for even air distribution over all evaporator slabs
S If OK proceed to Step 24.
24. Move sensing bulb further down suction line.
S If problem not corrected, replace valve
Pseudo Evaporator Superheat
Instructions
The Pseudo Evaporator Superheat calculates the superheat
at the outlet of the evaporator with known and available
information. Because there generally is not a pressure port
on the vapor line at the indoor coil, this procedure allows the
service personnel to evaluate the evaporator superheat with
the vapor pressure port at the outdoor unit.
The method requires the following information:
S Suction line temperature at the outlet of the
evaporator (F).
S Suction line pressure at the outdoor unit (psig).
S Outdoor nominal unit size (btuh).
S Suction line equivalent line length (ft).
S Suction line pressure drop from tables (Table 5 and
Table 6).
S Pressure- Temperature relationship for refrigerant
used (P- T Chart).
If system uses a vapor line the same size as vapor service
valve fitting or larger AND the line set equivalent length is 80
feet or less, the pressure drop in vapor line of line set can
be ignored.
1. Take suction line temperature at outlet of evaporator
at indoor unit.
2. Take suction service valve pressure at OD unit.
3. Determine lineset vapor line equivalent length and
tube diameter.