Troubleshooting with Superheat, Basic Diagnostics

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.)

SSet the subcooling at value listed on rating plate if standard lineset

SSet 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.

SIf low (< 2°F), proceed to Low SuperHeat section.

SIf between 2 and 20°F/11_C valve is probably operating properly.

SIf 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.

SFor 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.

SFor longer (greater than 80 ft) and small diameter . linesets (less than service valve size), correct pressure (add to gauge pressure reading) per Table 6 and Table 7.

SIf Pseudo Superheat is greater than 15, proceed to High SuperHeat section.

SIf Pseudo Evaporator Superheat is between 2 and 15, TXV appears to be functioning properly.

SIf 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.

SLow Superheat with Normal or Low Suction Pressure If OK proceed to Step 2

2. Check superheat at Vapor Service Valve	and	Pseudo
Evaporator Superheat.

SIf both are less than 2°F, TXV likely not controlling properly, i.e. stuck open -> REPLACE VALVE

SIf superheat is higher than 15°F, proceed to Step 3

3.Perform TXV function check.

SWith 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

SIf greater than 15°F between circuits, distributor or coil has a restriction.

SIf 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)

SIf OK proceed to Step 6

6.Check airflow, sensing bulb tightness, orientation on vapor tube and ensure bulb is properly wrapped.

SIf 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?

SIf OK proceed to Step 8

8.Recheck Airflow and Subcooling.

SIf 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.)

SIf OK proceed to Step 10

10.Check for restriction in suction line (kink, restricted suction filter drier etc.))

SIf OK proceed to Step 11

11.Check power element cap tube is not broken

SIf OK proceed to Step 12

12.Check that equalizer tube is not kinked or plugged

SIf OK proceed to Step 13

13.Check that inlet screen (R-22 systems) is not restricted

SIf OK proceed to Step 14

14.Replace Valve