This checklist will assist in the quick diagnosis of faults on vapour compression systems.
Visual fault finding
Compressor not running
1 Main supply isolator open.
2 Fuse blown.
3 Overload open circuit.
4 Control circuit open (low pressure switch, high pressure switch or thermostat).
Fixture temperature too high
1 High evaporator load.
2 Defrost heater(s) energized.
3 Condenser blocked with dirt.
4 Restricted air flow over the condenser or restricted water flow.
5 Condenser fan inoperative.
6 Evaporator blocked with ice.
7 Evaporator fan inoperative.
8 Vapour bubbles in sight glass (possible shortage of refrigerant).
9 Frosting liquid line (blocked drier).
10 Broken or loose drive belts.
11 Expansion valve bulb loose on suction line or broken capillary.
System noisy
1 Evaporator/condenser fan blades touching fan guards or loose on the motor shaft.
2 Compressor oil sight glass empty (shortage of oil or oil entrainment).
3 Compressor head frosted (compressor pumping liquid refrigerant).
4 Loose motor pulley, compressor flywheel, compressor mountings.
Pressures
If the fault is not obvious after visual inspection:
1 Attach compound and pressure gauges.
2 Check that the compressor is pumping.
3 Observe operating pressures.
High discharge pressure
1 Overcharge of refrigerant.
2 Air in the system.
3 Dirty condenser, poor air supply.
4 Inadequate water flow.
5 High load imposed on the evaporator.
Low discharge pressure
1 Shortage of refrigerant.
2 Compressor inefficient.
High suction pressure
1 Compressor inefficient.
2 Overcharge of refrigerant (capillary systems only).
3 Expansion valve defective.
4 High evaporator load.
5 Defrost system operating when compressor is running.
Low suction pressure
1 Shortage of refrigerant.
2 Blocked or defective expansion valve.
3 Blockage in liquid line, drier solenoid valve or shut-off valve.
4 Blocked evaporator (excessive ice build-up).
5 Inoperative evaporator fan.
6 Defective water pump or blocked water filter on a chiller system.
Note: a temperature difference across any component in the liquid line is an indication of a partial restriction.
Advanced diagnosis
Expansion valve capacity too small
1 No subcooling of the refrigerant liquid.
2 The pressure drop across the expansion valve is less than that for which it was dimensioned.
3 Incorrect expansion valve bulb location (too cold or being cooled).
4 Large pressure drop across the evaporator.
5 Expansion valve blocked by ice or foreign objects.
6 Incorrect expansion valve external equalizer location.