Different types of refrigerant produce different head pressures. In air-cooled condensers, a normal head pressure should correspond to a temperature of about 30°F higher than the temperature of the air passing over the condenser. In the case of a water-cooled condenser, the head pressure must correspond to a temperature of 20°F above the temperature of the return water (see fig. 87), considering the type of refrigerant being used.
Keeping these rules of thumb in mind, you will have a general idea what head pressure to expect. When the head pressure goes higher than normal, the cooling efficiency of the unit will drop as most of the heat does not dissipate from the refrigerant by the time it leaves the condenser.
Higher-than-normal head pressures are caused by the following:
1. Restriction in the sealed system. (As the compressor discharge line keeps pumping against the restriction, it creates high pressures in the condenser and the liquid line.
2. In the case of water-cooled condensers, poor water circulation or presence of air bubbles in the water.
3. Too much refrigerant in the system. Some of the refrigerant must be released by using the manifold gauge.
a. Connect the high-pressure gauge hose to the access valve on the discharge (high) side of the compressor while the valves on the manifold gauge are closed.
b. Turn on the unit.
c. Turn the high-pressure gauge valve on and off in short intervals and check the gauge.
d. By repeating this procedure a few times, enough refrigerant is released to bring the reading within normal range.
4. Ambient temperature above 85°F. It is normal to have a higher head pressure in the summer with higher ambient air temperatures.
5. Dirty condenser. When a condenser gets covered with grease, lint, and dirt, the necessary heat transfer to change the refrigerant from vapor to its liquid state does not take place. Consequently, head pressure goes higher than normal, and the unit no longer cools.
In places where pets such as dogs and cats are kept indoors, the condenser requires cleaning more often as hair shed by the pets is drawn into the fins and coil and restricts airflow.
6. Inefficient condenser fan. A fan that does not run (or runs too slowly) due to worn bearings or an internal short reduces (or stops) air circulation over the condenser fins and prevents the refrigerant from losing its latent heat and changing to its liquid state. This causes the head pressure to rise, and if not corrected, the compressor will burn out.