Do Line Restrictions Cause High Head?

Common liquid line restrictions include clogged filter-driers or metering device restrictions. Many techs think that refrigerant will exit the compressor and back up against the source of the restriction, which would raise the head pressure. However, that’s not true.

Our diagnostic sheet shows that liquid line restrictions cause low suction pressure and normal to low head pressure, not high head pressure. That’s because the compressor requires refrigerant of a specific density to pump refrigerant into the high side of the system; when the suction pressure is low, the compressor can’t move as much pressurized refrigerant to the high side, so a liquid line restriction doesn’t have a huge impact on the head pressure when you have reduced amounts of high-pressure refrigerant in the liquid line in the first place.

Some thought experiments that can help you grapple with that idea include thinking about refrigerant in a tank and the pump down cycle (used to prevent refrigerant migration). Note: don’t pump down a microchannel condenser!

However, when you have a restriction, its location has a significant effect on the head pressure. The amount of charge also matters, as overcharged systems are more likely to have a significant impact on the head pressure (raising it and causing hydrostatic pressure buildup within the system).

The most common liquid line restrictions are clogged filter-driers, metering device inlet screens, a kinked liquid line, or a TXV with an empty bulb. These very rarely cause high head for more than a short period of time.

The most common causes of high head pressure include high load, overcharge, and poor heat rejection (such as from dirty or obstructed condensers).