Checking Superheat

CHECKING SUPERHEAT

What is superheat?

Superheat refers to the number of degrees a vapor is above its saturation temperature (boiling point) at a particular pressure.

How do I measure superheat?

Superheat is determined by taking the low side (suction) pressure gauge reading, converting that pressure to temperature using a PT chart or the gauge's temperature reading, and then subtracting that temperature from the actual measured temperature at the point where the pressure was taken.

(Measured Line Temperature) - (Saturation Temperature) = Superheat

Example:

• The low-side suction pressure on an R-22 air conditioner is 61 psig.
• The saturation temperature at that pressure (shown on the gauge) is 35 deg. 
• The actual measured suction line temperature is 65 deg.

(Measured Line Temperature) - (Saturation Temperature) = Superheat

In this case: 65 deg - 35 deg = 30 deg Superheat

Why is it important to know the superheat of a system?

Superheat gives an indication if the amount of refrigerant flowing into the evaporator is appropriate for the load. If the superheat is too high, then not enough refrigerant is being fed resulting in poor refrigeration and excess energy use. If the superheat is too low, then too much refrigerant is being fed possibly resulting in liquid getting back to the compressor and causing compressor damage.

What is a normal superheat reading?

The superheat of an air conditioner or heat pump will vary based on several factors, the most important being the type of metering device being used in the system.

Cooling Mode:

Expansion Valve - Systems that use an expansion valve as the indoor metering device will typically have a normal superheat range of 8 - 20 deg.

• For accurate charging with an expansion valve, you should also look at sub-cooling. (See "Checking Sub-Cooling")

Fixed Metering Device (Orifice or Cap-tube) - On units with a fixed metering device, the superheat will vary greatly with changes in outdoor and indoor air temperature. A normal superheat range for a fixed metering device system can be anywhere from 3 to 45 deg. (See "Superheat Chart")