Tennessee Technology Center Center at Pulaski

Superheat & Sub-cooling A Technician’s Guide to HVACR Diagnostics

Introduction The ability to properly and accurately measure superheat and sub-cooling and interpret the results is very likely the single most important skill that you, as an HVACR service technician, can acquire As you study this program, keep one very important thing in mind ……….

No Test Is Valid If The Air Flow Is Not Correct!!!

Superheat A vapor is said to be “superheated” when its temperature is higher than its saturation temperature at the same pressure Sub-cooling occurs to a liquid when its temperature is below saturation for the same pressure

Checking Superheat Allow the system to operate for 15 –20 minutes to stabilize Attach an accurate thermometer to the suction line near the sensing bulb on TEV systems or near the suction service valve on fixed restrictor systems

Record the suction line temperature Connect a manifold gauge set to the suction service valve and record the suction pressure Use a temperature/pressure chart to obtain the saturation temperature for the suction gas at the pressure recorded Subtract the saturated temperature from the actual suction line temperature – the result is the operating superheat

Example 1 Suction line temperature =55 F Suction pressure = 68.5 psig 68.5 psig = 40 F

55 F – 40 F = 15 F superheat

Let’s examine some common system problems and see how they affect superheat and sub-cooling

High Superheat Possible causes and remedies for HIGH SUPERHEAT

Excessive or HIGH superheat is an indication of insufficient refrigerant in the indoor coil for the heat load present This could be from insufficient refrigerant entering the coil or from an excessive heat load crossing the coil

LOW REFRIGERANT CHARGE If there is insufficient refrigerant in the indoor coil, all of the refrigerant will evaporate in the first few passes of the coil. The excess sensible heat picked up by the refrigerant vapor causes a higher than normal suction gas temperature

Discharge pressure will be lower than normal Suction pressure will be lower than normal Superheat will be higher than normal Sub-cooling will be lower than normal Current draw will be lower than normal

Liquid Line Restriction A restriction in the liquid line will not allow sufficient refrigerant to reach the evaporator coil This causes many symptoms similar to a low refrigerant charge Often there is a noticeable temperature change at the point of the restriction

Suction pressure will be lower than normal Discharge pressure will be normal to lower than normal Superheat will be high Sub-cooling will be high Current draw will be low

Evaporator Air Flow Too High Excessive air flow reduces the latent capacity of the coil thus increasing the sensible heat load. This additional sensible heat results in higher than normal suction gas temperatures and pressures

Discharge pressure will be high Suction pressure will be high Superheat will be high Sub-cooling will be lower than normal Current draw will be higher than normal

Excessive Load Conditions Excessive indoor coil loads will cause a higher than normal heat content in the air crossing the coil This excess heat will cause the refrigerant liquid to boil away sooner allowing the vapor to pick up additional superheat Most commonly caused by internal gains such as an increase in occupancy load

Discharge pressure will be higher than normal Suction pressure will be higher than normal Superheat will be high Sub-cooling will be lower than normal Current draw will be high

Metering Device Not Feeding Properly A restriction in a capillary tube, orifice or TEV will reduce the amount of liquid refrigerant entering the evaporator. Symptoms are the same as for a liquid line restriction

Discharge pressure will be lower than normal Suction pressure will be lower than normal Superheat will be higher than normal Sub-cooling will be higher than normal Current draw will be lower than normal

Low Superheat Possible causes and remedies for Low SUPERHEAT

LOW SUPERHEAT Low superheat indicates an excess of liquid refrigerant in the evaporator coil Liquid refrigerant is very likely entering the compressor This results in reduced compressor life and possible imminent compressor failure

Refrigerant Overcharge An overcharge forces excessive refrigerant into the evaporator due to increased pressure differential There is not enough heat present to completely vaporize the excess refrigerant Compressor failure is likely

Discharge pressure will be higher than normal Suction pressure will be higher than normal Superheat will be lower than normal Sub-cooling will be higher than normal Current draw will be higher than normal

TEV Overfeeding Many symptoms similar to an overcharge Sensing bulb not insulated or not secured properly Improperly sized valve Wrong valve for the application

Discharge pressure will be higher than normal Suction pressure will be higher than normal Superheat will be lower than normal Sub-cooling will be lower than normal Current draw will be higher than normal

Low Evaporator Heat Load Most common cause of low superheat Low air volume (dirty coils, filters, restricted duct, etc.) Reduces the heat available to vaporize the refrigerant Liquid refrigerant may enter the compressor

Discharge pressure will be lower than normal Suction pressure will be lower than normal Superheat will be lower than normal Sub-cooling will be higher than normal Current draw will be lower than normal

Improper Metering Device The wrong orifice A capillary tube the wrong size (or that has been shortened) An improperly sized TEV Symptoms identical to device overfeeding

Discharge pressure will be higher than normal Suction pressure will be higher than normal Superheat will be lower than normal Sub-cooling will be lower than normal Current draw will be higher than normal

Equipment Oversized When a system is greatly oversized there is not enough heat to vaporize the refrigerant present in the evaporator Symptoms are similar to a low charge, except that a low charge will have a high superheat and run excessively Oversized unit will likely short cycle and have a low superheat

Discharge pressure will be lower than normal Suction pressure will be lower than normal Superheat will be lower than normal Sub-cooling will be higher than normal Current draw will be lower than normal

Condenser Air Flow Low condenser air flow or recycled condenser air will increase condensing temperature thus increasing condenser pressure Increased pressure drop across the metering device results in a flooded evaporator

Caused by: Dirty coil Bad motor or blade Shrubs, bushes or other obstructions Low overhangs Other equipment too close

Discharge pressure will be higher than normal Suction pressure will be higher than normal Superheat will be lower than normal Sub-cooling will be lower than normal Current draw will be higher than normal

SUB-COOLING A liquid is sub-cooled when its temperature is below saturation at the same pressure Measuring sub-cooling is a good method of confirming your diagnosis based on other tests TEV systems MUST be charged by sub- cooling in the absence of a known charge quantity

Measuring Sub-cooling Allow the system to operate for 15 –20 minutes to stabilize Attach an accurate thermometer to the liquid line near the inlet of the metering device whenever possible The condenser outlet may be used, but will be in error by the amount of liquid line pressure/temperature losses

Record the liquid line temperature Using a gauge manifold, obtain the liquid line pressure Discharge pressure may be used, but allowances must be made for condenser pressure drop

Using a temperature/pressure chart, convert the pressure reading to saturation temperature Subtract the line temperature from the saturation temperature The difference is operating sub-cooling In the absence of manufacturer’s data, a sub- cooling reading of 10 degrees or more is usually acceptable

Sub-cooling Losses Long liquid lines Liquid lines exposed to high ambient temperatures (un-insulated) Low condenser air flow Inadequate condenser size Long vertical lifts

Long Liquid Lines Long liquid lines cause increased pressure drop due to friction losses Use the shortest lines possible Relocate equipment if necessary

Liquid lines exposed to high ambient High ambient increases liquid line temperature Exposed liquid lines should be insulated Heat exchangers or auxiliary sub-coolers may be considered.

Low condenser air flow Low condenser air flow reduces the condenser’s ability to reject heat Causes increased condensing temperature Clean condenser coils and clear any obstructions

Inadequate condenser size A condenser coil that is too small uses all of the available space for condensing leaving no room for sub-cooling May be caused by the presence of NCG’s (non-condensable gasses)

Long vertical lifts The weight of the refrigerant in long vertical lifts causes a pressure drop HCFC-22 looses about ½ PSIG for every foot of vertical rise Reduce the lift, use a heat exchanger or artificial liquid line amplification

Practice Exercises

Exercise # 1 2 ½ ton package gas unit 5 years old No previous service Not cooling enough odb = 95F Idb = 80F Iwb = 68F SP = 55 PSIG ST = 36F DP = 210 PSIG LT = 96F Current = low

LOW LOAD Superheat is low Sub-cooling is low to normal Both pressures are low Current draw is low

Exercise # 2 2 ½ ton package heatpump 15 years old No previous service Not cooling enough odb = 95F Idb = 80F Iwb = 68F SP = 55 PSIG ST = 76F DP = 180 PSIG LT = 96F Current = low

LOW CHARGE Superheat is high Sub-cooling is low Both pressures are low Current draw is low