The “5 Pillars” of Residential A/C Refrigerant Circuit Diagnosis

Taking all five of these calculations into account on every service call is critical. Even if further diagnostic tests must be done to pinpoint the problem, these five factors are the groundwork before more effective diagnosis can be done. I would also add static pressure as an important reading that should be checked regularly (Keep TESP between .3″wc and .7″ wc on most systems) but I would still place it slightly below these five.

Some of these are “rules of thumb” and obviously are for reference only. Refer to manufacturer recommendations when setting a charge.

Suction Pressure / Low Side
Suction pressure tells us several things. The first thing it tells us is what the boiling temperature of the refrigerant in the evaporator is. If the suction pressure is below 32° saturation temperature, the evaporator coil will eventually freeze.

As a general rule, the higher the temperature of the air passing over the evaporator, the higher your suction pressure will be. A good rule of thumb for suction pressure is 35°  saturation below indoor ambient +/- 5° (Return temperature measured at the evaporator coil). This temperature differential is often called an evaporator split or design temperature difference (DTD),

This means that when the temperature of the air passing over the evaporator is 80°, the low side saturation temperature should be 45° when the system is set for 400 CFM per ton output. Remember the temperature scale next to the pressure scale on the gauge represents saturation or if you don’t have the correct sale on (or in your gauge if you have a Digital manifold) you would need to use a PT chart.

This 35° rule only works at 400 CFM per ton, when a system is designed for 350 CFM per ton the DTD will be closer to 40° +/- 5° 

Make sure you know the actual CFM output of the system before you calculate DTD. It can vary significantly based on the setup of the particular blower.

Head Pressure / High Side
When used in conjunction with liquid line temperature, we can know what state the refrigerant in the liquid line and that the compressor is pumping / creating the required compression ratio. We can also know something about the state of the metering device as to whether or not refrigerant is “backing up” against the metering device. A good rule of thumb for head pressure is a 15° – 20° saturation above outdoor ambient +/- 3° for most modern systems. These saturation / ambient calculations are only indicators; they are not set in stone. Keep in mind, when I say ambient; I am talking about the air entering the evaporator for suction pressure and the condenser for head pressure.

Jim Bergmann brought to my attention that different equipment efficiencies will have different target Condensing Temperature Over Ambient (COTA) readings.

6 – 8 SEER Equpiment (Very Old) = 30° COTA

10 SEER Equipment = 25° COTA

13 / 14 SEER Equipment = 20° COTA

15 SEER+ Equpiment = 15° COTA

Superheat
Superheat is important for two reasons. It tells us whether or not we could be damaging the compressor and whether we are fully feeding the evaporator with boiling, flashing refrigerant. If the system has a 0° superheat, a mixture of liquid and vapor is entering the compressor. This is called liquid slugging and it can damage a compressor. A superheat that is higher than the manufacturer’s specification can both starve the evaporator, causing capacity loss, as well as cause the compressor to overheat. So how do we know what superheat we should have? First, we must find out what type of metering device the system is using. If it is using a piston or other fixed metering device, you must refer to the manufacturers superheat requirements or a superheat chart like the one below.

If it is a TXV type metering device, the TXV will generally try to maintain between an 7 – 17 degree superheat on the suction line exiting the evaporator coil (12° +/- 5°) 

TXV target superheat setting may vary slightly based on equipment type.

Subcooling
Subcool tells us whether or not the liquid line is full of liquid or not. A 0 degree subcool reading tells us that the refrigerant in the liquid line is part liquid and part vapor. An abnormally high subcool reading tells us that the refrigerant is moving through the condenser too slowly, causing it to give up a large amount of sensible heat past saturation temperature. A high subcool is often accompanied by high head pressure and, conversely, a low subcool by low head pressure. Subcool is always a very important calculation to take because it lets you know whether or not the metering device is receiving a full line of liquid. Typical ranges for subcooling are between 5 and 14 degrees, but always check the manufacturer’s information to confirm.

Evaporator Air Temperature Split (Delta T)
The evaporator air temperature split (Delta T) is a nice calculation because it gives you a good look at system performance and airflow. A typical air temperature split will be between 16 and 22 degrees difference from return to supply. Keep in mind, when you are doing a new system start up, high humidity will cause your air temperature split to be on the low side. Refer to the air temperature split and comfort considerations sheets for further information.

For systems that are set to 400 CFM per ton, you can use a target Delta T sheet like the one shown below

If the leaving temperature/delta T split is high it is an indication of low airflow. If it is low it is an indication of poor system performance / capacity.

Again, this only applies to 400 CFM ton. 350 CFM per ton or less are more common today than ever and in those cases the above chart won’t apply.

Diagnosing With The Five Pillars
The way this list must be utilized is by taking all five calculations and matching up the potential problems until you find the most likely ones. A very critical thing to remember is that a TXV system will maintain a constant superheat, and a fairly constant suction pressure. The exceptions to this rule are when the TXV fails, is not receiving a full line of liquid or does not have the required liquid pressure/pressure drop to operate. This situation would show 0° subcooling and in this case, will no longer be able to maintain the correct superheat. Before using this list, you must also know what type of metering device is being utilized, then adjust thinking accordingly. Also remember, in heat mode, the condenser is inside and the evaporator is outside.

Low Suction Pressure
• Low on charge
• Low air flow /load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.
• Metering device restricting flow too much – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Low ambient (Low evaporator load)
• Kinked suction line (after the kink)
• Internal evaporator restriction
High Suction Pressure
• Overcharge
• High return temperature (Evaporator Load)
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Too much airflow over the evaporator (Blower tapped or set too high)
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction
Low Head Pressure
• Low on charge
• Low ambient temperature / low load
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Wet condenser coil
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing (heat pump units)
• Kinked suction line
High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on condenser, bushes too close to condenser
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• High ambient temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Non-condensables in the system
Low Superheat
• Overcharge
• Low air flow / load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Low return air temperature
• Abnormally low humidity
• Internal evaporator restriction
High Superheat
• Low on charge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• High return air temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
Low Subcool
• Low on charge
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Dirty condenser coil – not a very low subcool, but low considering how high the head pressure will be
• High ambient surrounding the condenser
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, bad or broken crank
• Reversing valve bypassing
High Subcooling
• Overcharge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• Low ambient surrounding the condenser
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Wet condenser coil
• Dirty Condenser Coil on New High Efficiency Condensers (Increased Condensing Temp Can Actually Result in Higher Subcooling)
• Having an H.R.U. in the discharge line (old school I know)
• Internal evaporator restriction
• Kinked suction line
High Evaporator Air Temperature Split
• Low air flow – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Abnormally low humidity (WB Temp)
• Blower not running the correct speed or running backward
Low Evaporator Air Temperature Split
• Undercharge
• Severe Overcharge – because saturation temperature is increased with overcharge
• Metering device not functioning properly – restricting too much flow or allowing too much flow
• Too much airflow through the evaporator – blower not running correct speed
• Heat strips running with air
• Abnormally high humidity
• Liquid line restriction
• Kinked suction line
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing

 

This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities. Diagnosis is an art as well as a science.

— Bryan

Leave a Reply

Scroll to top

Daily Tech Tip

Subscribe to our daily tech tip to receive daily tips and advice!
Email address
Name