Superheat and Subcooling Defined

  • 2 of the most important pieces of information in troubleshooting an A/C system are Superheat and Subcooling.
  • Let’s examine exactly what both are, how they apply to A/C and  and how they can be used as diagnostic tools.
  • Definitions:

Superheat: Heat added to a vapor above its boiling point

Subcooling: Heat removed from a liquid below its condensing point

  • Let’s use water as an example to show what is going on…
  • We all know at sea level water boils at 212°F right?

At 212°F there is both liquid and vapor
At 213°F there is only steam
At 211°F there is only water…Please see below

  • So…Any heat added to the steam makes it hotter- that is SUPERHEAT..Heat added to a vapor…Simple right?

If we add 5° to the steam it now has 5° of superheat and will be 218°F

Please see below…

  • Now let’s see it from a different view and what happens when we remove heat…

  • So…Any heat removed from the water will make it colder…that is Subcooling…Heat removed from a liquid…Simple right?

If we remove 5° from the water it has 5° of Subcooling  and  will be 206°

Checking Superheat

  • Let’s pretend you are in front of a condensing unit, you have your gauges on it and a clamp on temperature probe (yes you NEED it and yes go buy one) is on the bare copper suction line. You record the following off the low side gauge.

Suction pressure  140psi at 48°F, that 48° is found on the gauge in line with the pressure

Suction line temperature 58°F

This system has 10° of superheat (58-48)…GREAT !!!...What does that mean to you ??

  • The boiling point of the refrigerant is at 48°F, as the refrigerant flows through the evaporator it is boiled off from return air being blown across it from the furnace blower.
  • Once the liquid turns to vapor that vapor will still absorb heat as what ?..SUPERHEAT !!
  • Superheat serves 2 purposes:

    1. It guarantees no liquid is getting back to compressor which would cause failure.
    2. It “Tells” us what is going on in the evaporator. Low superheat shows the boiling point is occurring later in the evaporator, High superheat shows the boiling point is occurring earlier in the evaporator.

If Superheat is low:

System overcharged  ,Fan speed too low, Plugged air filter, Frozen evaporator, TXV bulb not insulated, Restrictive ductwork(high static pressure), indoor ambient too low.

If Superheat is high:

System undercharged, Fan speed too high, Filter drier or TXV restricted, indoor ambient to high, Short between Y and W bringing on heat with A/C.

 

SUPERHEAT GENERALLY IS 8°F-12°F, CHECK WITH MANUFACTURER TO VERIFY

Checking Subcooling

  • Let’s pretend you are in front of the same condensing unit with your gauges still attached. Remove the temperature probe from the suction line and place it on the liquid line.

You record the following off the high side gauge.

Discharge pressure 350 psi at 106°F, That 106° is found on the gauge in line with the pressure.

Liquid line temperature is 92°F .This system has 14° of sub cooling

(106-92 )…GREAT !!!…what does that mean to you ??

  • The condensing point of the refrigerant is at 106°. As the vapor flows through the condenser it gives up heat and turns to liquid.
  • Once the vapor turns to liquid the liquid will still give up heat as what ??..SUBCOOLING !!
  • Subcooling is imperative as a solid  column of liquid is needed at the TXV for correct operation and “Tells” us what is going on in the condenser. High subcooling shows that the condensing point is occurring earlier in the condenser while low subcooling shows that the condensing point is later in the condenser

If Subcooling is low:

  • System may be undercharged, System may have a restriction, Dirty outdoor coil, Failed condenser motor.

 

If Subcooling is high:

System may be overcharged, System may have a restriction, Outdoor air temperature too low

SUBCOOLING IS GENERALLY 5°-15°, CHECK WITH MANUFACTURER TO VERIFY

 

So now you can see why knowing what and why superheat and subcooling are essential in proper diagnostics/troubleshooting. If utilized they will become second nature to calculate, reduce wasted time and eliminate callbacks due to “taking a guess”.

SUBCOOLING is used to charge systems with a TXV

SUPERHEAT is used to charge systems with a fixed oriface

The days of “gassin’ her up”  are over. Modern residential systems demand a qualified technician to make repairs.Use SUPERHEAT and SUBCOOLING  as  ways  to set yourself apart from your competition plus you can buy some cool new tools for the toolbox too !