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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.
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°
- 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:
- It guarantees no liquid is getting back to compressor which would cause failure.
- 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
- 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 !
How Much Refrigerant Do I Need to Add?
- This question is asked quite often from technicians. Usually it is related to an A/C installation that was performed over the winter
- In an effort to eliminate spending time having to go back to a customer’s home in late spring or early summer for a startup some simple procedures can be performed.
Rheem ships its condensing units with enough refrigerant for a 15’ line set.
- Any length over that needs to have the charge adjusted…BUT WAIT…There’s more to it!
- The factory also ships a liquid line filter drier with each outdoor unit and guess what?, it requires a charge adjustment too…BUT WAIT…There’s more!
- Any 90° or 45° whether long radius or short radius requires a charge adjustment too …!
- The charts below are from any Rheem condensing unit installation manual
LIQUID LINE diameter and adjustment p/ft:
Let’s look at a common scenario and calculate how much additional refrigerant is required:
- 3/8 x ¾ x 35 ft lineset
- Factory supplied liquid line filter drier
-2 ¾” long radius elbows
35’ – 15’ = 20’ @ .6 oz p/ft
Filter drier 6 oz.
LR elbows = 2.6 additional ft
So now we have 22.6 ft x .6 = 13.5 oz + 6 oz = 19.5 oz of additional refrigerant
- After seeing this one example one would agree there are a lot of systems undercharged in the field.
- Using this method of weighing in additional charge will get you close to where you need to be as long as the fan speed is set for the proper tonnage and static pressure, the duct work is satisfactory and proper installation procedures were adhered to.
How to Check a Single Phase Compressor
SAFETY IS ALWAYS OF THE UTMOST CONCERN. BE MINDFUL THAT YOU WILL BE EXPOSED TO HIGH VOLTAGE AND HIGH AMPERAGE WHILE TROUBLESHOOTING COMPRESSOR CIRCUITS.
There are several reasons as to why a compressor fails to operate
The most common are as follows:
- Contactor not pulling in/ Contactor Failure
- Open winding(s)/ Open internal Overload
- Shorted winding(s)
- Locked rotor
Contactor not pulling in/ Contactor Failure:
Check for 24vac across Y and C at the contactor coil. If 24vac not present there is an open or short circuit in the 24vac or 120vac wiring. Further investigation is required. If 24vac is present at Y and C and the contactor will not pull in replacement is necessary.
Open windings/Open internal overload
Verifying the integrity of the windings is a simple task and requires a DMM (digital multi meter). Remove disconnect to break line voltage to the condensing unit (verify with meter). Take photo of compressor wiring for future reference. Remove 3 wires (Common, Start, Run). Set meter to lowest ohm value (Ω). Record resistance values for the following C-S, C-R, S-R. On an operational compressor you may see readings as follows C-S 3Ω, C-R 1Ω, S-R 4Ω.. Common- Start will always have a higher resistance value than Common - Run as the start windings are heavier to deal with the in rush current required to start the compressor. Start-Run will always be the added value of C-S and C-R.
If any ONE of these windings are open (OL) the compressor needs to be replaced. If you find C-S and C-R are both open the compressor is off on internal overload. The overload is an internal safety device and is in series with Common This condition generally happens when the compressor is exposed to excessive heat/amp draw. Once the compressor cools the overload
should close, if not, the compressor needs to be replaced. If this is the case the technician still needs to find the reason(s) why it opened. Some common reasons are failed outdoor fan, plugged condenser, under-overcharge, locked rotor…etc…
This condition happens when the compressor motor insulation has failed and the winding physically is in contact with the shell of the compressor or another winding.. Generally the circuit breaker for the condensing unit will be tripped and cannot be reset. To verify you will need to scratch the paint off the side of the compressor to bare metal. Set your DMM to the highest ohm value (Ω). Place one meter lead on the bare metal and the other across C, S and R. If there is a short there will be very little resistance to ground, typically 1-2 ohms. If the compressor is shorted it will need to be replaced.
This condition exists when the rotor of the motor is physically locked in the stator. When the compressor tries to start it will hum , draw LRA (locked rotor amps) and either trip the circuit breaker or open on internal overload. You can verify with a clamp type ammeter . Exercise caution as the amp draw can be temporarily quite high. The locked rotor rating is on the compressor data plate. If the compressor is locked it will need to be replaced.
All of the aforementioned conditions require investigation as to why there was a failure in the first place. To simply replace the compressor without doing so would be a disservice to the customer.
ABR Wholesalers, Inc.
Tools used in this post: Multimeter and Amp Meter
ABR Wholesalres, Inc. is a wholesale distributor of HVAC and Hydronics serving contractors and businesses for over 50 years.
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