Air Conditioning System Superheat & Subcooling Procedures for Optimal Charging
     - with Darrell Udelhoven - HVAC RETIRED - udarrell


The ordered procedure that must be followed to achieve an optimally charged efficient operating air conditioner.
Do not leave out any of the steps and always do these procedures in the order illustrated.

First, Check to see that there are NO air leaks in the Supply and Return Air duct system.
Next, Check to see if Indoor Squirrel Cage Blower wheel blades are free of lint or other build-up & Filter.

Check for a dirty lint clogged Evaporator Coil fins then check the Condenser Coil fins, check both coils on the air entering sides as well as between the fins, -- clean if needed.

Take a look at the ductwork for proper sizing and for leaks, check the indoor CFM Airflow, then outdoor condenser discharge air Temperature split (delta-T), then indoor Delta-T, then after 15 minutes of run time before any charging adjustments are made.

Check the Superheat & the Sub-Cooling as outlined below and always compare to the charging instructions that are with the equipment as some use the Approach Method & other methods may vary the operating figures & Target figures somewhat from SH & SC methods!

Those varied methods will usually be close to Super-Heat (SH) & Sub-Cooling (SC) results. I would always use the SH & SC method in conjunction with the mfg'ers method to trouble shoot  the refrigerant system.

Let's say with a TXV according to the Target Super Heat chart & your collected data indicates a  starved evaporator coil & a normal or slightly elevated Subcooling, even if you are using a Mfg'ers Approach Method do NOT automatically believe that it is undercharged.

One company is reportedly having problems with TXVs starving E-Coils, or it could be a partially plugged TXV strainer/screen or other restriction.

Optimize Evaporator BTU/hr Heat Input  1st always - "Optimize evaporator airflow heatload."

To Determine Super Heat (SH):

1.  Take the Suction Saturation Temperature (SST) reading from your manifold gauge.

2.  Then take the Suction Line Temperature (SLT) as close to the condensing section just before the serve valve.

3.  Take the difference between the above readings (Suction Line Temp 'minus' Gauge Saturation temp reading) = Superheat

4.  When ambient air temperature (Outside air temperature) is 85 degrees or above the Superheat should be 8-12 degrees.  Thermostatic Expansion Valves (TEV / TXV) should be set for a minimum 8-F Degrees Superheat.

Some Heat Pumps with TXV's are set at 7 to 9-F Super Heat because they have Suction Line Accumulators to store any spill-over liquid, which protects the compressor.

Superheat should be checked as close to the inlet of the evaporator refrigerant metering device as possible.

For TXV Subcooling, take the pressure of the liquid line note the gauge saturation temperature. Compare it to the actual temperature obtained near the same point the pressure was obtained. Thermostatic Expansion Valves (TEV / TXV) should be set for a minimum 10-Degrees Superheat.

This linked page is strictly a SUPERHEAT TABLE  Print these Tables & use them!

Print this Two linked pdf pages: Target Super Heat Chart and this Target Temperature Split for Airflow Chart

Here is a formula for getting the Super Heat Target - Within normal perimeters:
((IWB) Indoor Wet Bulb X's 3 - 80 -  (OAT) Outdoor Ambient Temp) / divided by 2 = Superheat Target

5.  If Superheat is low then the evaporator is flooding. Note:   Do NOT adjust charge YET.

6.  If Superheat is high then the evaporator is starving. Note:   Do NOT adjust charge YET!

7.  Do not adjust charge UNTIL Sub-Cooling is checked.

Note:  When charging a system using Superheat, you are charging the unit to the amount of air (CFM) and total heat load that is crossing the evaporator coil (Thus, the amount of latent and sensible heat load being absorbed by the evaporator coil).

Note:   Do not adjust charge based on Superheat on systems with Thermal Expansion Valves (TXV, TEV's), (use Liquid Line Sub-Cooling. TEV’s control the superheat; you should check the superheat to see if the TEV is working properly. Thermostatic Expansion Valves (TEV / TXV) should be set for a minimum 8-Degrees Superheat.

To Determine Liquid Line Sub-Cooling (SC):
1.  Take the high side pressure and convert it to temperature using chart or gauge.

2.  Then take the temperature of the liquid line as close to the condenser as possible..

3.  Take the difference between the above readings. (Saturation Temp – Liquid Line Temp.). Note: liquid line temperature at the evaporator should be within 2 degrees of liquid line temperature at condensing unit. If not, could be a restriction or line set too long.

4.  Sub-Cooling with a TXV, should be around 9 to 15-F degrees, always check with the mfg’ers for correct SC

5.  Then using the information from Superheat and Sub-Cooling we can have some idea where to look for a problem.
Example:
Suction Line Temp is ------- 60 degrees @ condenser
Gauge Suction Pressure is ------76-psig ---- 45 degrees, Read Gauge Suction Saturation Temperature (SST)
60 degrees – 45 degrees = 15 degree Superheat - Adjust charge to the mfg'ers  Super Heat settings

Liquid Pressure is  ------------226-psig --------110 degrees, Read Gauge - Liquid Saturation Temperature (LST)
Liquid Line Temp (LLT) is -------------95 degrees
110 degrees – 95 degrees = 15 degree Sub-Cooling - Adjust Refrigerant charge to the mfg'ers  SC settings
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It is important to understand that "equipment ratings are only the 'potential efficiency' of that component of the system under perfect conditions." Over half of the system’s efficiency depends on correct equipment sizing run-time, on the duct system sizing, i.e., on the quality of the complete field-installation!

Especially if your system is oversized or there are a lot of low AC load days use an adjustable differential room TH.
TH Differential: Differential is defined as the difference between the cut-in and cut-out points as measured at the thermostat under specified operating conditions. For example, if the thermostat turns the COOLING EQUIPMENT on at 78-F & OFF at 76-F that is a 2 degree differential setting; one has a 4-F adjustable differential. This is a good way to control high humidity problems & also improve SEER performance.


What you want & need is right sized equipment operating at its optimal ratings within varying conditions, for your optimal comfort and savings.

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TROUBLE  SHOOTING  TXV  VALVE SYSTEMS 

Bulb location: Some Mfg’s have there preferences, but a good rule of thumb is 10 or 2 O’Clock, away from headers and heat exchangers, on a smooth clean surface. 

Also, make sure the cap tube is on top (horizontal or vertical and never upside down).

Pressure drop: TXVs like to have at least 100-psi pressure drop across them to operate correctly. A solid column of liquid (at the valve) is also a requirement.

Flood back: Always make sure you have "the correct CFM airflow" (clean coils," Clean fan blades & fans running on correct speeds and in the right direction) before you try adjusting a valve.

No flow: A plugged screen is rare in air conditioning, but happens often in refrigeration. I have seen the external equalizer tube leak through in liquid form and give the bulb a false reading (which causes hunting more so than no flow).
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TXV Partially plugged, downstream from service port, filter-dryer or screen at Compressor Inlet, therefore TXV is Wide Open flashing some vapor & cooling coil is starved of liquid refrigerant:

Suction Low-Side
Suction-PSIG -
Normal to High
SUCTION
Super-Heat -
HIGH
  High-Side
Head-Pres. -
LOW
Liquid-Line
SUB-Cooling -
NORMAL
Cond.-Unit
Amp-Draw -
LOW
In HVAC-TALK  These Posts Illustrate and Discuss these Test Results

Lennox TXV Subcooling - Approach Method -  pdf P- 8
http://www.davelennox.com/pdfs/installation_maintenance/Lennox-12ACB-IOM.pdf

Always check both SH & SC for trouble shooting comparison to normal parameters!
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There is a concern that we need more accurate means to achieve an accurate target sub-cooling temperature under varying conditions.

First, before any operating performance feedback data is recorded, the following tests & corrections must be performed. We need to make certain that the airflow is checked to be within the proper parameters and that the ductwork is properly sized and sealed with return air grilles in every room at the ceiling level. In addition, if possible for the cooling mode, Supply Air Diffusers should also be at the ceiling level.

Accurate tests should also be made, to determine whether proper airflow CFM is being delivered, as well as into each room.

The manufacturers could be of great help in this respect, if they would list the Delta-T of the condensing unit at different BTUH load output levels, also at various outdoor ambient temperatures. .

I believe that with adequate test data feedback the subcooling could be targeted within plus or minus 1 or 2 degrees Fahrenheit, which would be a two to four degree differential.

That would be more accurate & concise a temperature target than present subcooling temperature targeting methodologies.

Additionally, I would consider using the Lennox Approach Method to help select the Subcooling Method.

The Lennox Approach Method subtracts the Outdoor Ambient Temperature from the Liquid Line Temperature (LLT), whereas, the subcooling temperature targeting method subtracts the Liquid Line Temperature near the evaporator from the Condenser Saturation Temperature (CST). I do not see why the Lennox Approach Method would not help pinpoint the subcooling target on other systems. The Indoor Heatload has to be part of the equation; there are other factors to incorporate as well.

Possible Diagnosis using Super-Heat and Sub-Cooling:
If Superheat is high and Sub-Cooling is low:
Charge must be adjusted. System is Undercharged.

If superheat is low and sub-cooling is high: 
Charge must be adjusted. System is Overcharged.

If Superheat is very high and Sub-Cooling is a little high: 
Could have blockage in coil, TXV strainer screen - settings, etc., orifice, filter dryers etc.

If Super-Heat is low and Sub-Cooling is low:  
Piston orifice could be too big, or some, in backwards, there is no orifice in the unit or the orifice is stuck and refrigerant is bypassing it.

To Determine Delta T  (Td) (Temperature difference across the coil):
1.  While unit is running take the temperature of the air in the supply plenum near the coil (approx. 12 inches.)
2.  Then, while the unit is still running, take the temperature of the air in the return plenum near the unit.
3.  Then take the difference between the above readings.
4.  Should be around 15-18 degrees.  Use linked Chart above!
5.  If to low then coil might not be seated in pan correctly - air bypassing cooling coil. (Assuming superheat and Sub-Cooling are OK.)
6. A TXV's normal Superheat setting is between 8-F to 12-F.  There must be a full liquid stream to the TXV!
With a TXV metering device if Superheat is too high say,  20-F or above — look for, suction line restriction, plugged cap tube/orifice./liquid line, hot gas discharge line restriction, filter dyer, downstream of suction service port, or compressor screen restriction or inefficient compressor.
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What are the proper methods to determine operating superheat, sub-cooling?

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb). Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb. Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible noting the difference.
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Carl Bergt, Principle Engineer at Rheem  writes:

Mr. Wolok: Your inquiry concerning subcooling as been forwarded to  me.  From the information that has been provided to me, it appears that you are searching for a recommended subcooling level for residential products.  As you  can tell from the variety of responses you have received, subcooling, at the outdoor condensing unit cannot be clearly defined.  Simply, subcooling  is a function of many factors that includes; the outdoor unit, line size between the outdoor condenser and indoor coil, total refrigerant line lengths, number of bends in the refrigerant lines, refrigerant utilized, ambient, vertical separation between the outdoor and indoor components, and flow control.

To my knowledge, all split residential products require subcooling at the outdoor unit.  This includes systems that utilize TXV, capillary  tube, or fixed orifice indoor flow controls.  If you are looking for a simple solution, I would suggest you measure the subcooling level at the indoor coil before the expansion device.  Assure you have at least 4-6 deg F subcooling and you should find proper operation for any given installation.  This assumes that you will measure subcooling at ambients or operating conditions that are somewhat close to your normal operating conditions and your refrigerant line sizes and lengths are within the manufacturer's recommendations.

Measuring subcooling at the indoor coil takes into account many of the variables noted above.  The ultimate goal is to assure you have liquid refrigerant at the expansion device using reasonable subcooling levels that allow for efficient unit operation.  What you will discover is  that the subcooling at the outdoor unit will vary depending on the installation and application.  If subcooling is measured at the outdoor unit, you will have to account for the variables noted earlier to determine the correct level.
-----------------------
My response to an HVAC Forum question on BTU & Tonnage Ratings:
Three ton is 36,000 BTUs.
The units are Rated in Nominal Tons per hour.

However, the nominal BTU/hr rating of some range from 36,000 down to around 34,000-BTU/hr.

Additionally, with high indoor temperatures & very high humidity a nominal 36,000-BTU/hr could go considerably higher.

Example, Goodman Expanded Data: a 3-ton condenser 13-SEER GSC130363A, with a 4-ton evaporator coil:
1434-cfm or 478-cfm per ton of cooling
85 OAT Outdoor Ambient Temp
80 IDB Indoor Dry Bulb
71 IWB Indoor Wet Bulb or 63% Relative Humidity
Nominal BTU/hr of 39,500
At 75 OAT outdoor Ambient Temp
other figures the same, nominal listed @ 40,500-BTU/hr. (At ARI Conditions)

Moderate outdoor temps coupled with high indoor temps results in a high latent humidity heatload through the evaporator coil which boils refrigerant at its fastest rate, which transfers more heat outdoors per unit of time.
- udarrell
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That is why we should NOT be upsizing equipment for latent heat removal; because the A/C system increases its latent capacity to handle that load. When the unit is upsized the run-time operating-cycles can be way too short for effective latent heat (humidity) removal.
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Gurgling sounds at TEV: Low evaporator heat-loads lead to reduced liquid line mass and increased evaporator mass could be due to airflow problems. Eliminate low evaporator heat-loads before looking into adjusting the refrigerant charge.  Gurgling - pulsation noises in Liquid Line at the expansion device can be caused by low evaporator circuit heat-loads, low charge, and/or non-condensibles and moisture in the system. Unbalanced airflow through the various distributor circuits of the evaporator coil will cause the TEV to close down refrigerant flow starving the coil. Piston-flow-rators will make it impossible to properly charge the system and cooling will be greatly compromised unless you eliminate the cause! "Put your ear on the liquid line at the evaporator coil."

On every Rheem condenser cover it lists "non-condensibles and or moisture" as causes for a gurgling or pulsating noise at the expansion device. The entire evaporator circuits, may not become active for various reasons, - "the entire coil must become fully active for efficient performance."

The purpose of these recommendations is to provide liquid refrigerant at the expansion device and provide efficient operation. Hopefully, this will aid your research.  If I can be of additional assistance, contact me.
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Too many do not properly purge & evacuate contaminated central air conditioning systems.

The Triple Evacuation Method is normally done on refrigeration systems, R-410a systems require it on central air conditioning systems:

First, remove any valve cores with a special  valve core remover this will speed up the evacuation time. Back service valves two turns off their back seat.

1) Re-claim unit charge (Recover all the refrigerant)

2) Charge system to 150 PSIG with dry nitrogen and leak test

3) On contaminated systems replace the filter dryers. Then Repair all leak(s)

4) Evacuate system to 500 microns valve off & see if it holds 500 microns for ten minutes, if it holds, break the vacuum with dry nitrogen

5) Evacuate system to a deeper 400 microns, valve off vac pump, & again break the vacuum with dry nitrogen

6) Evacuate system to 400 microns and & then Check to see if it holds. (Recharge with fresh clean refrigerant)

7) Check to see if the Supply and Return air ducts were correctly sized & sealed by the original installer.

If a vacuum pump will not evacuate a system below 1500 microns there is a problem with the pump itself, a leak in the system, or moisture in the system. Moisture is most likely because water vaporizes at 1500 microns.

Many HVAC contractors will consider this excessive time & effort for contaminated residential air conditioning systems, however it is a must for low temp applications.

The “micron” is a metric unit of measure for distance. The micron is a unit of linear measure; one micron equals 1/25,400ths of an inch. Modern high capacity vacuum pumps help speed up the evacuation process.

When a system has been evacuated below 500 microns, the pump is valved-off with the micron gauge connected, if the vacuum rises to 1500 microns and stops, there is moisture remaining in the system. If it rises above 1500 microns & continues to rise there is a leak. You should allow at least 15 minutes after the pump has been shut off an accurate micron gauge reading. When a system will not evacuate below 1500 microns there is either a lot of water or there is a system leak.
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Check Return Air (RA) at grille & at entry of blower for heat gain, due to hot Return Air leaks.
Where airhandlers' set over Return Air Chambers check for air leaks through the sheet rock & down the wall studs from  the attic - this is a fairly common condition that will overload the AC system!

Below is an outstanding PDF "Basic AC Overview - Specifications VS. Reality"
by John Proctor, P.E., Proctor Engineering Group, LTD:
HVAC TECH PERFORMANCE RATINGS  "AC Specs vs Reality" PDF - It's Worth Your Time

EFFICIENT INDOOR COMFORT  - An EXCELLENT SITE  for You
FREE HVAC Resources for Professionals

There are a multiplicity of things that affect subcooling, first, airflow must be correct; Supply Air Return Air sealed & correctly sized!
I would also add that we need effective ways to determine the BTUH that the system is delivering while we are recording superheat & subcooling temperature data. We need helpful condenser-temperature-split  performance data from from all of the mfg'ers to make this operating BTUH data more accurate & easy to acquire. See my other pages for this test method.  - Darrell

Study the Failure Rate Graph in the pdf above!

It is a well-known fact in the industry that a large percentage of compressors being replaced are replaced due to improper diagnosis, NOT compressor failure! 

Focus on Energy - Efficient Heating & Cooling Initiative - Target Temperature Split for Airflow - PRINT Charts
On above linked page scroll down and Click  - Airflow and Refrigerant Tables - the Super Heat Table does NOT comport with other methods!  - Darrell  - udarrell

DISCLAIMER:
Any of the HVAC companies I list on any of my web pages have nothing to do with the information I post on any of my Web pages nor do I assume any responsibility for how anyone uses that information.
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.

This information is for the edification of contractors and techs. I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven (U-dl-hoven)

Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired HVAC Contractor 
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