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Superheat and Subcool charging method

Lextex on Tue September 25, 2007 10:04 AM User is offline

Year: 1998
Make: Toyota
Model: Sienna

I have another general charging procedure question.

Most HVAC techs use the superheat and subcool method for charging a system. I don't see this procedure discussed much in mobile AC forums. Can automotive systems be charged using this procedure?


NickD on Tue September 25, 2007 11:39 AM User is offline

I don't fool with it on MVAC systems, these are suppose to be well engineered systems with all kinds of data. Key issue is cleaning up the system first, removing all the debris from the condenser, evaporator, heater core, and radiator, making sure the fans are operating correctly, then checking pressures.

Each HVAC system is unique with plenty of variables, there a guy should fool with super heat and cool, wet bulb, etc., for optimum performance making corrections in the system size, duct work, blower speed, etc. These corrections are not even possible in a vehicle, unless you are going to tear the vehicle apart and mold all new components. Then you won't make much improvement anyway, because always other stuff in the way, so you would practically have to redesign the entire vehicle.

DNT on Tue September 25, 2007 2:03 PM User is offlineView users profile

naw not worth messing with in MVAC. Problem is that the variables in MVAC vary so wide that most superheat/subcooling numbers would be useless. You are utilizing a variable speed/output compressor a widely variable blower speed plus widely variable ambient and interior temp range (can you imagine cooling your crib down after it was 135f inside) plus the underhood temp variations. If you think about it it is actually pretty amazing that a vehicles system functions at all!

Anyone who gives up freedom for the sake of safety, deserves neither freedom nor safety!

Chick on Tue September 25, 2007 2:55 PM User is offlineView users profile

All the guesswork has been done for you on MVAC..The underhood tag give you the amount "best suited" for that vehicle under the wide ranges of temps and humidy you'll run into. The superheat method is used to "check" the systems operation after the known/tag amount of refrigerant is added, and used more for retrofits where the amount varies widely vehicle to vehicle..Hope this helps..

Email: Chick


Freedoms just another word for nothing left to lose

Lextex on Tue September 25, 2007 7:15 PM User is offline

Thanks to all for the replies. There is always a lot of good info on this forum from "real" experience.

NickD on Thu September 27, 2007 9:03 AM User is offline

Charging by superheat is one of the shortest and best articles I can find on the web explaining the procedure. Basically, the key variable is how much do you charge an HVAC system for maximum efficiency. Too much charge with a low superheat value can liquid lock the compressor causing damage and not enough charge results in poor cooling and higher electrical bills. Four distinct temperatures are measured, condenser and inlet evaporator temperature with the latter using a wet bulb procedure that is to compensate for humidity that is the target superheat. The other two are the boiling point for the refrigerant near the orifice that may be complicated if a TXV is used and the outlet temperature of the refrigerant at the evaporator.

With my own HVAC system the manufacturer was kind enough to provide P-T charts at various humidity levels so I just have to deal with pressures, insufficient charging results in evaporator freeze up, but that is better than liquid locking the compressor so I tend to charge on the low end of the scale.

What I find confusing about the superheat charging method is that not much is said about the ambient temperature and humidity variations, if you are a contractor, can't pick the weather. I pick my time waiting for a 90*F day for tuning up my own system, but the temperature outside can vary between 40 to 125*F, that has to have some influence on the target superheat value. I also add a true power wattmeter to the compressor so I know it's electrical loading is within specifications

Typically in MVAC systems and mostly for domestic vehicles, look in the manual, if the manual states dump in 3.1 pounds, you dump in 3.1 pounds. I do not feel this is accurate because of production variations, and in those manuals that do give any type of performance chart, you see huge variations in low and high side pressures as well as vent temperatures. Like my Toyota Supra manual as precise low and high side pressures are given with the various temperature/humidity variations and the variable is changing the charge in weight a few ounces either way to get optimum performance.

These charts are based on a clean system, always the first step with properly working fans and blower motors and using the designed refrigerant. Lots of guys run into problems converting an R-12 system to R-134a and using these low side only tire pressure gauges is a very bad joke. High side pressures can vary over 200 psi depending upon how clean the system is. And never removed a blower motor yet without finding tons of debris on the evaporator core, least on some newer cars, they are adding filters, way overdue, but nothing to keep bugs out of the condenser.

Figure about 90% of my time is spend doing janitorial work on vehicles, but is the only way to do the job right. Best way to charge with R-134a on a R-12 system is to put in about 80%, run the engine at 1,500-2000 rpm, doors open, AC on, blower at max and monitor the vents temperatures on one open vent until adding more charge does not decrease the vent temperature while very carefully monitoring the low and high side pressures. But this is after you cleaned up the system first. That step is always ignored.

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