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Old 06-18-2015, 07:42 PM   #1
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Subcooling/Superheat


How accurate is measuring subcooling/superheat for correct charge of refrigerant as compared to evacuating and charging system based on weight of refrigerant called for on manufacture nameplate. I am assuming the required amount of refrigerate on the nameplate does not take into account the line-set length and dryer volume.

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Old 06-18-2015, 09:56 PM   #2
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Subcooling/Superheat


Weighing in is the most accurate. Superheat/subcooling can be affected by the amount of airflow, fan speed, how clean the indoor and outdoor coil are. Weighing in does not care and is the best. Dryer volume does not matter and you measure the line and add the correct amount in ounces if it is over 15 feet.

We do use superheat and subcooling for charging especially with the manufacturers charts to fine tune it.

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Old 06-18-2015, 10:06 PM   #3
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Subcooling/Superheat


I've seen charts which specific head pressure at a given suction psi and outdoor temp - those take airflow into consideration.
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Old 06-18-2015, 10:17 PM   #4
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If you read the fine print it states they have a 5% allowable margin. Lennox does. Weighing in is exact and you use superheat/subcooling to fine tune it. No chart knows what size the ducts are, type of filter, fan speed etc etc.

Refrigeration is not an exact science and only when you have refrigerators or units that have no field piping to vary in length can you stay with a exact charge. With any other units you adjust for the field conditions.
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Old 06-18-2015, 10:25 PM   #5
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Subcooling/Superheat


Superheat and subcooling are actually two different measurements/tests performed depending on the system being tested.

Subcooling is performed mostly on systems with variable expansion valves (TXV's) while superheat is used mainly on fixed orifice systems (capillary tubes).

Subcooling tests should be carried out on new installs (with TXV's) since the TXV can be out of adjustment regardless of a proper fill or not. The long and short... for best results you need to perform a subcooling test as well as make sure there is a proper fill on the system.

Most r410 systems are coming precharged now, and that charge usually includes enough for a 15 foot line set. You need to add about .6 ounces for each additional foot.
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Old 06-19-2015, 04:11 AM   #6
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Subcooling/Superheat


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Originally Posted by HotBama View Post
How accurate is measuring subcooling/superheat for correct charge of refrigerant as compared to evacuating and charging system based on weight of refrigerant called for on manufacture nameplate. I am assuming the required amount of refrigerate on the nameplate does not take into account the line-set length and dryer volume.
Manufacturers(conventional systems) include enough charge for 15 foot of line set. If you are adding an additional LLFD to the system, then you need to add for it.

Keep in mind that its made in a factory, which can make mistakes, and not put enough or could have put to much charge in the condenser when it was made. So SC and Sh should be checked on a new install, even if the line set is only 15 foot.

Some units will also spec a SC(Payne/Carrier/Bryant often spec a 15 SC).
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Old 06-19-2015, 04:46 AM   #7
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Subcooling/Superheat


Just back from work. Thanks for the info and discussion. On a fixed orifice system, some tradesmen take superheat and subcooling measurements. What is the subcooling measurement used for? Just more info on checking out a system?
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Old 06-19-2015, 05:18 AM   #8
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Helps to determine if the system is over charged to compensate for a restricted LLFD.
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Old 06-19-2015, 07:53 AM   #9
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In theory you should add the volume of a additional filter drier. I don't have to as Lennox has them built into their units. All depends how much theory you want to get into. I weigh in the charge and then use subcooling as we use TX valves on all our new units.

With repairs I weigh it in, check the line length and then fine tune it with the subcooling. An ounce or 2 either way does not make a huge difference. With minisplits it does as the liquid line is the metering device.
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Old 06-19-2015, 08:33 AM   #10
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Subcooling/Superheat


Please correct me if I am wrong .

My understanding is a super heat measurement is to indicate if too much liquid refrigerant is going into the A-Coil . If this happens , there is a possibility some liquid refrigerant will make it all the way back to the compressor and possibly damage the compressor ?

And sub-cooling is the temperature difference between the " hot " air exhausted by the condenser fan and the ambient air temperature ? Or is it the temperature of the liquid line leaving the condenser unit ?

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Old 06-19-2015, 09:06 AM   #11
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Google: refrigerant superheat and refrigerant subcool.

Google: superheated steam to understand what superheat really is about.

There is a lot to explain.

Basically you want to superheat the refrigerant so it is 100% vapor and has no liquid in it which can return to the compressor and damage it. Subcool makes it more efficient as the temp of the liquid going into the TX valve is lower so it can then absorb more heat when it enters the coil. This is a basic description. Google for more info and some sites have pics etc.
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Old 06-19-2015, 11:08 AM   #12
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If a manufacture's target superheat and subcooling values are not available, what happens then? Do you just call up the manufacture with the model number? I suspect most homeowners do not have their A/C papers. Or, is there a range of values that will work for all HVAC units?
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Old 06-19-2015, 12:30 PM   #13
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Subcooling/Superheat


There are generic charts according to the SEER on the net.

A highly experienced tech if careful will monitor the Delta T temp diff across the evap coil. After 1/2 hr of running and once it reaches 16 F and slows down going further to the 20 which is usually max then he stops adding refrigerant. If it goes 16>17>18 and then stops you don't keep adding refrigerant to try get 20 as that is where most techs screw up and overcharge it. Along with the superheat. You need some superheat irregardless of metering device so you don't slug liquid to the compressor. After many yrs of doing it you figure out how to "make" a system work. VERY rarely are the ducts large enough or perfect sized and you have the exact airflow the manufacturer recommends for those charts. You learn how to make a system work if you take enough interest which most techs don't.

Charging refrigerant is NOT an exact science or some cut and paste technique you find on the net as there are a dozen or more variables which can affect it. ( Hope you are not a new tech and I am scaring you or confusing you ).

There are variables like humidity and depending on what part of the Country you live in different methods work or techniques. I live in a medium humid zone. 50% average.
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Old 06-19-2015, 03:31 PM   #14
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Quote:
Originally Posted by WyrTwister View Post
Please correct me if I am wrong .

My understanding is a super heat measurement is to indicate if too much liquid refrigerant is going into the A-Coil . If this happens , there is a possibility some liquid refrigerant will make it all the way back to the compressor and possibly damage the compressor ?

While it indicates how flooded the evap is with liquid, the only purpose SH serves is to protect the compressor fro liquid flood back.

And sub-cooling is the temperature difference between the " hot " air exhausted by the condenser fan and the ambient air temperature ? Or is it the temperature of the liquid line leaving the condenser unit ?

Thanks ,
Wyr
God bless
SC is the difference between saturation temp and actual liquid line temp.
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Old 06-19-2015, 04:41 PM   #15
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Quote:
Originally Posted by beenthere View Post
SC is the difference between saturation temp and actual liquid line temp.
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