Subcooling Calculation

[chevaughn]

How do you properly calculate for subcooling if your unit has no liquid line service valve? I have a discharge pressure valve, but that is superheated vapor, before it enters condenser. But if that is the only valve I have on high side, how do I calculate for subcooling?

[Randy S.]

Measure the high side pressure and then take the temperature of the liquid line leaving the condenser.

[RichardL]

Very simple....It is the mirror opposite of a superheat calculation.

[lynn comstock]

There will be a couple of pounds pressure difference. Next several times you have both dicharge and liquid pressures, measure the difference. Keep that in mind.

The same thing occurs on the suction side. The pressure reading is at the condensing unit and the TXV is at the Air handler. Pay attention when you can also get a reading at the coil (some coils have pressure ports) and compare to the suction pressure measured at the outdoor unit. Pay attention to the piping and get an idea of the difference that is "normal".

Sometimes the problem is baffling and you really need to know what is really going on. Use a line tap valve to get a reading. I have seen condensors with blockage internally and other strange situations where I needed to see pressure drops in lines or coils in order to make any sense out of the behavior of the system. Concealed line kinks are an example. When finished I would usually be able to remove the line tap valve and keep it as a tool.

[GT Jets]

There is no "proper way" to check for subcooling using the discharge pressure...Period.

A good work around is to use the temperature differential across the condenser coil.

And no, it is NOT the mirror image of the superheat procedure.....:spitball:

The reason you may not have a liquid tap is because the metering device is either a fixed orifice, metered orifice or cap tube....Subcooling is not used in charging, superheat and ambient temperature is employed.

GT

[dan wong]

How do you properly calculate for subcooling if your unit has no liquid line service valve? I have a discharge pressure valve, but that is superheated vapor, before it enters condenser. But if that is the only valve I have on high side, how do I calculate for subcooling?

We run across this type of problem usually on package unit- no liquid line valve after the condenser.

For checking purpose; I use the approach method. See page 10 on attached pdf. Although the attachment is for a Lennox system. However, the value does represent a typical a system out in the field (R22 system only)

[flange]

pump down the unit or recover charge, add a tap, evacuate and recharge to specs.

[chevaughn]

What is the BEST way to charge an ac unit? I was told that the ONLY way to charge a system w/txv is to use sub-cooling method. If this is true; why? How do you know what the sub cooling should be for various units? I was told that sub cooling tells you the charge and superheat tells you how efficient the unit is. It appears to me that no 2 technicians do anything the same. What is the industry standard for a proper charge?

[MM#7]

What is the BEST way to charge an ac unit? I was told that the ONLY way to charge a system w/txv is to use sub-cooling method. If this is true; why? How do you know what the sub cooling should be for various units? I was told that sub cooling tells you the charge and superheat tells you how efficient the unit is. It appears to me that no 2 technicians do anything the same. What is the industry standard for a proper charge?

use sub cool for TXV, and super heat for fixed orifice or piston type. usually mfg, tell you what they want, depending on out door temp. indoor db/wb,
sometimes amp pull,and your right you talk to 20 techs they all approach a call from a differnant perspective!

[XcelTech]

I would say,
Technician Certification

EPA has established a technician certification program for persons ("technicians") who perform maintenance, service, repair, or disposal that could be reasonably expected to release refrigerants into the atmosphere. The definition of "technician" specifically includes and excludes certain activities as follows:

Included:

* attaching and detaching hoses and gauges to and from the appliance to measure pressure within the appliance;
* adding refrigerant to (for example "topping-off") or removing refrigerant from the appliance
* any other activity that violates the integrity of the MVAC-like appliances, and small appliances.

In addition, apprentices are exempt from certification requirements provided the apprentice is closely and continually supervised by a certified technician.

The Agency has developed four types of certification:

1. For servicing small appliances (Type I).
2. For servicing or disposing of high- or very high-pressure appliances, except small appliances and MVACs (Type II).
3. For servicing or disposing of low-pressure appliances (Type III).
4. For servicing all types of equipment (Universal).

Technicians are required to pass an EPA-approved test given by an EPA-approved certifying organization to become certified under the mandatory program. Section 608 Technician Certification credentials do not expire.

And unless you are the owner of the equipment you should get a certified HVAC Tech. so you don't break the law.

Good luck. Hope everything comes out alright for you!