# Thread: The theory of charging an R22 system without a TXV

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## The theory of charging an R22 system without a TXV

It is my understanding that the correct way to charge an R22 system without a TXV is by getting the actual superheat to match the target superheat. It is also my understanding that the target superheat in Fahrenheit is determined by the equation:

Target superheat = (3 x Indoor Wet bulb temperature – 80 – Outdoor ambient temperature)/2

And the actually superheat:

Actually superheat = Suction line temperature – the refrigerant boiling point for the measured suction line pressure.

I’ve found different sources that claim this is the correct method including the manual for my condensing unit. Nonetheless I have yet to meet an HVAC technician who agrees with this or for that matter even had a web bulb thermometer or digital psychrometer in their possession. The last technician said it was good enough to get around ten degrees of actual superheat no matter what.

What is the truth?

2. The truth is there are many "techs" who can't or won't properly charge a system. It's called "beer can cold". Get the suction line sweating and they're down the road.
Yes, a fixed orifice system should be charged to target super-heat, plus or minus 3 degrees. TXV system to mnf specs of sub-cool.

3. Ok... Hold steady here comes a barrage of replies... loll

4. all systems have a target superheat or and subcool. I never come up with some algebraic equasion, I go by what the manufacture suggests. Of course they had to use some equasion to come up with it, but they are engineers and scientists, and I just fix their design flaws.

all techs must have someway of knowing wetbulb temperature of the area being conditioned.

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An ideal charge is done by weight. Charge according to the data plate on the equipment and add .6 oz or R-22 per additional foot of line set beyond 15 feet for a 3/8 X 3/4 line set. On older equipment the factory charge accounted for a 25 foot line set, so check with the manufacturer if it more than about 10 years old.

When charging by weight isn't the best option (after a leak has been repaired) then charge by superheat for fixed orafice systems and by subcooling for TXV applications. The system should have a charging chart that will give the required superheat or subcooling data for the system and a qualified tech should be able to read and accurately interpret the chart and its data to charge the system properly so long as they have some kind of digital thermometer with a strap on temperature probe and a sling psychrometer or a digital psychrometer.

It never ceases to amaze me the number of "qualified technicians" who dont know the correct way to charge a system.

6. Originally Posted by mofotech
Ok... Hold steady here comes a barrage of replies... loll
I have a psychrometer and know how to charge to +/- 3 degrees.......

where have you been finding your "techs"?

some days 20 degrees of superheat is correct. 2 months ago I had to take 12 lbs out of a system that only held 9 lbs- must have liked really cold beer

7. Yep, Beer can cold, +/_ 3*.........

8. What if there is no chart on the condenser or its been ripped off by erosion. What base Superheat and Subcooling one should aimed for? Is having Superheat 12-15 degrees normal? And having 9-12 degrees normal for subcooling?

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if you like english beer beer can cold can be prety close

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the formula for superheat. seems I,m often shooting for 20-30 up here in the north country

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Originally Posted by mofotech
What if there is no chart on the condenser or its been ripped off by erosion. What base Superheat and Subcooling one should aimed for? Is having Superheat 12-15 degrees normal? And having 9-12 degrees normal for subcooling?
Contact the local distributor for said brand and get the # for tech support. Call them with models and serials and they can give you all the information you need (and then some more....)

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Originally Posted by 2old2rock
The truth is there are many "techs" who can't or won't properly charge a system. It's called "beer can cold". Get the suction line sweating and they're down the road.
Yes, a fixed orifice system should be charged to target super-heat, plus or minus 3 degrees. TXV system to mnf specs of sub-cool.
This is the truth and nothing but the truth.

There is something else that MUST be factored in, but if your techs don't have psychrometers, they aren't checking that, either......

13. Originally Posted by sneuberg
It is my understanding that the correct way to charge an R22 system without a TXV is by getting the actual superheat to match the target superheat. It is also my understanding that the target superheat in Fahrenheit is determined by the equation:

Target superheat = (3 x Indoor Wet bulb temperature – 80 – Outdoor ambient temperature)/2

And the actually superheat:

Actually superheat = Suction line temperature – the refrigerant boiling point for the measured suction line pressure.

I’ve found different sources that claim this is the correct method including the manual for my condensing unit. Nonetheless I have yet to meet an HVAC technician who agrees with this or for that matter even had a web bulb thermometer or digital psychrometer in their possession. The last technician said it was good enough to get around ten degrees of actual superheat no matter what.

What is the truth?
You are correct. Another correct answer is to use the manufacturer charging chart which usually requires a WB measurement. If you apply that formula to the manufacturer's charts, you will find that its dead on or less that 3° different than any target SH chart. That formula was derived by hvacrmedic, a pro member here.

Keep looking for a tech. There are good ones out there.

What field are you in?

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