Charging a Refrigerant System

The refrigerant charge is how much refrigerant a system holds. Too little starves the evaporator and overheats the compressor. Too much floods the compressor, raises pressure and wastes energy. Charging is not a guess, and it is never a way to fix a problem you have not found. This guide covers how to weigh in a charge and how to fine-tune it.

What You Will Be Able to Do

• Decide when charging is the right move.
• Weigh in a charge, including line set adjustments.
• Pick superheat or subcooling based on the metering device.
• Charge blends correctly.
• Handle a critical-charge system.

Before Any Refrigerant Goes In

1. Confirm airflow is correct. A dirty filter or coil makes a good charge look bad.
2. Confirm the system has no active leak. If it was low, find out why.
3. Read the data plate for refrigerant type and factory charge.
4. Check the cylinder label, because color alone is not enough.
5. Use a scale that reads in ounces, set to zero with the cylinder on it.

Adding refrigerant to a stationary system is work for a technician certified under EPA Section 608, or an apprentice closely supervised by one.

Weighing In a Charge

Weighing is the most accurate method after a system has been evacuated, and it works in any weather.

1. Find the factory charge on the data plate. That charge usually covers a set line length, often 15 feet.
2. Add or subtract for line length using the manufacturer's rate.
3. Calculate the total to add.
4. Charge through the manifold into the evacuated system until the scale shows the correct amount.
5. Start the system, let it settle at least 15 minutes, then check subcooling or superheat.

Worked example: the data plate says 5 lb 8 oz, which includes 15 feet of line. The actual line set is 25 feet. The manual adds 0.6 oz per foot of 3/8-inch liquid line.

• Extra length: 25 minus 15 = 10 feet.
• Extra charge: 10 x 0.6 = 6 oz.
• Total: 5 lb 8 oz + 6 oz = 5 lb 14 oz.

Fine-Tuning by Metering Device

Superheat method. Measure indoor wet-bulb and outdoor dry-bulb temperature, find the target on the manufacturer's chart, then compare it with your measured superheat. If superheat is too high, add a small amount. If it is too low, recover a small amount. Wait 15 minutes between changes.

Subcooling method. Find the target on the data plate, often 8 to 12°F. Add refrigerant to raise subcooling, or recover some to lower it. Wait 15 minutes between changes.

Add refrigerant slowly and in small amounts. Pressures take time to catch up, so it is easy to go past the target.

Warning: Never remove refrigerant by bleeding it to the air. Use a recovery machine. Venting is illegal.

Charging Blends

Blends with glide, such as R-407C, can separate if you draw vapor from the cylinder. Always charge blends as a liquid. R-410A, R-454B and R-404A are also charged as liquid.

To do that safely:

1. Turn the cylinder as the label directs, or use its dip tube, so liquid comes out.
2. Charge liquid into an evacuated system, with the system off, through the high side when the manufacturer allows.
3. When the system is running, feed liquid into the low side slowly through a metering tool or a throttled manifold valve, so it flashes to vapor before it reaches the compressor.

Never let liquid slug into a running compressor.

Topping Off After a Leak

If a system lost part of its charge to a leak, you have two choices once the leak is repaired:

• Near-azeotropes such as R-410A: topping off with the same refrigerant is generally accepted after a small loss, around 10 to 15% of the charge. These blends barely separate.
• Blends with larger glide, or a big loss: recover the whole charge and weigh in a fresh one, because the mix left in the system may no longer be correct.

Always use the same refrigerant. Never mix types.

Critical-Charge Systems

Some systems hold such a small, exact charge that a few ounces matter. Mini-splits, window units, small package units and many small refrigeration products are in this group. They are called critical-charge systems.

For these systems, do not top off. If one leaks:

1. Recover what remains.
2. Repair the leak.
3. Evacuate.
4. Weigh in the exact factory charge, plus any line-length adder the manual lists.

Inverter systems also change speed constantly, so gauge readings shift during a test and do not tell you much about charge.

Heat Pumps in Heating Mode

Charging a heat pump in heating mode is hard to do by superheat or subcooling. Use the weigh-in method, or use the manufacturer's heating chart if there is one. When the weather allows, switch to cooling mode and use the normal chart.

Records

Write the refrigerant type and amount added on the invoice and in any required log. Commercial systems above certain sizes require records of each refrigerant addition. On A2L equipment, write the total charge on the required label.

Common Mistakes

Check Your Understanding

A TXV split has 3°F of subcooling and the data plate says 10°F. Airflow is good and the leak has been fixed. What do you do? Add refrigerant slowly and wait 15 minutes between changes, until subcooling reaches the target.

A piston system's chart calls for 12°F of superheat at today's conditions and you measure 4°F. What do you do? Recover a small amount of refrigerant, since low superheat on a piston suggests an overcharge. Recheck after it settles.

A mini-split lost refrigerant and now blows warm air. The owner asks you to add a little. What is the correct process? Recover what is left, fix the leak, evacuate and weigh in the exact charge.

The data plate lists 7 lb 2 oz at 15 feet of line. The line is 15 feet. How much do you weigh in after evacuation? 7 lb 2 oz, then check subcooling or superheat after it settles.

Key Takeaways

• Never charge before you confirm airflow and repair any leak.
• Weighing in is the most accurate method after a full evacuation.
• Fine-tune by superheat on fixed devices and by subcooling on TXV or EEV systems.
• Charge blends as liquid, and keep liquid away from a running compressor.
• Do not top off a critical-charge system.