Master How to Add Refrigerant to an AC Unit in 12 Easy Steps

Adding refrigerant to an AC is a routine task for most HVAC technicians, but that doesn’t mean you have every step and detail memorized. That proves especially true if you’re new to the field or a long winter season has muddled your recollection of what colored manifold hose goes to which pressure side. Whether this guide functions as educational material or simply a point of reference, we’ll walk you through the steps on how to add refrigerant to an AC unit safely, helping restore its cooling efficiency without taking up too much of your time. 

Supplies You’ll Need

• Refrigerant 
• Manifold gauge set 
• Refrigerant scale 
• PPE  

How to Add Refrigerant to an AC Unit 

Before adding refrigerant to an AC unit, ensure you’ve run through all the diagnostic criteria first, including checking the thermostat, air filter, circuit breaker, air vents, condenser and evaporator coils, blower motor and fan, etc. Once you’ve determined that the decreased cooling efficiency is caused by a low charge, follow these 12 straightforward steps to learn how to add refrigerant to an AC unit.

Step 1. Don the Proper Safety Gear 

Refrigerants can pose several health and safety risks that can be mitigated with the proper personal protective equipment. When performing maintenance and repairs that expose you to these hazardous chemicals, be sure to wear the following:

• Face shield 
• Goggles 
• Long-sleeved shirt and pants 
• Closed-toed, non-slip shoes
• Gloves 
• Respirator 

Step 2. Check for Leaks 

Since refrigerant flows in a closed-loop system, you shouldn’t need to recharge the AC system unless there’s been a leak or after certain repairs. The unit might exhibit several symptoms, including:

• Frozen evaporator coil
• Longer cooling cycles 
• Short cycling 
• Hissing or bubbling noise 
• Warm air coming from the vents 
• Reduced cooling efficiency 
• Higher utility bills 

Refrigerant leaks should always be verified with either a soap method, UV dye, electronic leak detector or pressure test.

Step 3. Confirm the Refrigerant Type 

Different air conditioners use different refrigerant types, so you must select the right one before adding it to the HVAC system. Charging the AC with the wrong type of refrigerant can cause overheating, fire, explosions, injury and damage to the equipment or surrounding property. 

Taking steps to consult the manufacturer’s manual or the nameplate on the indoor or outdoor unit will prevent the accidental mixture of incompatible refrigerants and keep the unit operating safely.

Step 4. Check Outdoor & Indoor Temperatures 

You’ll need to take the ambient indoor and outdoor temperature to convert the pressure readings to saturated temperature, which is vital to understanding how much refrigerant to add to the AC unit. If the saturated temperature is below 32℉, it could mean that there’s a low refrigerant charge, low indoor airflow or a liquid line restriction. 

It’s also not advisable to charge HFCs and HCFCs refrigerant at an outdoor ambient temperature below 55℉ because it can cause inaccurate pressure readings, leading to potential overcharging and system malfunction.

Different refrigerants require specific pressures related to temperature, so you should always reference a pressure-temperature chart before adding refrigerant to an AC unit.

Step 5. Turn off the AC Unit 

For safety, turn off the AC at the thermostat and circuit breaker. It is possible to purge the air out of the lines while the system is running, but you’ll want to be careful to release air from the vapor side before the liquid side.

Step 6. Connect the Manifold Gauges to the AC

There are a few things you need to check before you’ll be ready to connect the manifold gauges to the AC. Also, take note of which hoses attach to which side of the suction and discharge line:

• Step 1. Make sure the manual loss valves are in the off position. 

• Step 2. Make sure all the gauge set valves are closed. 

• Step 3. Remove the port caps. If locking caps are installed, you may need a key to remove them.

• Step 4. Attach the high-pressure side hose to the liquid line. 

• Step 5. Attach the low-pressure side to the vapor line.  

Step 7. Purge the Hoses 

Next, you’ll need to purge the air out of the lines. Follow these steps before adding refrigerant to an AC unit:

• Step 1. Open the valves on the manifold gauge set and release the air through the refrigerant hose. Wait a few seconds before closing the valve.

• Step 2. From there, attach the refrigerant hose and open the valve.

• Note: Make sure the refrigerant tank is upside down if the system needs to be charged as a liquid. The tank should be clearly labeled to indicate the correct orientation.

• Step 3. Slightly unscrew the manual loss valve on the refrigerant tank to purge the air from that line. 

• Step 4. Don’t forget to shut the valves on the manifold gauge set.

• Step 5. Open the refrigerant tank. 

Step 8. Weigh the Refrigerant Tank 

Place the refrigerant tank on the scale and zero it out. Then, change the unit of measurement to ounces if you haven’t already done so. This will help you keep track of how much refrigerant is added to the AC unit and how much to charge the customer.

Step 9. Attach the Temperature Sensors 

Once you’ve connected and purged the hoses, attach the temperature sensors to the vapor and liquid lines. Once again, you’ll need to know the temperature to determine how much refrigerant to add to the HVAC system. Depending on whether the system uses a thermostatic expansion valve (TXV) or a fixed orifice or capillary tube, you’ll need to monitor the subcooling or superheat.

• Measuring superheat – Superheat = Actual Suction Line Temperature – Saturation Temperature 

• Measuring subcooling – Subcooling = Saturation Temperature – Actual Liquid Line Temperature 

Step 10. Turn the AC Back On 

Turn the AC unit back on at the thermostat and circuit breaker. Wait at least 15 minutes for the pressure gauges to stabilize, otherwise, the readings may be inaccurate.

Step 11. Add Refrigerant to an AC unit

Slowly add refrigerant by opening the valve on the gauge set to the low-pressure suction line. It’s important you only add a little at a time to avoid overcharging the system and damaging the compressor. If you hear a fluctuation in the compressor’s operating frequency, this is a sign that you’re adding the refrigerant too fast.

Note: If the evaporator coil is frozen due to low refrigerant levels, you may need to turn off the outdoor condenser unit and wait for it to melt. 

Step 12. Disconnect the Manifold Gauges 

Once the HVAC system has reached its optimal pressure/temperature, as indicated on the chart, disconnect the manifold gauge and hoses. Close the refrigerant tank before closing the vapor and liquid line valves and disconnecting the low-pressure, high-pressure and refrigerant hoses. Remove the temperature probes, and don’t forget to reinstall the end caps. And that’s how you add refrigerant to an AC unit like a pro!

Safety Tips 

Never attempt to add refrigerant to an AC system unless you are a trained and certified HVAC technician. Not only does the mishandling of refrigerants pose significant health risks, but the Environmental Protection Agency has outlawed unlicensed individuals from purchasing and handling ozone-depleting substances. If you are a professional HVAC technician, be sure to remember the following safety protocols:

• Always wear the proper PPE – Safety goggles, respirators and gloves all PPE that can protect your skin, eyes and lungs from the toxic effects of refrigerant leaks and accidental spills. 

• Work in a well-ventilated area – Whether in its gaseous or liquid state, refrigerant can cause severe physical injury, such as burns and asphyxiation. Working in a well-ventilated space while wearing a respirator and other PPE can help mitigate the risks of refrigerant poisoning. 

• Follow the manufacturer’s guidelines – Make sure you double-check the manufacturer’s manual, product label and material safety data sheet regarding the refrigerant type and charging procedures specific to the AC unit. 

• Keep refrigerant away from heat or open flame – Refrigerants can be flammable, so it’s important to keep them away from heat sources to prevent ignition and explosion.