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AC Unit Blowing Warm Air - DIY Check & Add More Refrigerant?


“Low on Freon.” You’re probably familiar with the phrase and if you’re like most homeowners, this sounds like a problem that’s fixable with a topoff or refill.

Right now, you may be wondering how to check the refrigerant levels in your AC or even how to add refrigerant to your home’s unit. However, determining whether or not the refrigerant is the issue isn’t quite as simple as reading a gauge, and your AC can’t simply be “topped off” with coolant in the way most people imagine it. Finally, if low refrigerant is the source of your AC woes, it’s likely you’ll require more than just a freon recharge – you’ll need professional repairs.

Let’s talk more about why.

Why AC Refrigerant Can’t Be “Topped Off”

When people ask us if they can add refrigerant to their home AC, the short answer is “not likely”. That’s because unless you’ve been trained in HVAC, you’ll spend more time, resources and money learning how to than you’d ever spend on professional diagnostics and repairs.
Asking how to put refrigerant in an AC unit is a little bit like asking how to put jet fuel into a plane; it’s not as simple as pulling up to a pump and grabbing a nozzle.

To add freon or another coolant to your AC, you’ll require professional protective wear in addition a variety of monitors, hoses, gauges and other specialty equipment that will allow you to pump refrigerant into your unit. You’ll need to know how to safely diagnose a coolant issue, how many pounds of refrigerant to add so the unit isn’t over- or under-charged, how to monitor the charging process, and so much more.

How To Check AC Coolant Levels

Your freon “levels” are indicated by the differences in temperature of coolant vs. water during evaporation and condensation.
Normally, these differences lie within 10 Kelvin for evaporation (here known as superheating) and 5 Kelvin for condensation (subcooling). Differences outside of that would indicate a problem with the refrigerant.

Here’s how to calculate the temperature differences during subcooling and superheating:

First, determine what type of refrigerant your AC uses. Freon is one type of refrigerant but there are many others. Information about your system’s coolant should be located on the installation plate.

Next, check your unit’s pressure gauges, which will indicate the suction and discharge (high and low) pressures. Some systems may not have gauges, in which case you’ll need to locate the official documentation for your unit.

To determine the evaporation and condensation temperatures specific to the coolant used in your unit, you’ll use a refrigerant slider tool or a refrigerant slider app on your phone. Using the “dew” slide for for evaporation and “bubble” slide for condensation, select the correct refrigerant type, and enter the suction/discharge pressures. Document the resulting temperatures.

Then, using a digital thermometer, measure the temperature of the refrigerant suction line at the compressor inlet on the outdoor unit, as well as the temperature of the line just before the expansion valve at the evaporator unit or indoor air handler. Document these as well.

Finally, subtract the evaporation temperature from the suction pipe temperature to determine the difference during superheating. To get the subcooling temperature difference, subtract the discharge pipe temperature from the condensation temperature.

Too much superheating and too little subcooling are indicative of refrigerant levels that are too low.

Why You Need a Professional If Your Coolant Is Low

Even if you’re able to determine that your refrigerant is low, adding more is a complicated and potentially hazardous process. Perhaps even more importantly, it doesn’t fix your underlying problem.

The refrigerant inside your HVAC system doesn’t get used up, it simply transforms from a liquid to a gaseous state and back again, always remaining at the same level as it was when it was first added – unless you have a leak.
In addition to poor cooling, signs of a refrigerant leak include hissing sounds coming from the unit, leaks in the attic or around your closet unit, spikes in electrical costs, and ice on your evaporator coil. When left unchecked, low refrigerant will not only lower the cooling ability of the AC, it can severely damage the compressor, a costly repair.

Coolant is critical to the air conditioning process, so it’s logical to suspect a coolant-related problem if you’ve recently noticed rising temperatures or additional humidity in your home. However, your air conditioner relies on more than coolant to do its job effectively. Your AC might not be cooling adequately due to a variety of issues including lack of maintenance, clogged vents or filters, damaged or dirty coils, a damaged compressor, electrical problems or other faulty components.

Whether you suspect a coolant leak or something else, we recommend calling or scheduling an appointment as soon as possible. A professional will be able to quickly and accurately diagnose your problem and provide the best solution in terms of repairs or replacement.