Why is the AC Hissing?

Air conditioners produce liquid water by design and by the simple nature of physics. Sometimes this harmlessly leaks out around the air conditioner, such as with window units, and sometimes it leaks when a drain gets blocked. There is however, a second leak an air conditioner can develop: refrigerant leaks.


The Cooling Compound

Air conditioners work by exploiting physics around state-changes. When liquid turns into a gas, it can absorb heat.The effectiveness of the state change varies from compound to compound. For air conditioning, we tend to use things like R134a (freon), R12 (phased out/illegal in much of the world now), and even propane. These are all chemicals which have particular properties ideal for cooling. For example, they won’t turn solid at 0 degrees C like water, so they won’t clog up the air conditioner’s tubing and fittings.

These chemicals though have some downsides we can’t really escape. Propane is outright flammable and probably capable of turning your air conditioner into a flaming set piece in the next post-apocalypse movie. R12 destroys the ozone. And R134a is toxic. It causes a wide range of symptoms from headaches to hallucinations and death in the worst case exposures.


The Hissing Leak

When the air conditioner is running or has recently been run, the refrigerant will be highly pressurized. In order for us to make it work, we compress it. We’re cramming a lot of material into a small space, which creates pressure. The refrigerant ‘wants’ to escape into an area of lower pressure. During normal operation, this helps the refrigerant flow and lets us turn it into a gas through an expansion valve.

In the event of a leak, however, this pressure causes the refrigerant to get out into the room. Most leaks are generally very small, which can cause a whistling or hissing sound. High pressure material in small holes tend to be quite loud, just like a high pressure water leak or that whistle on your tea kettle.

This type of leak can pretty much happen anywhere. Refrigerant is needed at all parts of the system, in the outdoor condenser and the indoor evaporator. There’s typically coolant lines connecting these systems together, running through the facility. Anywhere there’s a component of the air conditioner, there is the potential for a leak to occur. This issue is slightly mitigated with rooftop, packaged systems though, where the entire system is outside the building, in a single unit.

Failures like this can come from just about anything. Wear and tear on the fittings or a careless accident with a power tool are the more common causes. Copper pipe after all, is hard to puncture without some beefy tools. Evaporator and condenser coils however, are pretty easily broken, and fittings are held together by just solder. There are opportunities for failure.


Dealing with a Leak

In the event of a leak, evacuate the building. This may call for using the fire alarm or just yelling through the door for everyone to get out. Emergency services aren’t necessarily required unless there are people already experiencing poisoning or trapped inside. It is however, important to clear the building quickly. Refrigerant exposure is not immediately fatal, but it is possible for it to remove all the air in a room. Most people would not recognize the danger before it’s too late.

Once the building is evacuated, leave as many doors open as possible, shut off the HVAC system if it is safe to do so, and contact your HVAC specialist to assess the situation. Depending on the leak location, this could be fixed that day or there may be a wait for parts. Your contractor will have a better idea of when it is safe to re-enter the building. In many cases, your contractor will have gas detectors which indicate what’s in the air around them.

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