Why is there ice on my air conditioner pipes?

In cooling mode, the indoor coil runs cold but normally stays above freezing. When less air flows across it, from clogged filters, a dirty coil or a slow fan, it gets colder, and the moisture it pulls from the room freezes instead of draining away. Low refrigerant does the same thing: pressure drops and the coil and pipes run below freezing. The ice then blocks more airflow, and the problem feeds itself.

Frost on the outdoor unit in heating mode on a cold July morning is different. That's normal, and the unit runs a defrost cycle to clear it, sometimes with a puff of steam and a short pause in heating. Ice that stays, or ice on the indoor unit or pipes while cooling, isn't normal.

Running a unit with ice on it strains the compressor, and meltwater can overflow the drain tray and run down the wall. Turn it off at the remote and let the ice melt on its own, with a towel under the indoor unit. Don't chip or scrape the ice, which can puncture the coil or pipes. Once it has thawed, clean the filters if you can reach them safely, then try it again.

Refrigerant circulates in a sealed loop and isn't used up, so a system that is low has a leak. The Australian Refrigeration Council says topping up a system before it has been fully tested and every identified leak repaired is illegal, and the gas would escape again anyway. The proper fix is to find and repair the leak, pressure test the system, then recharge it to the maker's specification. Refrigerant work needs a licensed technician.