Ice on a rooftop unit looks wrong. It is wrong. A system that is supposed to cool a building should not be growing ice on the equipment, and when it does, it means something has gone sideways in the refrigeration process.
Here is what is happening and what you can do before the technician arrives.
Why Ice Forms on HVAC Equipment
Ice forms on the evaporator coil when the coil temperature drops below 32 degrees Fahrenheit. That should not happen in a correctly operating system. When it does, the cause is almost always one of three things: airflow is too restricted, the refrigerant charge is low, or the system is short cycling.
Restricted airflow is the most common cause in commercial buildings. A filter that has not been changed in four or five months in a commercial space can restrict airflow enough to cause coil icing without any other problem. A failed or slipping blower motor creates the same restriction. So does a blocked return air path.
Low refrigerant charge changes the pressure-temperature relationship inside the evaporator. When the charge is low, the evaporator runs colder than designed. Combined with even moderate airflow restriction, that cold coil ices over fast. Short cycling, where the system starts and stops in short intervals instead of running through normal cycles, does not allow enough warm return air to flow across the coil between starts. The coil stays cold and ice accumulates.
What to Do Right Now
Turn the system to fan-only mode. Most commercial thermostats have a setting that runs the blower without the compressor. This moves air across the coil without adding cooling, which allows the ice to melt without flooding the drain pan.
Do not shut the system completely off if you can help it. The ice needs to melt, and melting it with the fan running is more controlled than letting it sit and thaw on its own. Check the filters. If they are visibly clogged, change them now if you have replacements on hand. Two minutes and it may solve the problem entirely.
Check all return air grilles and supply vents in the building to make sure nothing is blocked. Furniture against a return grille, a storage box in front of a duct, a closed damper in a section of the building, all of these contribute.
Watch the condensate drain. When the ice melts, that water needs somewhere to go. If the drain pan overflows or the condensate line is backed up, you can get water damage separate from whatever caused the icing.
What Not to Do
Do not pour hot water on the ice to speed melting. Do not chip the ice off. Do not run the system in full cooling mode while it is iced over. Running a compressor trying to push refrigerant through an ice-blocked evaporator puts serious stress on the compressor and can convert a fixable problem into a compressor failure.
After the ice melts, run the system briefly in cooling mode with someone watching it. If it starts building ice again within an hour, shut it down and wait for the technician. A heavily iced coil typically clears in 30 to 90 minutes in fan-only mode, sometimes two to three hours if the ice buildup is significant. Fan-only mode keeps air moving through the building without conditioning it, which is not comfortable in summer heat but is not dangerous. If changing the filter resolves it and the system runs normally afterward, you may not need an emergency call, but do schedule a maintenance visit soon. A filter dirty enough to cause icing means the maintenance schedule needs attention, and if the problem returns after the filter change, there is a refrigerant or mechanical issue that needs a technician regardless.
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