Why Does My AC Ice Up at Night? (Denver HVAC Expert Explains)

You wake up in the morning, the house is warm, and you check the AC to find ice coating the refrigerant lines or the indoor air handler. What’s happening, and why does it always seem to happen at night?

There are three primary causes of AC icing: low refrigerant, dirty evaporator coil, and restricted airflow. But Denver has a fourth compounding factor that makes overnight icing particularly common here — our summer nights regularly drop to 55–65°F, and running an AC when outdoor temperatures are that low creates conditions perfect for coil freezing, even in systems that are otherwise healthy.

Why Ice Forms on an AC System

The evaporator coil — located inside the air handler in your basement, closet, or attic — is where refrigerant absorbs heat from your indoor air. Under normal conditions, the coil surface stays at about 35–50°F. That’s cold enough to condense moisture from the air, which drips into the drain pan. It’s not cold enough to freeze that moisture — the steady flow of warm air across the coil keeps surface temperatures above freezing.

When something disrupts the heat exchange — too little refrigerant, too little airflow, or too dirty a coil surface — the coil temperature drops below 32°F. The moisture that condenses on it freezes instead of dripping. Ice accumulates, blocks more airflow, makes the coil even colder, and the problem escalates rapidly. Within hours, a coil can be fully encased in ice, completely blocking airflow.

Denver Cool Nights: Why Overnight Icing Is More Common Here

Denver’s summer climate has a characteristic that’s unusual for a hot-weather city: dramatic day-to-night temperature swings. July and August days reach 90–95°F, but nights drop to 55–65°F regularly. This large diurnal temperature range is a product of Denver’s elevation and semi-arid climate.

Here’s the problem: most AC systems have a minimum operating temperature of about 60–65°F for the outdoor air. When outdoor temperatures drop below this threshold, the refrigerant in the outdoor condenser doesn’t release heat as efficiently. Refrigerant pressures drop, the evaporator coil gets colder than normal, and ice formation is likely — even in a system with perfect refrigerant charge and clean coils.

Denver homeowners who run their AC overnight when nighttime lows hit 55°F are inadvertently creating perfect ice conditions. The symptoms are classic: they go to bed comfortable, the AC runs all night, and they wake up at 7am to a warm house with a frozen air handler because the AC iced up at 2am when it was 58°F outside.

Low Refrigerant: The Most Common Cause

Low refrigerant causes icing by creating a pressure drop in the refrigerant circuit. When refrigerant charge is low, the evaporator coil operates at below-normal pressure — and lower pressure means lower temperature. The coil drops below 32°F and moisture freezes on contact.

The key diagnostic tell: a system that ices up even when outdoor temperatures are well above 65°F is almost certainly low on refrigerant (or has a coil/airflow issue). A system that only ices up on cool nights (when outdoor temps are below 60°F) may simply be a Denver climate issue rather than a refrigerant problem.

If your AC is low on refrigerant, remember: refrigerant doesn’t deplete — it leaks. Adding refrigerant without finding and repairing the leak is temporary and increasingly expensive with current R-410A prices of $80–$120/lb.

Dirty Evaporator Coil

The evaporator coil surface needs to be clean for efficient heat transfer. As dust, pet dander, mold spores, and particulates accumulate on the coil fins, they act as insulation — reducing the coil’s ability to absorb heat from passing air. This causes the coil surface temperature to drop below what it should be, eventually reaching below-freezing territory.

Denver’s dry climate means that dust in the air doesn’t settle with moisture — it circulates freely and accumulates on the evaporator coil faster than in humid climates. Wildfire smoke adds an additional layer of fine particulate coating that’s particularly problematic because the particles are extremely small and pack densely on coil surfaces.

A dirty evaporator coil also causes higher humidity in the space (because the reduced coil contact area means less dehumidification), reduced cooling capacity, and higher electricity bills as the system runs longer cycles. Evaporator coil cleaning is part of an annual AC tune-up; never skip this service in Denver.

Airflow Restriction

The third cause of coil icing is restricted airflow — not enough warm indoor air moving across the coil to keep its surface above freezing. Common causes:

• Clogged air filter: The #1 DIY-preventable cause. A filter so clogged that airflow is severely restricted will cause coil icing within days. Change your filter. In Denver, every 60 days is the standard recommendation — more frequently during wildfire smoke season
• Closed supply or return registers: Many homeowners close registers in unused rooms to “save energy.” This actually increases static pressure and reduces system airflow, causing coil problems. Keep all registers open
• Undersized return duct: Some Denver homes, particularly older construction, have return ducts too small for the installed system. This is a chronic airflow restriction that gradually degrades system performance
• Blocked return air path: Furniture placed over floor return registers, or doors closed to rooms with supply registers, restricts air return
• Failed blower motor: If the indoor blower motor is failing or has seized, airflow drops dramatically and coil freezing follows quickly

What to Do Right Now If Your AC Is Frozen

• Step 1: Turn the thermostat to FAN ONLY (not OFF, not COOL) — this runs the blower to defrost the coil with warm indoor air without running the refrigerant circuit
• Step 2: Check and replace your air filter immediately — if it’s visibly gray and clogged, this may be the primary cause
• Step 3: Open any closed supply or return registers throughout the house
• Step 4: Place towels around the indoor unit base — when ice melts, it can overflow the drain pan and cause water damage
• Step 5: Wait 2–4 hours for the coil to fully defrost before switching back to COOL
• Step 6: After defrost, if it ices up again within hours (especially in daytime with outdoor temps above 70°F), call us — refrigerant or coil service is needed