Why Does My AC Freeze?


It’s still hot outside, temperatures have passed the hottest part of the year but your air conditioner is still running through the hottest parts of each day.  Then you realize that the house is growing warmer and the air is rather humid.  A quick check of the vent confirms that lack of cool, refrigerated air and a glance outside shows the strangest sight, your air conditioning unit is covered in a thick layer of air.  It’s still too hot outside for this to be natural, and the air cycling through your vents is warmer than it should be, so what’s causing a premature winter only on your HVAC system?

The Problem

1. Lack of Air Flow – The simplest fix, and one of the most common causes for an iced HVAC unit is an airflow restriction or blockage.  The premise of how an HVAC system works is that warm air is cooled by the super-cooled refrigerant before it’s cycled back into the home.  Warm air is constantly being pushed over the evaporator coils, turning the refrigerant into a gas, before the now-warm refrigerant heads back to the compressor.

When airflow is blocked, that cold air can’t escape.  The refrigerant becomes too cold for the system to absorb heat quickly enough, leading to freezing on the condenser coils.

2. Lack of Refrigerant – Heat transfer within an HVAC unit is based around a pressurized refrigerant.  When a leak develops, the pressure drops over time, leading to lower pressures overall throughout the system.  Since there is less refrigerant, it can’t transfer heat as effectively. The compressor has to overwork to reach the right pressure level (greater energy loss), and the difference in pressure drop at the condenser coils leads to an extreme change in temperature, which causes water in the outside air to condense and freeze on the unit.

Again, once this happens, the ice acts as an insulator, making it extremely difficult for the condenser to shed heat, piping it back into the home.

3. Lack of Warmth Outdoors – There’s an obvious trend in what causes ice on your condenser coils:  inability to transfer heat effectively.  With airflow, it’s a lack of heat being added while with refrigerant the pressure change tries to draw in too much heat from the surrounding air.  This last measure is usually unlikely, but with enough people in the home, it can happen.  HVAC units have trouble operating when the outside air is cooler than 60ºF.  If the outside air is too cold, the pressure change will once again lead to freezing on the unit since there is not enough heat outdoors to prevent ice build-up.

How to Fix It

Long story short: fix the heat transfer problem.  To do this, always defrost your unit first.  Don’t switch the HVAC or heat pump over to heat, simply turn it off.  While your unit is defrosting, check for airflow blockages.  Do you have multiple vents and air registers closed throughout the house? Too many closed vents create backpressure on your system, preventing proper airflow.  Have you changed your air filter within the past three months?  The more you use your air conditioning, the dirtier the filter will get.  A dirty filter makes it difficult for air to pass through.

Sometimes a freezing unit can be fixed with a simple filter change out.  Of course, if the temperature outside is nice enough, try opening the windows instead of running your AC.

If the weather is hot outside, and your filters and vents are all free-flowing, then you’re looking at a refrigerant issue.  Any time you have an issue with the pressurized systems in an HVAC unit, you need to call a professional technician.  For example, at Service Legends, our technicians not only replace and refill the refrigerant used in your unit, we inspect the entire system to find where the leak is to prevent the issue from reappearing.  A technician has the tools and know-how to find where the root problem, not just the symptom, lies and resolve it for future use.

Heating and Cooling Des Moines Since 1997

At Service Legends, we take pride in heating and cooling Des Moines and the surrounding areas. Our HVAC services include furnace and air conditioner installation, repairs, maintenance, and tune-ups. Our lineup of Indoor Air Quality solutions includes whole-home humidifiers, air purifiers, and air filtration systems. We can even help with HVAC financing.

Call 515-657-6634 to contact our Home Comfort Heroes today. We’re standing by 24/7.

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