“Why is Ice Forming on My Inside Air Conditioning Unit?”

You’re concerned.

Your central air conditioning isn’t cooling your home very well. So you go to check the inside unit and, lo and behold, your central AC’s inside unit is frosted over! (Which you probably find strange considering it’s summer).

What could be the problem?

There are 2 main issues causing this.

1) Weak/Reduced Airflow Over the Evaporator Coil

Reduced airflow to the inside unit will cause the refrigerant coil to freeze up.

First, we’ll tell you what usually causes reduced air flow to the inside unit and then explain the science behind why reduced airflow would cause the unit to freeze up.

What to Do:

  • Change the air filter — A dirty air filter blocks air from returning to the inside unit.
  • Unblock return grills— Move curtains or furniture blocking return vents that block air from returning to the inside unit.
  • Open closed supply vents in unused rooms— Closing supply vents in unused rooms does not save you money. It causes several problems instead, like a frozen inside unit.
  • Have a professional look at the blower — The coil will freeze if the blower’s motor malfunctions in the inside unit and isn’t pulling in enough air.
  • Have a professional clean the evaporator coil— Dirt insulates coils from heat, meaning that they can’t absorb heat from the air being pulled over it by the blower motor.

OK so those are some “to-dos”. But why do you need to do them?

Why Restricted Airflow Causes the Coils to Freeze Up

The evaporator coil (tubes forming the A-shaped part of the inside unit) has cold refrigerant flowing through it. A blower pulls in air from your home and over that coil, cooling the air.

But when there’s little to no air flowing over the cold evaporator coil, it quickly frosts over because there’s less heat to absorb.

When it’s frosted over, air can’t easily flow through the coils (it’s a solid block of ice now), reducing airflow to your home.

Here’s an illustration: Imagine you have a cup of water and every hour you add 5 ice cubes into it. You also have a hair dryer blowing hot air over the cup. Obviously, the ice would melt after you add ice into the cup.

Now imagine that you put something in between the hair dryer and the cup while you’re still adding ice to the cup. Because there’s something blocking airflow to the cup, the ice isn’t melting. This would continue until the cup is completely full of ice.

That’s basically what’s happened to the inside unit.

Follow our instructions from before to make sure there’s enough airflow going over the coils!

2) Low Refrigerant

This is the most common cause of a frozen evaporator coil.

When your AC system is low on refrigerant, pressure drops causing the evaporator coil to get abnormally cold.

So when returning air hits the coil, humidity/moisture from the air beads ups on the coil and quickly freezes. This continues until the evaporator coil is frozen.

If you’re low on refrigerant you also have a refrigerant leak.

That means you’ll need a professional to find the leak and fix it before adding more refrigerant.

Worst-case scenario: The leak can’t be fixed and one of the refrigerant coils needs replacing. This is an expensive air conditioning replacement. So if your AC is 15+ years old, you should consider replacing it.

What You Can Do ASAP:

  1. Turn off your air conditioner
  2. Let the AC defrost
  3. Change your air filter (If it’s dirty)
  4. Open all the closed supply vents your home
  5. Ensure no furniture or drapes are blocking the return vents

If the filter was not dirty and there were no problems with the supply or return vents, you most likely need to call a professional for AC repair help.

AC Experts in Framingham, MA & Beyond

Nicholson Plumbing, Heating and Air Conditioning has a team of expert air conditioning contractors ready to help! If you live in the Framingham-area, contact us to schedule an appointment for AC repair!

Call (508) 881-1500

Our level of commitment to our customer and superior service is the reason you can find comfort in choosing Nicholson Plumbing, Heating & Air Conditioning.

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  • Francis from Nicholson Plumbing and Heating was excellent. He was here to evaluate HVAC in our home. He was very patient and answered all my questions. He is also polite and easy to communicate with him. He explained technical concepts to me in layperson terms!

    - Shelley S.

  • Nicholson offered what is possibly the best customer service I have dealt with in a home repair situation.  I called them because I was looking to replace my very old furnace.  They came out to do an initial estimate, and then came back when I had some further questions about my options.  After installing the furnace, they ran into some problems with my air ducts.  This happened on a Friday, and the problem was fixed by Monday afternoon.  In addition to their excellent service, they did quality work at a fair price.  I would recommend them to anyone looking for an HVAC company in the Framingham area!

    - Melissa T.

  • Recently used these guys because my HVAC unit AC wasn’t working (first hot day of summer).  Showed up on time, wore booties, fixed the unit.  During the visit we talked about how my unit was having problems in the winter – which was resulting in NSTAR service calls of maybe 3-4 times per season (I am on a service plan with them).   He was the first guy to tell me WHY the problem was happening, and HOW to fix it.  I find it strange that all of the NSTAR guys etc. no one took time to point out how to fix the source of the problem…Very professional, smart.  I would use them again, and plan to for the fix on my heating unit!

    - Ben J.

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