Well, it’s been hot lately in Louisville. Real hot.  Certified Comfort has been running full speed six days per week.  My running and training schedule has been put on hold. Good chance your air conditioner has been running hard, too.

Some of you have experienced a frozen coil. The big line between the indoor and outdoor units is covered with ice. If you removed the coil panel, you would see a big blog of ice there as well. Next step is a big puddle of water under the furnace when you stop the unit from running.

So, what causes a frozen coil? Two things primarily cause it: low load and low refrigerant (R-22, R-410a, Freon, Puron, etc.) charge.

Low load. What is that? Well, it’s usually an extremely dirty super high efficient allergy preventing cat hair catching filter that was just changed six months ago. Yep. My favorite. 3M filters. Paul Harvey and Rush promote some nice near-cardboard dense filters, too.

Seriously, I’m not a fan of the high dollar 1″ thick filters. Very few duct systems are designed to handle the pressure drop of the filter. This high pressure drop causes low air flow across the indoor coil. In turn, this makes for a very cold coil. When then filter loads and gets plugged, the airflow can be reduced to a point that the coil temperature is less than 32°F. With a coil that cold, the moisture in the air freezes on the coil surface. If the system runs long enough, you will have a block of ice.

Side note: unless you have an EAC or 5″ media filter, you really only need the cheapo blue filters. Change them monthly when you get your LG&E utility bill. Buy them by the case at your favorite big box store.

The other cause: low refrigerant charge. Now, you may be thinking “Rob when the refrigerant charge is low, the house gets hot. How can this possibly freeze the coil?” Well, here’s the answer.

Low refrigerant charge reduces the pressures of the indoor coil. The lower pressure corresponds to a lower temperature of the refrigerant as it enters the coil. The freezing process starts there. As the system runs and runs and runs, the ice builds and builds. It does take some time to build this ice block.

So, what causes a low charge? Two choices again: a leak or improper charge. Refrigerant should never need to be added to a system unless it wasn’t charged properly at installation.  All systems come charged from the factory with enough refrigerant for a 15′ lineset (the copper pipes between the units). The installing contractor has to add a certain amount of refrigerant for each foot of copper. If not, you’ll get a frozen coil when the unit runs long and hard in the first heat wave.

Well, I hope that you now understand a little better about the causes of a frozen evaporator (indoor) coil. If you have any questions, feel free to ask. Love to help you with questions even if I’m not working for you at this time.