How to Diagnose & Repair a Broken Air Conditioning (A/C) Unit
I’m excited to let you know that today’s Pro-Follow is the work of a new contractor named Chuck Thompson. Chuck is the owner of County Comfort LLC and an experienced HVAC specialist. Look for his information to appear on our Meet the Pros page soon and more Pro-Follows to come. Chuck received a service call that a homeowner’s air conditioning wasn’t working, and he let me shadow him on the job.
A homeowner’s A/C can quit for several different reasons. In this case, the homeowners realized that there wasn’t any cold air circulating, and on further inspection, they found the indoor air coil wasn’t even cold. This is the information Chuck had when he arrived.
Diagnosing the Problem
Chuck performed a few diagnostics to determine what the problem was. Here are the steps he took (most of which homeowner’s can do too).
Step 1: Check the Air Filter
Chuck started with the easiest diagnostic of examining the air filter. The homeowners was using a 3-month filter, and it was almost due to be changed. As you may expect, it was quite dirty which hampers air flow causing the unit to work even harder to regulate the temperature.
Instead of trusting the manufacturer’s instructions, Chuck suggests visually inspecting the filter periodically or simply changing it monthly.
Pro-Tip: Inexpensive filters, changed monthly work just fine to protect your HVAC equipment. Consider upgrading to pleated filter to better remove allergens.
Step 2: Check HVAC Fuse
Since the outdoor unit was attempting to cool air, it was unlikely that the fuse had gone bad. Even so, Chuck grabbed his multimeter and checked the fuse and the fuse box.
Chuck checked the fuse by setting the multimeter to the ohm function and touching the probes to either side of the fuse. A good fuse will read zero (or about that) resistance, and a bad fuse will read infinity or OL on some multimeters. He did the same for the fuse box and found both were functional.
Step 3: Check Freon Levels
The next diagnostic Chuck performed was a coolant pressure check to look for any leaks. He connected a pressure gauge to the high-pressure and low-pressure lines.
Pro-Talk: The small diameter line is called the high-pressure or liquid line, and the large diameter, insulated line is called the low-pressure or suction line.
Next, he compared the readings to the pressure charging table found on the side of the unit.
Chuck found that all the readings were within the normal range.
Step 4: Check High-Pressure Line Temperature
The next step was to check the high-pressure line temperature and compare that to another chart. He placed his temperature probe along the high-pressure line and wrapped it with an insulated tape. Chuck found the temperature was within range too.
Pro-Tip: The new freon, R410A, is actually composed of three different types of freon.
Step 5: Check Air Temperature Change
Back inside, Chuck measured the temperature of the air after the coil and the temperature of the air in the return, and ideally the difference should be 18° (16° – 20° is acceptable). He found this temperature difference was 17°.
Pro-Tip: You can check these temperatures with a simple, digital cooking thermometer.
Step 6: Check HVAC Drain Line
The last thing Chuck examined was the HVAC drain line to make sure it wasn’t clogged and that the trap wasn’t too large (inhibiting drainage). While a little nasty, he found it working properly.
Pro-Tip: During the winter months, water in the drain line can stagnate and create a blockage. That’s why it’s recommended to pour a little bleach through the line.
Pro-Tip: In some areas, an emergency drain line is required, and if you ever see your HVAC draining through the emergency line, you know there’s a problem with the primary drain.
Pro-Tip: Some drain lines feature a float that will automatically shut off the unit should the drain line get backed up.
The Problem and Solution
Ultimately, the dirty air filter was the culprit, and after changing it out, the HVAC system functioned properly. It doesn’t seem like such a mundane thing could be the source of the problem, and hopefully this is a reminder of how important simple maintenance can be.
OnTime Air Filters is a subscription service that delivers replacement air filters. All you have to do is pick your air filter type and how often you’d like them to ship. When OnTime sends a new filter, that signals it’s time to replace the old one. Find out more at the OnTime Air Filters website.
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