The Most Common Cause for an HVAC Service Call…that you can fix yourself (Nerd Alert)
By David Muir
Have you ever experienced this? For no apparent reason, your HVAC system goes dead. This happened at my house a few weeks ago. The condenser and air handler units stopped working and the thermostat screen was blank. Hmmm, the circuit breaker must have tripped, I feared. But no, the A/C breakers didn’t trip but I reset them off and then back on, anyway. But the HVAC stayed dead.
Next, I turned to the service breakers alongside the units, one in the closet near the air handler and the other outside near the condenser. I tripped them, but still dead. What else could it be?
Maybe the thermostat failed? Although there was no error code, it was easy enough to pop it from the wall bracket to look for a reset button, or something. There was none. But while the wire clips were exposed, I pulled out my trusty multi-meter and discovered – What? – there’s no power to the thermostat. How can that be?
My aging memory finally kicked in, remembering when years ago a service technician resolved the very same issue very simply, before handing me a hefty invoice. I was vexed for not recognizing the issue immediately, and here it is:
It’s common knowledge that air conditioners cool and dehumidify. The coils in the air handler produce condensation that drips into a collection tray inside the unit which drains to a pipe, ending outside the house in the soil. Often this “drip line” gets clogged from dust, mold and mildew. When that happens, the water in the collection tray in the A/C handler backs up and overflows onto the floor, often in an indoor closet or attic, so that’s a real problem.
Newer HVAC models have a condensation float or overflow switch. When water backs up, a float inside the unit rises and tells the circuit board to shut off electricity to the thermostat, effectively causing a total shutdown. That prevents water damage but can cause a comfort crisis during the hot, humid Dog Days of Summer. What can you do? Well, you can take the hit for an expensive service call, maybe on the weekend. Or, fix it yourself.
First, turn off the system at the thermostat or the breaker. Next, confirm the overflow switch was activated and caused the shutdown by finding a conspicuous device attached to the air handler with a thin wire coming out of the top (Figure 1). (Figure 2 shows a vintage arrangement). Have no fear, it’s a low voltage switch.
Pull the top up or open the hinged lid and look inside (Figure 3). If you spot any substantial water in the cavity (i.e. enough to raise the float), that’s confirmation the drainpipe is clogged. (Note: Most clogs are not complete blockages … they backup slowly and drain slowly, so even a little water in there is telling.)
This drip line’s outlet (Figure 4) is usually near the outside condenser unit and the most common way to unclog it is by vacuum. You will need a wet-dry shop vac. Don’t forget to take the filter canister out of the vacuum tank and empty any dry content before using it wet. Take off any hose attachments, turn it on and place the nozzle against/around the drip line outlet. If there isn’t a snug fit grip your hand around it to make a better seal for full suction. Immediately there will be gurgling and glugging sounds as water and gunk are sucked from the pipe.
Continue the clean out until that stops, usually only a minute or two. That should do the trick! While you’re at it, the next step is to clean the entire line of any remaining gunk microorganisms. The common cleanser of choice is white distilled vinegar. (There are also cleansers formulated for clearing AC drain lines.)
Locate the port with a cap in the drainpipe near where it is attached at the air handler (Figure 1). Remove the cap and slowly pour a cup (8 oz) of vinegar into the port. Replace the cap. Let the vinegar work for a half hour. Then, with the shop vac still outside and ready to go, suck out the vinegar. Then, pour an additional pint of water into the drip line port and vacuum that out too for a thorough flush. Now, go inside, turn the system back on and check that the AC is again functioning well and, thereafter, cancel that pending service appointment.
My house has another newer, more hi-tech HVAC system for the second story. Over the past week the temperature upstairs was fluctuating, not following the programmed schedule and it took hours for it to cool down to the sleep-time temperature setting. Also, the air handler fan was running very slowly 24/7, not as programmed. Now what? Reluctantly I called in a service technician who performed a routine diagnostic procedure and concluded that the drain was clogged. What? I saw water dripping from the drip line outside and there was very little fluid in the overflow switch when I looked a week ago when the trouble started with the other unit. He pointed to the water in the overflow switch. Wait! Wouldn’t that shut down the entire system like previously? He said that doesn’t happen with the newer, more sophisticated units. Instead of shutting down, the system goes into a protection mode that minimizes de-humidification/condensation and allows for the backed-up water to drain out. At that point, the unit starts slowly cooling again. Well, that was another learning experience, which cost me a service call.
From now on, I’ve got reminders on my calendar for my preventative drip line clean outs, especially throughout the Summer. I will also, as a simple preventive measure, pour a cup of vinegar into the drain port now and then, and 30 minutes later flush it with a cup or two of water. Remember, as with all drip line applications, first cut the power to the air handler, or turn the system to OFF with the thermostat, or just set the AC to a very high temperature so it won’t turn on during the procedure. Just to be sure.
