The Most Common Cause for an HVAC Service Call…A Revised DIY.

By David Muir

It’s that time of year again when Air Conditioners are running full-tilt and problems arise.  The most common issue for HVAC failure, especially in Florida, is a clogged Drain Line.  Due to safeguards built into your HVAC circuitry to preclude water damage, a clog will result in a total shutdown and thermostat blackout.  This shutdown can also be intermittent with a partial Drain Line clog.

We’re all aware that air conditioners not only cool but also dehumidify the air in the house.  When that happens, the cold coils in the Air Handler cause condensation to drip into a collection tray inside the unit which leads to a pipe that runs to the outside of the house where the water then drains harmlessly into the soil. 

A common occurrence with this “Drain Line” is that it gets clogged and backup due to a buildup of of dust, mold and mildew and the overflow spills out onto the floor.  Most air handlers are in an indoor closet or attic, so that’s a real problem.  Years ago, a new code was put into effect for HVAC installation to prevent the overflow of condensation when the Drain Line becomes clogged.  This simple device is commonly known as a condensation float switch or overflow switch.  The wired switch involves a float inside a housing within the drain system.  When water backs up, the float rises and signals the circuit board to shut off electricity to the thermostat, causing a total shutdown.  That’s good, it prevents water damage. But it can be a comfort crisis during the Dog Days of Summer.  What can you do?  Of course, call in your HVAC Service.  They love this all-to-common service call, because the solution is so simple. 

To avoid the service charge, here’s how to unclog the Drain Line yourself.  The same procedure is used for preventive Drain Line maintenance.

(Note: Many recently installed HVACs also have a Drip Pan under the inside Air Handler that might have another float switch – that when activated by water accumulation will shutdown the HVAC. That usually occurs because the main Drain Line is clogged so the solution is the same…along with vacuuming the water from the Pan.)

First, turn off electricity to the HVAC system by flipping the circuit breaker.  The Drain Line Outlet is usually aside the outside Condenser Unit.  The most common DIY way to unclog it is by vacuum.  You will need a wet (or wet/dry) Vac.  A decent battery powered wet Vac is convenient, otherwise look for the nearest outlet and grab an extension cord. If using a wet/dry Vac, don’t forget to take the filter canister out of the vacuum tank and empty its dry content before using it wet.  Take off any brush attachments, turn it on and place the nozzle against/around the Drain Line Outlet (Figure 1).  If there isn’t a snug fit try an attachment, use duct tape or grip your gloved 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 until that stops, often less than a minute.  That’s usually sufficient to unclog the Drain Pipe. 

If you stop there the next thing to do is add water to the Drain Pipe Port (Figure 2) at the inside Air Handler to reestablish a water seal in the J-trap* before turn the HVAC on again.  Here’s a link to a smart video demonstrating the basics (and all that most Services do): https://www.youtube.com/watch?v=cFdN2XANBOQ

Unclogging the Drain Line is easy enough but, while you’re at it, it’s best to perform a flush and even use a cleanser to better clear the gunk and kill causative microorgasms.  The Steps for a thorough cleaning are given below.  A flush and cleansing involves accessing the Drain Line Port at the Air Handler inside the house (Figure 2).  Remove the cap and slowly pour a quart of hot water into the Drain Pipe (a funnel helps).  It’s best to do this with the Vac attached and running at the outside Drain Outlet.  Otherwise, pour in the water and then go outside to vacuum it out.  The common cleanser of choice is white distilled vinegar.  (There are cleansers formulated for AC Drain Lines.) 

Next, slowly pour a cup (8 oz) of vinegar into the Port at the inside Air Handler.  Replace the cap.  Let the vinegar work for a half hour.  Then, with the wet Vac still outside and ready to go, suck out the vinegar.  While you are at it, pour an additional quart of hot water into the Drain Line port and vacuum that out. This provides for a thorough flush and confirms an unobstructed flow through the pipe.  It’s always satisfying to see the rendered gunk in the Vac when you empty it out. 

Lastly, add water to the Drain Pipe Port at the inside Air Handler to reestablish a water seal in the Drain Line’s J-trap before turn the HVAC on again.

I’ve got reminders on my calendar for my preventative Drain Line cleanouts, especially throughout the Summer.  Also, a simple year-round preventive measure is to pour a cup of vinegar (or formulated cleanser) into the Drain Port now and then and 30 minutes later flush it with a quart of hot water. As with all HVAC Drain Line applications, first cut the power to the Air Handler, or turn the system to OFF with the thermostat before you begin.

Steps for Flush and Cleansing an HVAC Drain Line:

1. Turn off power to the HVAC Air handler by flipping the circuit breaker.
2. Using a Wet Vac, suck out the Drain Line at its Outlet outside the house.
3. With the Wet Vac still attached and running, at the inside Air Handler pour a quart of hot water into the Drain Port.
4. Turn off and detach the Wet Vac from the Drain Line Outlet.
5. Slowly pour a pint of white distilled vinegar (or Drain Line cleaning product) into the Drain Port and let it work for at least 30 minutes.
6. Attach the Wet Vac to the Drain Line Outlet and turn it on.  With the Vac still attached and running, at the inside Air Handler pour a quart of hot water into the Drain Port.
7. Remove the Vac and pour a pint of water into the Drain Port to refill the J-trap.
8. Return power to the HVAC at the breaker.

*There various J-trap configurations in the Drain Line, some at the inside Air Handler, some at the outside Outlet, and others out of sight. They all work the same and are most often where a clog occurs.