Furnace Drain Trap Full of Water? How to Unclog It?

It’s not uncommon to run into trouble with complicated machinery or high-tech equipment with hundreds of moving parts. While a condensate pump on a furnace is simple by comparison, drainage can be a major problem with some systems. Whether you have a clogged condensate drain or just want to prevent one, our guide has you covered.

High-Efficiency Furnaces

High Efficiency furnaces have become more commonplace in the United States, and they are a great way to save energy as they are cheaper to operate. While a system must have an AFUE of between 95-97% to qualify for this class, these furnaces also produce condensation by design.

A traditional furnace only has one heat exchanger whereas a high-efficiency furnace has two. The intense heat from the exchangers will transform exhaust gas into liquid condensation, which is collected by a condensate pump.

A condensate pump will take excess moisture from any AC unit or furnace and collect it in a drain trap. When the water level reaches a certain point, a float inside triggers a switch to expel the water.

Tubing inside the pump connects to a drain line, which in turn sends the water through a pipe to drain our outside. Over time, dirt and dust will accumulate in the pump and become trapped in the lines. If too much build-up occurs, you can end up with a blockage.

Drainage Dilemmas

When a furnace water pump produces excess condensation, the water needs to have somewhere to go. Furnaces can have two drains inside as well – one for the vent and another for the heat exchanger. Water collected by the system goes into a trap or collector, and that’s generally where most blockages occur.

Another issue are systems with poorly designed drainage. All condensation lines should be sloped toward the drain with a ¼” slope per foot at a bare minimum to ensure proper drainage. Is the drain line in an area near an air outlet or somewhere that could cause water to freeze? That can be an unforeseen issue in the winter.

In most cases, clogs occur naturally over time as components become dirty and carry debris to the drain. Algae can also muck up the drainage system on a furnace drain trap. Depending on the location of your unit, this could result in water damage when the trap overflows. That includes wet drywall and the potential for mold along with mechanical failure on the furnace.

Cleaning the Furnace Drain Trap

There are a variety of ways you can clear out a furnace drain trap, and some can vary depending on the style of furnace in your home. Regardless of the model, the first thing you need to do is remove the access panel and locate the trap. Then you can choose one of the methods below depending on what you have at your disposal.

Shop-Vac Cleaning

A wet/dry vacuum, otherwise known as a shop vac is an incredibly useful tool. Not only can they suck up large debris, but these versatile vacuums can also deal with clogged drain lines. When using a vacuum on the drain trap inside a furnace, you need to completely cut power to the system and keep the pressure switches in mind.

The line from the heat exchanger to the switch needs to be disconnected beforehand or could be damaged by high negative pressure. Take the drain line and place it into the end of the vacuum. You don’t need to tape it in tight or do anything to increase suction as it doesn’t take much to clear a blockage in the trap.

With the blockage clear, reassemble your system and test it out. This video is the perfect example of clearing a trap on an older furnace. There’s also a great tip to help you get your furnace started again if the trap needs a little H2O to get going.

Air or Water Power

Airpower, in general, is the best way to clear the trap on a furnace, and the simplest method only requires a bit of lung power and a piece of PVC tubing unless you have an air compressor and a nozzle. The key is to get past the T and into the bend of the trap which forces air into the section to clear any blockages.

You want to force the air downward, but also need to maintain a tight seal. Depending on the unit, you could use a piece of PVC tubing with some duct tape on the end and manually blow to clear small blockages. Canned air doesn’t have enough power, but a small air compressor will do the trick. With that in mind, you don’t need an extreme amount of pressure, only 30-40 PSI.

Depending on the style of drain line and blockage, you can also use water pressure from a garden hose to clear light blockages of algae and mildew. A good example is the exterior PVC line in the photo below which can be detached from the coupling in seconds and cleared with a garden hose to ensure there are no blockages in the line.

Maintenance Tips

If you want to stop problems before they start, preventive maintenance is the best way to do that. That includes parts of your system like the trap as well, so you can follow these tips to keep your furnace drain running smoothly and the trap from clogging.

At least 2-3 times each year you should rinse out the trap. You can do this with nothing but hot water, to ensure no ports are blocked. A bit of bleach can clear up any mold, and you can simply blow through a line to clear a light blockage of dirt or grime.

While you are rinsing the trap, take some time to inspect the drain lines. Look for any leaks or clamps that are becoming worn unless the drain has been hard-piped. Your maintenance will be limited in that case, although you should still be able to access the trap for a light-duty cleaning as needed.

Final Thoughts

The drain trap is something that’s frequently overlooked by homeowners when it comes to routine maintenance, and as you can see, a part that shouldn’t be taken for granted. A furnace water pump with a blockage can lead to a variety of issues with your unit, and the area outside of it as well.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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