Rust in the Condensate Drain Line from the Furnace

                               
Nothing can destroy your furnace faster than a leaking / rusting condensate tray. 
                                 
When an air conditioning system or a heat pump is cooling, household air passing across the cold coil in the plenum causes condensation to form. This condensation is collected in a condensate tray . A condensate line carries the water from the tray to a floor drain, sink or pump.

Condensate line connected to plenum of furnace

If the water cannot flow by gravity, a condensate pump is installed to pump the water to a suitable location. The condensate should not discharge directly into a plumbing vent or stack, nor onto a roof.

While the condensate tray is not visible (in the plenum), there is sometimes evidence of a malfunction. Water stains can sometimes be detected on the top of the furnace indicating a cracked or broken condensate tray, a condensate tray which is not level, or rust in the drain line, or a condensate tray which has a plugged outlet  and is overflowing.

Condensate line draining to floor drain and contains rust

Rust in condensate line indicates a problem

It is essential that the condensate tray function properly or the excess water will drip onto the heat exchanger (the most critical component of the furnace). Water dripping on a heat exchanger can rust it prematurely, requiring furnace replacement.

The condensate line which takes the condensation from the condensate tray to a drain can be leaking, missing, broken, or plugged. On systems requiring condensate pumps, it is important that the pump be inspected monthly. While the pumps are relatively inexpensive and are easily replaced, they are prone to failure.

The image above illustrates what happens when a condensate tray leaks, risking costly damage to the furnace heat exchanger below. 

Some problems with the condensate collection tray inside the plenum (air handler) can be:

• the drain in the pan itself is clogged with debris - you'll need to turn off and look inside the air handler
• the drain pan is corroded through and leaks - it will need patching or replacement;
• the drain pan or the whole air handler is not properly leveled - it may be sloped away from the condensate pan drain opening so that condensate collects in and runs over the edge of the pan in another direction.

How to Clean or Clear an Air Conditioning Condensate Drain Line

There are plenty of articles on the topic:

How to Clean or Un-Clog an Air Conditioning or Heat Pump Condensate Drain Line or Trapp

A/C condensate drain cleaning video HOW-TO

How to clean your clogged ac condensate drain line in 5 minutes

Condensate Line Maintenance Tip: If the condensate line becomes plugged system condensate overflow will spill onto and rust and possibly seriously damage the furnace heat exchanger. During system operation you should periodically confirm that condensate is being discharged properly through the pipe.

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