While furnaces are relatively complex appliances, they tend to fail in fairly predictable ways. Components such as igniters or draft inducers often wear out, causing your furnace to fail to light or preventing the control board from passing its safety checks. While these problems can be frustrating, they're also relatively easy to diagnose and repair. If your furnace keeps shutting off, the underlying cause is probably a fairly common one. On the other hand, it may be due to one of these three unlikely issues that can stop your furnace cold.

1. Exhaust Vent Nests

Your gas furnace needs an exhaust vent (or flue) to remove harmful combustion gases from your home. Your furnace's primary job is to extract heat from this exhaust stream, ejecting the now relatively cool toxic gases from your house. Your furnace also uses a pressure sensor to ensure adequate flow through the exhaust flue, preventing carbon monoxide from backflowing into your home.

Unfortunately, small animals sometimes like to nest in the relatively warm flue. Their nests can block the exhaust path, stopping the pressure sensor from being able to detect an adequate exhaust flow. As a result, your furnace may frequently shut down or fail to ignite at all.

2. Draft Inducer Bugs

No, your draft inducer doesn't have a software glitch. Instead, physical bugs can sometimes make their way into the draft inducer assembly. This motor creates negative pressure to push gas through the exhaust flue, making it a critical part of your system. Unfortunately, bugs can crawl through your flue (especially in the winter) and enter the inducer motor or blower wheel.

A severe enough infestation can create numerous problems, including blocking up the blower motor and preventing it from spinning. Bugs can also damage the electrical connections inside the motor. As with a clogged exhaust flue, these problems can result in intermittent furnace operation.

3. Flooding and Rust

Water can be a major enemy for your furnace. Surprisingly, your furnace can also be the source of the problem. High-efficiency condensing furnaces extract so much heat from their exhaust stream that acidic water condenses and flows back into a drainage system. Clogs or other problems with the drainage system can allow this water to flood back into the furnace cabinet.

Most furnaces have safety switches to detect this situation and prevent flooding. However, frequent clogs can increase moisture levels in the furnace, causing critical components to rust. This moisture can damage everything from the draft inducer to the burners.

Contact a local HVAC service to learn more about heating system repair.