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Whenever our technicians are conducting a furnace cleaning, or even an air duct cleaning, it is standard practice for them to assess the condition of the furnace flue for cleaning as well. Many customers, though familiar with the importance of furnace and air duct cleaning, are unaware that the furnace flue needs to be cleaned regularly as well—for reasons of unit efficiency and more importantly for carbon monoxide safety.

What is a furnace flue?

The furnace flue, sometimes referred to as the furnace chimney, is a roughly 4-inch pipe that funnels exhaust from the furnace and ushers it safely out of the house through the roof. (It’s worth mentioning that in homes newer than 2010, the unit would likely exhaust by way of a PVC “power vent,” not the conventional flue this article refers to.) The furnace flue is often housed in the same masonry chimney as the fireplace flue, but not always.

The technician checks the flue by removing an access panel on the flue itself, or he creates an access point below, near the furnace. (He will clean from this same access point, and after the service, any access panel removed or point created will be safely reattached/closed.) There can also be indications in the furnace itself that the flue is dirty. When the technician finds rust scale or gas sediment inside the furnace, it’s a good sign that the flue itself is compromised, and that its buildup is getting inside the inducer motor, which then essentially “spits” it around the furnace. 

What do furnace flues get dirty with?

​Mostly what dirties furnace flues is, as indicated above, rust scale or gas sediment, which is created during the process of combustion. This sediment is the consistency of beach sand, and, left unchecked, it piles up and can clog the flue. The buildup, in sufficient amounts, can then work its way into the inducer motor in the furnace, causing it to malfunction and, in extreme cases, shut down. 

Various critters sometimes find their way into furnace flues as well, plugging it and potentially jamming the motor. Just a few days ago one of our technicians found a small bird jammed inside a furnace inducer motor, having found its way in through the flue—a messy and unhygienic scenario to be sure! Though in this case a furnace flue cap would likely have guarded against the bird’s entry, it is not a guarantee. Some types of very small or exceptionally flexible critters can squeeze themselves through the opening of even a well-made standard chimney cap. It is not the norm, but we’ve seen everything.

What happens when the furnace flue is clogged?

The biggest danger of a clogged furnace flue is carbon monoxide (CO). When the flue is plugged, pollutants generated during the process of combustion are unable to escape completely. These pollutants, including CO, will backdraft into the house, and this is of course very dangerous for you and your family, as the hazards posed to human health from CO are well known.

All of this risk is of course easily minimized with regular maintenance and checks on your heating appliances and their associated flues, including regular furnace flue cleaning. In this case, a little prevention really does go a long way.

Wanna learn more? Have a look at our Flue Cleaning service page.

View Flue Cleaning Page

Many thanks to our technician Roy S for providing the case notes and quotes for this article.