Why is my high-efficiency gas furnace venting hot exhaust?


Your high-efficiency furnace is supposed to keep your home warm during the winter by creating heat and distributing it throughout the house. You pay your energy bills with hard-earned money and you want to make sure as much heat as possible stays in the house and is not wasted. So why is your high-efficiency furnace venting hot air to the outside? Are you losing heat—and money—through the exhaust?

Let’s take a look at how your high-efficiency furnace, also called a condensing furnace, creates heat.

What is a high-efficiency furnace?

Your gas furnace is fueled by natural gas or propane. Every gas furnace has an energy efficiency rating—its Annual Fuel Utilization Efficiency (AFUE)—based how much fuel it can convert to heat. Newer high-efficiency gas furnaces have more than 90% AFUE, which means that 90% of the fuel is being converted to heat and only 10% or less is lost through exhaust. Compared to older furnaces with 50 to 70% AFUE, that’s impressive! This translates to energy cost savings for homeowners because more of their natural gas or propane fuel is transformed into usable heat.

How is it different from a traditional furnace?

The process of creating heat also creates byproducts or waste materials. In the case of a furnace, the byproduct is hot water vapor. Of course, you don’t need or want these byproducts sticking around so they need to be captured and disposed of somehow. Traditional furnaces typically route exhaust gases through a vent system in the roof or chimney. However, a significant amount of heat can be lost when the water vapor is vented through the chimney to the outdoors.

To do a better job of using all of the heat that is created during the combustion process, high-efficiency furnaces use two heat exchangers instead of one. The second exchanger captures heat from the water vapor that would otherwise be vented out of the home. The exchanger extracts usable heat from the vapor in a process that cools the vapor enough that it condenses or turns into a liquid. If left in your pipes, this acidic condensation can damage your pipework and create health and safety hazards.

For this reason, high-efficiency furnaces are specially vented using PVC pipes that are engineered to withstand the acidic condensation. Some pipes drain the liquid condensate into a floor drain and other pipes carry exhaust to the outside. The exhaust pipes must be vented to the outdoors and cannot (and should not!) be vented through your chimney. Since the exhaust pipes are now located on the side of your home instead of inside your chimney, they’re more noticeable to you. It may seem like you’re losing energy and heat through the exhaust, but it’s actually nothing to worry about.

When should I worry about a hot furnace exhaust pipe?

It’s normal for your exhaust pipe and the exhaust to feel warm. It’s also normal to see icicles or other icy build-up around the pipe during freezing weather. If your pipe is very hot, call Hoosier Indoor Air so we can have an experienced and qualified technician check it out. We’ll make sure your pipes are venting correctly and will take a look at your heating system to confirm everything is working as it should be.

Proper and regular system maintenance can help prevent damage from acidic condensate in your home. Our Comfort Guard Protection Plan can help keep your high-efficiency furnace running smoothly by providing service reminders, cleaning inspections and discounts on repairs, should you ever need to have your furnace fixed.

If you have questions about installing a high-efficiency furnace or are concerned about the way your furnace is running, give us a call. All of our employees are certified and professionally trained. We’ll give you our honest, expert recommendation on a solution that’s right for you and your family.

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