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Furnace Replacement Advice: Metal vs. Plastic Venting

Posted by Gregg on Dec 20, 2016 10:22:38 AM

furnace replacementIf you’re considering a furnace replacement, you may have heard that there are two types of venting: metal and plastic. But what does that really mean? And should you be considering one over the other? Here’s what you need to know about the difference between metal and plastic venting.

Operating Temperatures are Key

Whether your furnace is vented with metal or plastic depends on its operating temperature, which is related to the fuel being used. For example, wood and oil burn at very high temperatures, with flames reaching roughly 1100°F and the flue gas temperature (chimney temperature) ranging from 400° to 900° F when burning wood and between 350° and 500° F when burning oil. 

Gas burns at different temperatures depending on the type of furnace and its efficiency. An older furnace that runs between 65 and 82% efficiency burns between 275° and 350° F. That’s lower than wood or oil, but still hot enough to require metal venting.

Newer, higher efficiency furnaces, run much cooler. A furnace running at 90 to 98% efficiency burns at only 85° to 150° F. When these furnaces burn gas, they form a liquid and mild acid condensate (so they are called condensing furnaces). Not only do these high-efficiency models not need a metal vent due to their low burning temperature, but a metal flue would actually rust over time due to the mild acid that’s formed. As a result, the vent of a high-efficiency furnace needs to be corrosion resistant and liquid tight. Plastic — typically PVC or CPVC — is perfect. It would be possible to use high-quality stainless steel, however, that’s very expensive, so it is rarely used.

Types of Venting

A conventional brick or metal chimney relies on the buoyancy of the warm flue gasses to move the gasses up and out. Because the warm air rising only works if it can rise, it needs a vertical vent. This type of chimney is sometimes called "natural venting” or “gravity venting.”

By contrast, new furnaces have an electric-powered blower that pushes the flue gasses out. These go by several names — draft fan, draft blower, draft inducer, or inducer blower. The blower makes it possible to vent out the side of a house — sidewall venting or horizontal venting.

One advantage to the old-style venting system is that multiple appliances can be combined on the same vent. So, the water heater can be vented with the furnace, for example. In some cases, there could be three or four appliances using the same vent, as long as they all work with natural venting. The new PVC venting systems have to follow strict code and manufacturer guidelines and cannot be used for multiple appliances; each has to have its own vent. 

An Easy Way to Know Your Furnace’s Efficiency

Whether you have a metal or plastic vent is a simple way to determine the efficiency of your current furnace. If it’s metal, you know you’re running in that 65-80% range. If that’s the case, it may be time to consider replacing your system with a new, more efficient unit. There are many options available, and over a short period of time, you can recoup the cost of a new system with the energy savings you’ll get in exchange.

Of course, if you’ve got a plastic vent, you can rest assured that your furnace is running in that high-efficiency range of 90% or more. Congratulations!

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Topics: Energy Efficiency, HVAC