90% Venting in Furnace

The definition of venting is when you let something out, whether it's hot air of an appliance or hot air of emotions. In this tech tip we are discussing the hot air of a 90% furnace. A more accurate definition of venting pertaining to a furnace is: providing a passageway from the furnace to the outdoors.

The reason you want to vent the appliance is for both health and safety reasons. The process of burning natural or lp gases creates combustion byproducts that are unsafe for your health. If these gases are not vented properly and the byproduct replaces the oxygen, you run the risk of carbon monoxide entering your home, which at certain levels is deadly. Therefore, for many reasons these combustion gases must be vented.

An appliance vent system is rated by category. Always check the instructions to verify the furnace category, to insure it’s vented properly. Typically, 90% furnaces are category IV, but like everything else there are exception to all rules so check your instructions.  Category IV means the furnace operates with positive vent pressure. The temperature is typically at or below 140 degrees. Due to the low exhaust temps, the exhaust byproducts will condense inside the vent pipe. The exhaust will condense and run back towards the furnace drain system, so a proper drain is necessary.

Now that we know our furnace is a category IV furnace, we need to decide if we are going to vent it as a direct vent, non -direct vent, or a ventilated vent. In a direct-vent (2-pipe) system, all air for combustion is taken directly from outdoor atmosphere, and all exhaust products are discharged to outdoor atmosphere.

In a non-direct-vent (1-pipe) system, all air for combustion is taken from the area adjacent to furnace, and all flue products are discharged to outdoor atmosphere. Please be aware that with this system, the space that you are taking combustion air from must be adequate. There must be enough of air for combustion, and it must be uncontaminated.  

In a ventilated combustion air option, the vent terminates and discharges the flue products directly to the outdoors like a direct vent system. All air for combustion is piped directly to the furnace from a space that is well ventilated with outdoor air (such as an attic or crawl space) and the space is well isolated from the living space or garage. The furnace instruction may make that decision for you. For example, a furnace that is multiple stage and is 97% efficient is not allowed to be vented as a non-direct.

All three methods have one thing in common and that is the exhaust/vent pipe, which takes the combustion byproduct outdoors. This exhaust pipe cannot be installed half hazardly.  Horizontal portions of the venting system need to be supported to prevent sagging. Slope the vent and combustion air piping downward towards furnace a minimum of ¼ in per foot. Failure to support and slope the pipe can result in nuisance short cycling, frozen vent termination, and/or no heat situations.

Now, the type of vent system has been decided. We also know it needs to be pitched and supported, but what about the type and size of pipe to be used? Combustion air and vent pipe, fittings, primers, and solvents must conform to American National Standards Institute (ANSI) and the American Society for Testing and Materials (ASTM) standards. The material types are numerous, and you should consult the installation instructions. The most popular material used is schedule 40 pvc pipe. The installation instruction will provide a chart to indicate the correct size to use. The chart will give you maximum length based upon how many elbows there are. It’s important to use the correct size vent pipe, otherwise issues like flame disturbance or the pressure switch not closing can occur and lead to a no heat situation.

There is one last thing to discuss about venting and that’s the termination. The termination is very important and has specific requirements - location and clearances. Locate termination should be positioned where vent vapors will not damage plants/shrubs and air conditioners.  It must be kept from operable windows, doors, or intakes of any type. Consult the installation instructions for other items listed, such as electrical meter and gas service regulator.

Each manufacture has their own method of terminating the combustion air pipe and the vent pipe. These instructions should be followed. If not, you can cause damage to the heat exchanger and other furnace components. Two major furnace manufactures both say the two pipes coming out of the building structure should be right next to each other. The combustion air, which brings air into the furnace should point down and the exhaust should snorkel up and then elbow out. Keeping in mind the combustion pipe and exhaust pipe should be 12” vertically apart and they should be above anticipated snow levels. Shown here to the right is an illustration of the termination and some guidelines.

To learn more on topics related to 90% venting, check out our previous tech tips on the Importance of Draining Condensate and Condensate Draining in Gas Furnace.

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