Monday, November 16, 2009

Flue Season is Here!

No not the swine type, the furnace type! The weather is getting cold enough in many parts of the country that people are starting up their furnaces. Now is a good time to teach your students to check furnace flues during fall seasonal checks. The purpose of this article is not to discuss CO poisoning, but it deserves a mention since furnace flue problems can lead to carbon monoxide poisoning and CO poisoning shares many symptoms with influenza. One big difference is that influenza is normally accompanied by a fever and CO poisoning is not. For more information on CO poisoning check out the Carbon Monoxide Safety Organization web page.

A good place to start discussing furnace flues is to describe the four categories of vented appliances: Categories I, II, III, and IV. These categories are determined by the static pressure in the vent and the temperature of the vent gasses. For reasons of manufacturing and application limitations, Category II and III furnaces are rare. Most furnaces fall into either Category I or Category IV; 80% furnaces are category I while 90% furnaces are category IV. Category I furnaces are vented with type B gas vent. Category IV furnaces are usually vented using PVC. The combustion gas is cool enough to be safely vented through PVC and PVC is relatively easy to seal air tight.

In practical terms the vent gasses in a properly operating Category I furnace will not leak out small cracks because the vent gas pressure is less than the surrounding air. Vent gasses will generally not condense in a Category I flue because the temperature of the flue gas is considerably above dew point. Even though most 80% furnaces manufactured today have induced draft blowers, they still operate with a non-positive pressure vent because of the buoyancy of the hot combustion gas. However, the combustion gas coming from an 80% induced draft furnace is far more likely to condense in the flue than with older natural draft appliances. Oversized vents, single wall vents, masonry vents, or some combination of these can lead to condensation in the flue. Flue condensation can corrode metal vents and cause masonry vents to crack. Severe condensation can return water to the furnace and cause pre-mature heat exchanger failure. These situations most often occur when an older existing furnace is replaced with a newer, higher efficiency furnace. Even though the newer furnaces are designed for regular type B gas vent, they can not necessarily be connected to the old furnace flue. The extra heat in the combustion gas and the dilution air from the draft diverter of the older furnaces combined to make large single wall vent connectors and masonry vents work without condensation. The cooler combustion gas and lack of dilution air in the fan assisted furnaces makes their vent gas more susceptible to condensation. I have seen a single wall vent connector on an 80% induced draft furnace in a crawl space rust completely through and fall on the ground in a single year of operation. The furnace replaced a previous natural draft furnace that operated for many years without problems on the same type of vent. To prevent similar results when replacing an older furnace I recommend using only double wall vent and installing a metal flexible chimney liner when venting into a masonry chimney. An alternative to lining the masonry chimney is to vent horizontally using a power venter and not using the masonry chimney. More information on power venters is available from Field Controls

Another important step is to size the vent. The existing vent for the older natural draft furnace being replaced is often larger than is required for the 80% induced draft furnace. An oversized vent can also lead to condensation. You can download a pdf file on vent sizing from Hart & Cooley

Visual cues that furnace combustion or venting needs attention include: rust on metal vents, condensation weeping from vent joints, or carbon buildup anywhere in the vent system. Your students may run across non-condensing furnaces that were vented using a rigid plastic vent material called high temperature plastic vent, HTPV. This material has been recalled and should be replaced whenever it is found. HTPV recall If they see any of this material they should contact the furnace or vent material manufacturer or to find out what replacement vent material is recommended.

Fundamentals of HVAC/R can help your students prepare for flue season. Details on gas combustion can be found in Unit 37 Gas Fired Heating Systems. Furnace categories are discussed in Unit 38 Warm Air Furnaces. Vent sizing is discussed in Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation. Gas combustion and venting problems are discussed in Unit 41 Troubleshooting Gas Furnaces.


  1. Your furnace is what keeps your home comfortable all year long, so knowing a little bit about furnace maintenance is important for any homeowner.

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