Special Venting Requirements for a High-Efficiency Furnace


To reduce household energy costs, many homeowners replace their old, inefficient furnaces with newer, more efficient models. While this can greatly reduce your home's overall energy costs, replacing an old furnace with a more efficient, newer model requires some additional considerations in order to get the best performance out of the new furnace. Of particular importance is proper ventilation, since new furnaces have drastically different ventilation demands than do older, less efficient models.

Furnace Ventilation

  • To understand how the ventilation needs of newer furnaces differ from those of older furnaces, you must first understand some basics on furnace ventilation. Furnaces need fresh air to create combustion and generate heat. This combustion produces carbon monoxide and some other potentially harmful gases that need to be removed from the home so that they cannot put occupants at risk of exposure. Furnaces are typically ventilated using a chimney or chimneylike pipe; however, newer furnaces offer newer ventilation methods.

High-Efficiency Furnace Byproducts

  • The essential difference between older furnaces and newer ones in regards to combustion byproducts is that newer furnaces take more of the heat produced by combustion and use it to heat the home; indeed, this is precisely what makes them more efficient than older models. But because less heat is sent up the chimney, combustion byproducts are at a much lower temperature than those produced by older furnaces, so that there is often an extreme temperature difference between the gases inside the flue pipe and the air outside of it. This temperature difference produces condensation that can corrode the lining of your chimney and create other problems.

Ventilation Considerations

  • To accommodate this temperature difference, it is usually necessary to replace single-walled chimneys with double-walled ones. The extra layers can help mitigate the temperature differences between the air outside the chimney and the gases inside. Additionally, homeowners often need to replace their chimney lining when upgrading to a newer, high-efficiency furnace. Fireclay, masonry liner or a retrofitted metal flue liner needs to be installed on the inside of the furnace chimney and regularly inspected and maintained to ensure proper ventilation.

Safety Considerations

  • Regardless of the type of furnace you use, the most important part of designing proper ventilation for that furnace is to inspect the chimney at least once a year or oftener to check for any cracks, crevices, deterioration or other damage that could leak potentially hazardous gases into your home. Installing carbon monoxide detectors around the furnace can also help alert you to a furnace ventilation problem before it has a chance to put your family at undue risk. Be sure to check carbon monoxide detectors, and replace batteries regularly, just as you would a smoke detector.

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