This month I would like to discuss replacement gas fired furnaces. Often during inspections we encounter furnaces that are replacements for older gas forced air units. Technically, these replacements should be permitted and inspected by the local AHJ. However, we all know this may not occur.

One of the most common defects found with these installations is the size of the return air opening in the pedestal (common in hallway closet applications). Sometimes the hole is too small, but more often too big. The installer pulls the furnace forward to cover the hole and a gap is left at the back. This gap could allow products of combustion to be pulled into the circulation air system. Open the lower compartment door, stick your flashlight in below the pedestal top and look for light shining into the furnace closet. This typically means a gap somewhere in the return air plenum.

Another common item is the failure to replace a cement-fiber (AKA “Transite”) flue when a plus-80 (mid-efficiency) furnace is installed. Nearly every manufacturer installation manual I have read for these furnaces has a bold sentence “use of non-metallic flue is not permitted” or similar. This to me indicates cement-fiber or masonry flues are not allowed for these furnaces. The problem is the flue gasses from higher efficiency furnaces are cooler than the old furnaces and do not heat the non-metallic flues enough to prevent flue gas condensation. This condensation can result in failure of the flue and/or the new furnace.

Most mechanical codes require a furnace alcove to be 12-inches wider than the furnace. There is an exception for replacements in the IRC (section M1305.1.1) that allows the alcove to be lesser as long as the working space clearances are in compliance with the manufacturer installation instructions.

Many times an old gas furnace is replaced with a new furnace that has air-conditioning. We all know cold air is harder to move than warm air and larger ducts are almost always needed for proper air flow across the coil and into the rooms. Old steel ducts five or six inches in diameter are most likely not large enough to properly transfer colder air. I usually check the manufacturer instructions for guidelines on minimum size duct runs.

Speaking of ducts, often the connections from the new furnace feed plenum to the old ductwork are incomplete, resulting in leakage. Recommend duct mastic be installed at these locations.

We'll talk next month,

Mike Casey
Kaplan Professional Schools
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