Intake and Exhaust Diagnostic

Check your furnace intake and exhaust venting to ensure it's clear of obstructions.

Intake blockages

When it is exceptionally cold or snowy, high-efficiency furnaces with intake and exhaust pipes can stop working due to snow and ice blocking the intake pipe.

Ice blockage of the furnace intake is often caused by moisture-rich exhaust air being drawn back into the nearby intake pipe, where it condenses and freezes. The ice builds up over hours or days, eventually blocking the intake.

On very snowy days blowing snow drawn into the intake pipe can also cause a blockage.

High-efficiency furnaces that draw combustion air from the furnace room and have only an exhaust pipe are not affected by intake blockage issues.

How to diagnose and fix a furnace intake blockage

When the intake pipe is blocked, pressure switches in the furnace sense there is not enough combustion air flowing for safe combustion of the natural gas and force the furnace to shut down.

You might hear the inducer motor cycling on and off multiple times as the furnace attempts to start. There may be a “pressure switch failed to close” error code.

If you think there could be an intake blockage, visually inspect the furnace intake for obstructions. If there is snow in or around the intake, you may the able to remove it with your hand. If the intake is frozen solid with ice, do not attempt to break or remove it — contact a qualified furnace repair technician.

If intake blockages occur repeatedly, have a qualified technician inspect the venting to ensure that exhaust is being directed away from the furnace intake.

Exhaust blockages

Furnace exhaust blockages are typically caused by condensate water collecting at low spots in the exhaust venting due to damage or improper installation.

Properly installed, high-efficiency furnace exhaust venting should be back-graded — allowing condensate to flow back to the furnace, through the collector box, and into the condensate trap where it is drained off to a floor drain.

To prevent furnace exhaust venting from sagging over time most building codes require support strapping every six feet for horizontal runs.

How to diagnose and fix a furnace exhaust blockage

When the exhaust pipe is blocked, pressure switches in the furnace sense there is not enough exhaust flowing and force the furnace to shut down.

The furnace may start up normally and run briefly, before shutting down again. There may be a “pressure switch failed to close” error code.

If you think there could be a blockage but the intake vent, condensate trap, and inducer motor are operating normally, visually inspect the exhaust venting.

The furnace exhaust venting should be slightly graded back toward the furnace for the entire venting run. If there are any low spots, signs of damage, or water leakage, contact a qualified technician to ensure the vent is properly installed.

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Furnace repairs should only be performed by a certified technician.

This website is for informational purposes only. Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.

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