Problems with leaky ducts

The following diagrams show some of the problems with leaky ducts.

The first diagram shows a leak on the “return” side of the system.  This pressurizes the house, pushing interior air out through “natural” leakage paths.  Either to the outside air, or into the attic.  The diagram shows a summer situation.  In winter the air leaving the house would be warm and humid.  When moving through the walls, or into the attic, it will condense water inside the walls or in the attic space.

If the return leak is in a basement (or garage) pollutants will enter the air stream here. That’s a problem quite apart from the waste of energy in reconditioning the lost conditioned air.

The diagram shows just one leak.  In practice both the return and supply side will have leaks – unless the installation contractor has done a really good job.  What symptoms the house will show will depend on where the bigger aggregate leaks are.  Either way energy is wasted.

The next diagram shows a leak in the supply side of the distribution system.  The house is now at a lower pressure than the outside, and unconditioned air comes into the house through every possible pathway.  This diagram shows a winter condition.  The occupant will feel cold breezes and lower comfort.  The leak, often into an unconditioned space like the basement, garage or attic wastes heat.

I really should have another diagram – just being lazy – to show this same picture in summer.  Then we have warm, humid air hitting the relatively cool inside walls and ceiling.  Again, water will condense out against those cooler surfaces, and will, in all probability case mold growth.

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