How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is a jump duct?
Friday, July 27, 2018
In order to understand what a jump duct is and what it does, we first have to cover a basic principle of heating and air conditioning: when a register (ceiling vent) in each room to delivers air, there has to be a way for air to flow out of the room and back towards the air handler (indoor unit). Otherwise, the system is just pressurizing the room—similar to pumping air into a bicycle tire. Air flows easily out an open door but, If the door to a room is closed, air flow decreases and pressure builds against the walls and ceiling.
But, unlike a bicycle tire, houses have plenty of leaks. Some air returns through the undercut gap at the bottom of the door and gets back to the air handler, of course; but not enough to relieve the pressure, and so air is pushed through any tiny openings around windows, the back of electric receptacle boxes, and gaps between the baseboard and floor, into the outside.
Meanwhile, because the open areas of the house cannot make up the shortage of returning air, negative pressure builds in those rooms, which sucks outdoor air into the home through the same leakage points. This is not good.
Ideally each room of a home should have both a supply and return air register for a system that is perfectly pressure-balanced. But that adds major dollars to the construction cost and can create a maze of ducts in the attic. So a simpler alternative is a ceiling register in rooms with a closable door, like bedrooms, that simply connects to another register on the other side of the wall in an open room. That’s a “jump duct.” also sometimes called a “jumper duct.” It’s a passive duct that works like the old louvered transom vents over doors in South Florida houses of a bygone era, to create an air flow connection between two rooms. The photo at the top of this page shows a jump duct installed into the framing of a home under construction, before the drywall has been hung, and the diagram below also illustrates the concept.
Newer mobile homes and budget site-built homes often have a small louvered vent through the wall over each bedroom door, called a “transfer grille.” It is similar to a miniature transom vent, and is an even less expensive alternative than a jump duct for a return air connection to the open area of the house.
A minor disadvantage is that sound also travels through both a jump duct and transfer grille, so privacy within the room is somewhat reduced. But energy efficiency for the heating and air conditioning system is increased, and jumper ducts also help to reduce the incidence of rooms that are noticeably warmer or colder than the rest of the house.
To learn more about jump ducts, we suggest visiting the U.S. Department of Energy’s webpage about them at:
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To learn more about heating and air conditioning systems, see these other blog posts:
Photo and Diagram - U.S. Department of Energy
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