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What is a “spill-free” duct and how does it work?

Brad Hardie | Posted in General Questions on

Make-Up air conundrum……

I was planning to use a heat-pump dryer to eliminate venting issues with my clothes dryer. My wife has heard that heat-pump clothes dryers are maintenance nightmares, and not the good at drying clothes. (We have a big family, and our drying cycles can’t take 2-3 hrs). Happy wife, happy life! Soooooo…………..

I came across what some call a “spill-free” make up air duct…..

What is a “spill-free” makeup air duct and how does the “J” work?
Can I use this strategy for make-up air for my electric clothes dryer or my range hood?
Can I use this in combination with a sealed/gasketed motorized damper for the dryer? What about for the range hood?

What are the drawbacks to the “spill-free” duct? What are the hazards or warnings for using one? What should I be on the look out for if using one?

It’s a super tight and super insulated house in zone 6A.

Thanks – Brad

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Replies

  1. Stephen Sheehy | | #1

    We have used a Whirlpool heat pump dryer for almost three years. It works fine. Cleaning the lint filters is simple. Clothes take longer to dry, but more like 1 1/2 hours at most.

  2. Alan B | | #2

    I have one, there is no nightmare, i clean the main filter after each load, check the secondary after each load (and clean it when i can't see light through it and rinse the foam insert, every 2-4 loads). Once every couple months i use the vacuum to clean the areas where the filters sit. Thats it.
    The filters are rather flimsy so i am careful with them and loads take about 1 1/2 hours as mentioned. It also uses about half the electricity of a regular dryer. If you can still find one buy it.
    It dries very well and clothes come out softer then a regular dryer. I can't explain why but its like using fabric softener in the wash without using it. This has been documented elsewhere so i'm not imagining it.

  3. GBA Editor
    Martin Holladay | | #3

    Brad,
    I have never heard the term "spill-free duct." Maybe you can tell us where you heard about the term?

    I'm guessing that you might have heard about a technique that has no basis in science -- the idea that if you include a J-trap or P-trap in an outdoor air duct designed to admit makeup air in the house, then less outdoor air will spill into the house. The idea is false.

    After I explained why these makeup air ducts don't work, Scott Gibson summarized what I wrote for a Q&A Spotlight article:

    A well-respected home inspector ... once suggested a trap in a line for makeup air for the boiler. “He said that cold air would settle in the bottom of the trap and minimize air leakage,” Maines writes. “The concept makes logical sense to me but we didn’t do it on that house, and I’ve never seen an actual example. Is his logic flawed?”

    Indeed it is, says GBA senior editor Martin Holladay. “The ‘trap in the line’ concept works with plumbing drains, but it doesn't work with air intake ducts,” Holladay writes. “If air is leaking out from the top of the house, air will enter a leak at the bottom of the house — whether or not the leak at the bottom of the house has a trap in the line. A hole is a hole.”

    To verify his understanding of the P-trap myth, Holladay contacted Gary Nelson of the Energy Conservatory for a second opinion.

    “You're right,” Nelson tells Holladay. “The P-fitting maybe increases the resistance slightly, but not much. That ‘trap’ should not be called a trap; it should be called a siphon. It doesn't act like a trap at all. The reason some people like it is that when they install a so-called trap, they usually terminate the pipe so it aims at the ceiling.

    “A pipe without a trap is usually aimed at the floor, causing the incoming cold air to pool at the floor. The pipe with the trap — the one aimed at the ceiling — allows all that air rushing into the house to mix with the air in the room before it falls to the floor, so the temperature of the air doesn't feel as cold by the time it reaches the floor compared to a pipe without a trap aimed downward. But just as much air is coming in, in either case.”

  4. Brad Hardie | | #4

    Thanks for the feedback on the heat-pump dryers.....the more them merrier!

    Martin -
    While researching on the web for make-up air I found this (see below), and wanted to ask a trusted source (GBA) if it was legit and if there was any actual factual scientific data to back it up.....

    https://structuretech1.com/combustion-air-ducts-part-ii-problems-and-solutions/

    I wondered if the return loop (section that goes up) was longer if it would work? I mean my neighbors that live on a hill are always warmer in the AM than my neighbors who live below them on the bottom of the hill (like 10 degrees warmer!). That convection stuff is amazing!!!!

  5. GBA Editor
    Martin Holladay | | #5

    Brad,
    The article you linked to advises creating a J-trap with the outdoor air duct. As I wrote in Comment #3, this trick doesn't work. It has no basis in science. The blogger you linked to wrote, "I don’t have any hardcore proof that this makes a big difference, but I’ve convinced myself that it helps."

    It doesn't help. A hole is a hole. The force pulling air through this outdoor air duct is depressurization -- either depressurization caused by the stack effect, or depressurization caused by air being pulled through a combustion appliance connected to a flue. The depressurization pulls air through the outdoor air duct whether or not it terminates in the shape of the letter J.

    Ideas like this have floated around job sites for decades. Ideas like this are one reason the world needs GBA.

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