If I cut a neat 1 inch dia. hole in the 10" dia. supply air duct the new hole will act one way, ... or another. What determines if the new hole will act as a vacuum or a new push of air.
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In a positive pressure duct you would have to insert something to get negative pressure at an orifice. Or I suppose you could deform the side of the duct. My heuristic is wind on a house: Pressure on the windward side, suction on the roof and leeward side. This is all for the laminar flow.– pophamCommented Jan 25 at 17:54
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1Please see take the tour to learn how this site works, then consider migrating some of the lengthy explanation below to your actual question post.– isherwoodCommented Jan 26 at 15:23
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2 Answers
A supply duct is under positive pressure - the air handler is pushing air out through the ductwork to the rest of the house. Air will go out from a hole in the ductwork. As an extreme, consider if you cut a hole the size of a typical air vent (e.g., 4" x 10") - you would expect air to come out of it. A 1" hole does the same thing, but at a smaller scale.
A return duct is under negative pressure - the air handler is pulling air from the rest of the house through the ductwork and back to itself for heating/cooling. Air will go in to a hole in the ductwork.
answered Jan 25 at 17:08
manassehkatz-Moving 2 Codidactmanassehkatz-Moving 2 Codidact
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2The fluid physics of the thing aren't quite that simple, but it's probably correct. Commented Jan 25 at 17:13
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2@isherwood Agreed. I thought about the more complex physics (which I vaguely understand, but I am not a physicist - too much math) but I came to the quick conclusion that at these scales (1" as opposed to perhaps tiny pinholes, typical residential HVAC air speed and pressure and not jet engines, etc.) air should basically behave pretty much normally. Commented Jan 25 at 17:16
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In the course of installing and modifying duct work, especially the positive-pressure plenum right on top of an air handler, I've noticed that some gaps yield a leaking stream of air that can be sensed with a bare hand many inches away. The leakage through other gaps, often larger ones a foot or two away, might be barely perceptible. I've never tested whether one of those might be leaking in. That'll be interesting to test next time I have an opportunity. Commented Jan 25 at 19:53
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14 - manassehkatz-Moving 2 Codidact ... Thanks for the informative answer. It is as I thought but this occasional/random reading of a vacuum caused by holes in long runs of active supply ducts had me confused.– BABCommented Jan 26 at 14:52
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1"this is the real question", please @BAB, edit your original question to actually ask the question you're after.– FreeManCommented Jan 26 at 16:18
In a supply duct, you'd get air blowing out of the hole when the blower is running. However, a 1" hole won't give you much air flow.
When the blower is not running, it's likely to suck some air in as the air in the ducts cools and starts to drop (due to gravity) back down to the lowest level in the house.
