Where can I find a duct size
Where can I find a duct size chart to size a trunk duct supplying to multi size supply ducts to vents?
Sizes of heat pumps in regard to tons per unit? i.e. 3 ton unit or 11 ton unit! How big do they get?
How to figure return air requirements and size of air filters to fit? I know the cubic feet.
Thank you

David. Thank you for the
David. Thank you for the clarification. I have always reduced my trunk duct. I also think it is one of the best ways to design duct systems. I just wanted to get some clarification from you. I am always striving to be better. You have some helpful and knowledgeable articles. Keep up the good work. On another note, if you ever have time to give some information on HVAC systems at altitudes above 4000 feet, that would be awesome. There is not nearly enough information out there on this subject.

@Bob, running ducts out to
@Bob, running ducts out to the perimeter isn’t a problem, especially in the scenario you describe. I think Allison’s point is simply that it’s not necessary. I would argue that the air mixing benefit of perimeter diffusers is minimal at best for homes built to today’s energy code or better.

I wasn’t a blogger during
I wasn’t a blogger during your “Twilight Zone” article, but it sure drew a lot of response. Although it is 9 years later, I would like to make a comment about zoning systems. That article drew a lot of comments about equipment manufacturers liking or not liking bypass ducts. Please note that only the zoning system equipment manufacturers who don’t make the heating/cooling equipment support bypass ducts. I don’t know of any heating/cooling equipment manufacturers that support the use of bypass ducts or at least not the 3 HVAC OEM’s that I have worked for. We prefer to only heat or cool the air once before it is supplied to the house for equipment efficiency and reliability reasons.

@Greg wrote: “weve been doing
@Greg wrote: “weve been doing it wrong for years”

So have most contractors. And it’s still being taught… Case in point: ACCA Manual Zr (residential zoning, published in 2012). At least one prominent member of the Zr committee sought to address this issue head on in the manual, but it was not to be. Old habits are hard to break. On the other hand, Carrier (as well as Bryant and other affiliated brands) prohibits bypass with its Infinity Zoning system. And at least one state has banned bypass (a limited exception is allowed).

“We do use a bypass damper though”

All zoning systems with bypass have a bypass damper, no? But it’s the damper that causes many of the problems we see with zoned systems. A motorized damper that “fails open” (or shut), or even a mal-adjusted barometric damper can cause a big performance hit or worse. At best, a well implemented bypass on an air conditioner reduces both sensible capacity and efficiency.

Thanks for the link. Read it.
Thanks for the link. Read it.

Our jury-rigged solution was to keep the bypass attached but leave the damper of the smallest of 3 zones (finished attic) open. The bypass duct terminates into the return for this particular zone so the air moving through it at least experiences some mixing.

Cheers David,
Cheers David,
re: unducted returns…I’ll take your word for it though it won’t change how systems are done up here…
re: perimeter/glass/heat loss…yes the heat loss will increase but if it is not done the downdraft occurs which leads to cold floors and ankle draft…then there is the radiant asymmetry issue that designers ignore
re: condensation…yes…and a nice segue into recent advice by a certain well know health professional to increase humidity levels in homes…this picked up and distributed across Canada by the press without the caveat you noted…since we tend to pressurize our houses its not just condensation on windows its in the cavities…to add insult to injury people then setback theiur thermostats….so picture this….a pressurized house with an increase in moisture and decrease in ambient temperature…

Greg, sorry about that.  I

Greg, sorry about that.  I meant to put a link to my article on that topic but forgot.  It’s in there now as well as in the list of related articles at the bottom of this article.  And here it is, too:

Navigating the Twilight Zone: The Hidden Flaw in a Zoned Duct System