[Shophound]

In general, the duct should be inside the building envelope which means inside the insulation
Just because you said the crawl or attic space doesn't mean this is impossible since the current thinking is to "basementize" your crawl or "cathedralize " your attic.
The crawl conversion is probably the easiest so why not consider adding the insulation and vapor barrier and use the crawl space

Plus one.

My :.02: for the OP...if you have no plans to insulate your roof deck, keep the furnace AND the ducts out of the attic. It will save you $$ and comfort compromises over the long haul.

Ensure all duct joints are sealed, regardless of where you eventually decide to locate them. That means duct mastic on all duct joints, seams, connections, and plenums. Also, ensure wherever a supply or return duct pokes through your ceilings, walls, or floors, that the gap between the duct box and the drywall is sealed. A little time spent doing this will reap comfort and energy saving dividends for years to come.

My vote in your case would be for equipment and ducts to be in the crawl space, with the strong encouragement for you to "basementize" the crawl space (that is, insulate the crawl space walls and put plastic on the ground). If you live in a humid climate, ventilating a crawl space is an exercise in futility.

 

[Shophound]

I live near Los Angeles, so humidity is not a problem.

Our single story house previously had stratified air in winter, that often made it uncomfortable. The registers and return were both in the ceiling, so the cold air leaking into the house just sat on the floor. I know a tight envelope is supposed to fix that, but I've never experienced that, and I am quite paranoid about stratification now that we will be getting a 2 story home in a remodel.

Most contractors here recomment 2 separate systems, but that doesn't prevent stratification.

The idea is to have returns near the floor on the first story and near the ceiling on the second story. This should help to keep the air mixed year round, even if the ducts placement and velocity themselves don't provide very good mixing. As an added tweak, if each return is sized to be sufficient by itseld, I could, occasionally block one of the returns to take advantage of stratification when it suits (let the upstairs roast on hot summer days).

The crawlspace is tempting - it is the best way to minimize most duct runs. If we decide to put the HVAC in the crawlspace, I assume the registers go in the floor. Aren't floor registers more of a problem due to blockage?

Stratification within a dwelling, apart from the HVAC system, is caused by uneven heat gain or loss through the building shell, and air leakage. The thermal performance and comfort levels of a marginally insulated house can be significantly improved merely by tracking down and stopping the air leakage. The trick is to know where the leaks are, and how to stop them.

With your house being two stories, one system serving both floors offers a lot of challenges, whether you put the equipment in the crawl space or the attic. Floor registers are more difficult to work room furnishings around, and one must use caution to see they never become blocked by curtains, furniture, or other objects. Ducts for the level of the house furthest away from the air handler will need to be chased between floors, meaning either the second story gets floor registers, along with the first, if the equipment is located in the crawl space, or the second story gets ceiling registers, along with the first, if you choose to put the equipment in the attic. To properly chase ducts between floors, including returns, means not using wall cavities, and it means creating a proper chase to enclose and protect the ducts. Eats up a bit of floor space, which many people do not like.

There is no excuse for poor supply register velocity and volume if you are starting from scratch with new ducts. The registers should be located to where they can throw the design volume of air per room so that the air in each room is thoroughly mixed when the system is running. Floor registers should throw the air straight up (vs. deflected in a wide pattern as is common with floor register designs), and they work better if the air washes a wall and ceiling. Returns...have one per floor, and any room without a dedicated return and where the door will be closed a good part of the time, such as a bedroom, should have transfer ("jumper") ducts to allow the same amount of air to leave the room as is introduced to it by the HVAC. Again, do not use wall cavities as return chases. Repeat, do not do this. Use dedicated, sealed ducting only.