# HVAC duct routing advice sought



## Willin (Aug 20, 2012)

Am doing plans for a new two-story house on conditioned crawlspace, to be built in upper midwest. Plan is generally rectangular, with 2000 sf on each of main and upper floors, 4000 sf total.

Crawlspace design has four feet of overhead between rat-slab foor and bottom of 12" I-joist floor frame.

The crawlspace is built with ICF (insulated concrete form) walls and there will be a heavy membrane under the rat slab. The intent is to have a totally sealed conditioned crawlspace with supply and return to keep it same as house . . . warm and dry.

Should the main floor be supplied with under-floor ducting, coming through floor registers? Main floor ceiling height, and second floor also, is 10 feet.

For the second floor, do 12" (actually 11-7/8") I-joists generally provide adequate room for running the supply trunks and laterals, for an in-floor supply system?


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## overanalyze (Dec 28, 2010)

Not trying to sound prickish...but that size of a system needs to be designed by a professional HVAC installer that can also do the heat loss calcs to best size everything.


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## UpNorth (May 17, 2007)

I'm not designing the system, but I am designing the building the system is to serve.


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## thom (Nov 3, 2006)

The I joist depth is inadequate to run your trunk lines. You will need to install soffits. 

The real question is why I joists? I switched away from them specifically because running ducting and plumbing in engineered trusses is a far better way to go. 

Engineered trusses allow you to place chases that line up exactly and speed installation. Doing the design job properly requires designing the systems concurrent with designing the house. 

Installing a 24" engineered truss between floors will allow a single duct system for the two floors, floor registers on the upper floor, ceiling registers on the lower floor. That makes designing the system easier with less pressure loss throughout. The additional (small) cost of the engineered trusses is more than offset by savings in HVAC and plumbing installation with continuing energy savings from a more efficient design.

My plans included truss designs, exact lengths, exact chase locations, then I let the truss engineer fill in the structure. That way, even with varying lengths of floor trusses, the chases will always line up perfectly. Your HVAC contractor will love the ease of installation, your customer will never know how much you have saved him in continuing energy costs.


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## Willin (Aug 20, 2012)

For the upstairs floor supply and returns, I can drop soffits to handle the trunk or trunks. No problem.

I believe that a 14.5" w x 11-3/4" h joist cavity, into which supply air will jump from trunks, is sufficient size for running all the laterals out to the floor registers at room perimeters. No?

In the 4' high crawlspace, there is no reason to complicate things by going in-floor with anything. Trunks and laterals can run wherever it is convenient. No?

For the main floor supply stuff, it may be inconvenient and uncomfortable to work in the crawlspace to do this, and duct sections for trunks will be size-limited due to the single-hatch access and the 4' depth, but what else?


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## CompleteW&D (May 28, 2011)

Willin said:


> For the upstairs floor supply and returns, I can drop soffits to handle the trunk or trunks. No problem.
> 
> I believe that a 14.5" w x 11-3/4" h joist cavity, into which supply air will jump from trunks, is sufficient size for running all the laterals out to the floor registers at room perimeters. No?
> 
> ...


In a nutshell.... a properly designed, sized and laid out system - will make the space a lot more confortable, at much lower energy usages, resulting in the equipment working a lot less and lasting longer.


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## thom (Nov 3, 2006)

Willin, why the I joists between floors? You are increasing total costs while decreasing performance. 

Yes, you can install soffits for Ducts, but why? It costs more money and looks crappy. Yes, you can install two sets of trunk-lines, upper and lower, but why? It costs more and decreases performance. 

Finally, putting ducts in the crawl space means you are probably installing a counterflow furnace, the A coil for the A/C goes below the furnace in that installation, complicating condensate drainage. 

Bottom line, you will get a better system for less money if you install an upflow furnace on the lower floor with your trunk line and laterals installed within the trusses. 

Once you've decided to do the better job for less money by using engineered trusses, you will also have improved the plumbing system while making the electricians job easier. 

I like TJI's BUT, for this application, engineered trusses are a better choice.

When you design, always think about the tradesmen doing the job. The more difficult you make the job, the lower performance you will get. Your tinners will be frustrated trying to work in a crawl space. You are more likely to get kinks, poor joints and poor sealing. The insulation on the ducts will probably not be as well installed. In the future, individuals working in the crawl space are likely to damage the duct insulation and the ducts. Quite simply, installing ducts in the mid-floor is a better installation and it costs less money.


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## Dr Heat (Dec 25, 2008)

Use hydronic heat and minni splits and be done.


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## Willin (Aug 20, 2012)

Well, in speaking with a plumbing and heating outfit local to the site, one who has completed installations at a few new builds and major remodels, withing a few blocks of this job, there is a new approach possible. Actually a couple, and we will pursue both, and see how things compare.

One way, the fully-ducted way, is to use two units, one for main floor and one for upstairs floor. Both are to be located in adjacent attached garage, where location provides direct access to both the crawlspace for main and under-floor space for upper. The design of the house has ceiling drops, large false beam drops, and soffits, all in the main floor space, meant to accommodate trunks. They are actually part of the architectural scheme, and enhance the design.

And yes, the crawlspace will contain all the ducting for feeding to the main floor. This contractor has no issue at all with the work required in the crawlspace. His plumbers have to work there, too. They use 12"h scooter seats on dolly wheels to scoot around, say it is faster working that way than working off ladders up at 9 and 10 feet on main and upper floors.

The alternative approach we'll examine is heating with a high efficiency condensing boiler using baseboard units and euro-style radiators, then cool with a high-velocity system using 2" tubing.

Thanks to all for your input.


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## beenthere (Mar 5, 2009)

A shame to build a new house, and then put a high velocity system in that can't get the higher efficiency of regular split systems.

A well insulated and tightly constructed new home shouldn't need large systems. So the duct work of a conventional system should be that big.


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## flashheatingand (May 3, 2008)

my vote: two systems.


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## DBrownHVAC (Oct 25, 2012)

*Why not in the attic?*

Willin,
Is there an attic above the second floor? If so, why would you want to try to run the ductwork in the space? I'm asking this question because I assume that if you are using ICF for your crawl space that you would also be encapsulating your attic with foam insulation and no venting. If this is the case, then installing the duct system in the attic would be a no brainer, regardless of where the furnace is to be located. The key is that if you are going to locate the furnace in the crawl space or basement is that you will need to include a chase, or multiple chases that are large enough to accomidate the duct risers that will be required to get the air to the attic. While this will add additional length to the duct system, proper duct sizing will insure that adequate air flow isn't an issue.
http://ie3media.com/blog/encapsulated-attics-guidance-for-hvac-designers


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## Shouldz14 (Nov 21, 2014)

As far as ducting goes, can you concert an older gas furnace that vents out a chimney to a side wall vent?


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## beenthere (Mar 5, 2009)

On an 80%, if you use a power venter, you can sidewall vent it.


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