Duct design high eff house

[oldagpe]

I'm looking for thoughts on duct design for an energy-efficent house.

The project is a 2400 sf house in the Dallas TX area. Manual J calculations show a 2 ton air conditioner with 800 cfm. More normal houses would have a 5 ton unit.

All equipment and ductwork will be inside the building envelope; nothing in the attic.

A normal house with a 5 ton unit would use cheap stamped air terminals and plastic ducts. With 1750 cfm the air gushes out. With only 800 cfm to be divided in the energy-efficient house, terminal selection is going to be important. In order to get enough throw out of a terminal, terminal velocity targets would be 700 fpm.

Using Manual D at 700 fpm and x/800 cfm gives higher than usual duct friction values for the blower to fight. Is there a better way to design ducts than Manual D for this energy-efficient house? Would an engineered design be worth the effort to save blower electricity?

Your thoughts appreciated.

 

[NCDesign]

I hate to be a pessimist but I just have to ask, are you sure about those load calcs? Is the house partially underground? You would have to have a pretty stout thermal envelope to get away with 1200 ft2/ton. That is double a very generic rule of thumb of around 600ft2/ton for residential, which even for well insulated homes I have found to be a little on the short side. If the house is sealed that tightly are you bringing in some fresh air? In this area it would only take about 50 CFM of outside air to bump your load up to a 2-1/2 ton unit and without looking it up I am sure Dallas is much worse than NC.
To answer your duct question size the duct, including branch lines, for the airflow and size the terminals for the desired velocity. Using arbitrary sizes it might result in an 8" round duct routed to a 4x6 diffuser.
I hate to ask again, but are you sure?

 

[Golestan]

Hi,
I also repeat NCDesign's comments. I live and work in the DFW area and I know that some builders have been pushing the idea of well insulated homes with small AC units. I think they are "undersized". I use 400 sf/ton in the Dallas area for residentails. The owners are used to cold houses even in the summer and will not be happy with a small AC unit.

I know that the Electric bills will be expensive but people are willing to pay for it.

Regards,

 

[oldagpe]

Thanks for weighing in here guys. The numbers do look odd but were run against the house design with EnergyGaugeUSA and HVAC-Calc, an ACCA Manual J calculator. Ventilation from outside is at .35 air changes per hour, both winter and summer.

The only sure thing is death and taxes. So it remains to be seen if this really works.

One practical matter of this abnormal house is using off-the-shelf components. "Normal" air terminals seem to be designed for the 400-500-600 sf per ton. This abnormal design will be smaller air flows, but still needing velocity to get the air across the room. I wonder if anyone has spliced smaller diameter duct between the air terminal and the feed duct in order to raise velocity? Or maybe there are air terminals designed for the energy-efficient house?

Best regards

 

[krb]

Have you looked here? Might be right for your application.

http://www.unicosystem.com/products/four-system-sizes/

 

[oldagpe]

Thanks for the tip, KRB!

I am not familiar with unico as most systems here are of mass quantity. Quickly looking through their manuals, there may be some good ideas to use.

Appreciate it!