Need a little help folks.
Designing a +/- 4000 sf, two story house for a TCU proffessor. Normally around here (Ft. Worth, Texas) the heating/cooling units and the ductwork is located in the (hot) attic. This is not good practice but it’s cheap and easy. I’m trying to keep all of the equipment and ductwork out of the attic. This requires the ductwork to be located in furred down halls. chases and between the floor joist of the 2nd story. This now requires that I know the approximate sizes of the required ductwork.
I have calculated the heat gain (cooling is primary factor around here) for each area. Now I need a good rule of thumb determining the CFM and duct size to handle the BTU’s for various rooms. To do it right and by the book is tedious and time consuming and is not part of my expertise. I just need to know how much space to give the HVAC guy so he can do his job right.
Really. I need a lot of help. Thanks.
Replies
Keeping the duct work out of the attic is a very good idea. But, there are so many variables on the duct work, that probably the only person that can give you the answers you are looking for is the HVAC contractor.
A two story house usually calls for two seperate systems, and 4,000 sq. ft. in Ft. Worth, makes that almost a certainty. The two systems may or may not be the same size, and the location of the returns is going to be a big factor as well.
you say you have a "HVAC guy"
is he a licensed pro? why doesnt he have access to the design software to properly select equipment and size ducting?
do your professor client a favor and get a hvac professional involved, a 4000 sqft home is a big investment, do the hvac system right for him....
Hire a professional HVAC guy who uses Manual J and Manual D.
DG/Builder
Good Morning, Actually, the co who hosts this site has a set of Code books that ghive you the required minimum per building code. (I found at Home depot) one building basic ) or order specific book on HVAC only) has all trades and references the exact size required for heating only ( 2sqin per 1000/btus) for the supply and return air ducts, and also for AC and Heat pumps ( 6sq in per 1000/btus) you then reduce the sizes from the air-handler and reduce by the btu load and then convert btu's to cfm's ( ac= 400 cfm per 12,000 btu's) ask your cervice contractor and if you dont have one ,get one, if you keep your air handler parts clean it makes a big difference on energy costs, Good Luck, Ken H
Hey Blue. Thanks for the info. As it turns out I have thoes publications. At first glance it looks as if this will get me the info. I need.
To answer the other folks that posted. I'm sorry that I did not make it clear that was the designer only (been at it for over 35years) and not also the builder/GC. I am not sizing the system. I am just trying to give the HVAC contractor, who ever it may be, the space he needs to do his job without having to cut, chop and jury-rig things after construction is underway.
The house will have two systems. The first floor system will have two zones. The second floor will be a seperate system with a single zone.
Thanks to All
The problem is not having the HVAC ducts and equipment in attics, it is having them in unconditioned space.
What about using a conditioned attic design?
Bill. That is what I'm doing in the second floor attic. On the first floor I have a to get the ductwork from point "a" to point "b" below the floor joist. I can minimize the length of the runs by using furred down areas. It's a part of the design. Thanks.
For each room that you have calculated the cooling load , divide the btu/hr by 21.8 and round to the nearest 10. This will give you required air flow for supply at 20 degrees below the space temperature (standard residential conditions).
For supply ducts, size them at 0.08" of water column static pressure loss/100 ft of pipe to start with and drop the size when you get down to 0.05". You will need a ductulator for this, available from any HVAC supply house and most manufacturer's reps. For short ducted returns, use 0.05"/100' or 500 fpm.
Thanks Tim. I will do just that. Sounds about my tech. level. I still use pencil and paper.