General HVAC Design Issue

Robert Green

Greeting to All,
This is my first post to this site. My first question relates to an overall design issue. I am considering buying a 2 story Victorian home (circa 1900 with addition added sometime later) near the coast in the SE United States. Thus, cooling is more important than heating (summer temperatures often exceed 90 while daytime winter temperatures may approach 32 and as low as 0 during the night). In order to properly frame my question I have done preliminary calculations for heat gain/loss and equipment sizing. I will do exhaustive calculations prior to finalizing or installing anything.
The house is built over an open crawl space about 2.5 ft. high. The total area of conditioned space will consist of 1750 sq. ft on the 1st floor and 1750 sq. ft. on the 2nd floor. Ceiling heights on both floors are 10 ft. The unused attic peaks at about 10 ft. My preliminary calculations indicate two 3.5 Ton individual split system heat pumps for each of the two floors. The house is rectangular in design, with the older section being approx. 36 ft. by 32 ft. and the newer rear section approx. 32 ft. by 20 ft. I have the option to use adequately sized “utility” rooms at the rear end (36’ x 32’) section on either or both the 1st and 2nd floors to locate the air handlers. Optionally, I could locate one or both air handlers in the attic.
In this post, I will limit my question to the 2nd floor main trunk supply design. Some advantages of using a 2nd story utility room for the upstairs air handler are that the unit will be in a conditioned space, very accessible for maintenance and repairs, and less than 25 feet for the line set installation. The main disadvantage, as I see it, is that the length of the main trunk (using an incrementally reduced extended plenum design (starting at about 20” round sheet metal from the plenum) will be at least 60 feet long not counting either the run from the air handler into the attic (about 6 feet) or EL factors. What I would like to do, if I can use this design concept, is run a single main trunk from the utility room through the first 32 feet of the newer addition with a reduction at the 24 ft. point. Then, upon entering the older/larger attic portion of the structure, I want to split the trunk into two trunks with each one running parallel near each side of the attic. This split (each branch which will be further reduced at the 48 ft. point) will allow me to take advantage of the central attic space for storage. Overall, this would require a supply trunk length of about 75 ft. (not including EL calculations) from air handler to end cap on each of the two branches.
If, on the other hand, I centrally locate the upstairs air handler in the unconditioned attic, the individual trunk runs would be cut by about 50%. However, by doing this I would need 40 ft. of line set, as well as having the air handler in unconditioned space and incurring the associated repair/maintenance problems.
Please bear in mind that at this stage I am interested in deciding on an overall design concept and fully realize that much more information and thought will be required to determine a final design. What I want to know at this stage is whether or not placing the air handler at the end of the trunk in the 2nd floor utility room is a good and viable option considering the overall length of the trunk, or, alternatively, would I be better off centrally locating the air handler in the attic. Of course other design options do exist, but for the time being I am attempting to minimize changes to the architectural integrity of the home. Any feedback will be very much appreciated.
Sincerely,
Bob Green

beenthere

Centrally located in the attic with proper planking for later service may work best.

Whils having it in a conditioned space is great. if it adds a large amount of duct length, which means it must also be wider/larger. You lose some of your savings of having it in the conditioned space.

Robert Green

Many thanks beenthere for your really fast response. I do realize that my "long length" trunk idea would be more costly than the centralized attic plan, and believe me, cost is very important to me (I can barely afford the house). But I am very curious to know whether or not the longer system, properly sized, would work as well as the centralized plan. In other words, is there anything about the "long length" plan that inherently makes it a "bad" idea other than cost considerations. Again, thanks for reponding so quickly and thoughtfully. Also, if I do go with the air handler in the attic, is not the 40 ft. lineset a big problem?

beenthere

Pretty much all brands work fine with a 50' line set without any efficiency or capacity loss(or atleast not a measurable one).
Over 50 foot, and there are derates/capacity and efficiency losses. But a 70' line set is often only a 3% loss.

Using larger duct, increases duct heat gain in the attic. A 18"X8"X4'(48 sq ft surface area) duct with R8 wrap with a conditioned air temp of 50°F inside of it, in an attic that is 100°F. Gains 300 BTUs of sensible heat.

A 20"X8"X4'(53.3 sq ft surface area) duct with R8 wrap with a conditioned air temp of 50°F inside of it, in an attic that is 100°F. Gains 333 BTUs of sensible heat.

Thats an 11% heat gain increase, from just 4 foot of duct work. And only on a 2" increase in width. So if you have to increase width by 4". You'd have about a 20% gain in sensible heat for that same 4" length.

Multiply that gain by the additional length, and width of the duct and it adds up to a lot.

Robert Green

Thanks again beenthere for a particularly informative answer. I had assumed that my "cost" question related to upfront installation cost only, and had no idea you were referring to not only installation cost but a significant long term cost relating to overall system efficiency. Viewed in this light, the attic plan for air handler is clearly advantageous despite the drawbacks. It is indeed a pleasure to talk with someone who truly knows what they are talking about!

Because of your quick response, I will need to compose the second part of my overall design issue regarding the return system for the 2nd floor and post it ASAP. (The supply and return issues for the 1st floor design are more complicated, as I will explain later).

I realize, of course, that you are probably overwhelmed attempting to deal with the multitude of questions that get posted, yet hope you will be able to find the time to address my return ducting issues at your convenience after I get them posted. Your help is sincerely appreciated.