Sizing Recomendations needed.

[Vivid]

I am in the process of building a new house and I am trying to be as green as my budget will allow. I wanted to check with you guys what you though this house would need in terms of tonnage? I have heard rough rules of thumb that say an older house should get 1 ton per 500sq/ft but when reading about spray foam insulation the say you can use 1 ton per 1000 sq/ft, a very big difference. Let me point out a few important factors.


I am using open cell spray foam insulation in the ceiling and floor and using JM's Spider spray fiberglass in the walls. The exterior of the house will have house wrap that is taped to ensure airtightness

the windows are non-thermally broken storefront system with double pane, tinted, low-e and insulated glass.


the front of the house faces due south with minimal shade.


the house is 4000 sq/ft but 3300 sq/ft heated, the basement (1/2 basement really) is not going to be conditioned


Ceiling heights are 8' basement, 8' first level, 10'second level and 9'thrid level


the home is located in Atlanta GA


Front of the house (due south)
http://i55.photobucket.com/albums/g142/vividuk/House/Northeast.jpg
west side of the house
http://i55.photobucket.com/albums/g142/vividuk/House/West.jpg
North end of the house
http://i55.photobucket.com/albums/g142/vividuk/House/SouthWest.jpg
East side of the house
http://i55.photobucket.com/albums/g142/vividuk/House/southeast.jpg

thanks for any help,

Mark

[dash]

You need a Manual J (from wwww.acca.org )Heat Loss and Heat Gain Calclation,no guess or rule of thumb will be accuarte.

Then you need Manual S,D ,and T,to be sure you are comfortable.

These are the Standrads of our Industry,a real Pro will have no problem ,with doing these,if he's doing the install.

[flyin'gator]

i would highly reccomend you get a manual j heat load calculation done on this structure. the energy saving improvements you have listed will have a dramatic effect on the amount of a/c tonnage necessary. additionally, i would ensure that you leave adequate spacing for your ductwork between the floors. the ability of your a/c contractor to ensure proper return ventilation will be extremely important in comfort and efficiency. i am not familiar with atlanta, but here in jacksonville fla., we are required to do a load calc and submit it with the permit package before a permit will be issued.

[gonekuku]

Looks like I may be first to reply, and hopefully you don't follow rule of thumb advice...

I looked at the specifications for the spray in spider insulation. Per what I guess from your post, the walls will be framed from 2x4? If so, the Spider spray stuff will yield R-12 to R14. I have seen the old fashion Pink Panther in the same neighborhood of insulation values; therefore it appears that the Spider Spray stuff doesn't possess any magical insulation properties. (1000 sq ft / ton sounds out of reach).

That's a very interesting looking home you are about to build. Be sure you get the thermal loads properly calculated, the system well designed, and zoned if you have it in your budget.

Do you have AutoCAD drawings of this home? If so, get a hold of me. I can verify that you are not being quoted a wrong size system (when the time comes). Then again - after the home is built, this is a great place to get advice on what to do after your HVAC problems start.

What size system? Between 4-7 tons. Maybe less, maybe more.

[Vivid]

thanks for you replies guys... I will make sure to get the manual tests done, I don't want to leave it up to chance when it can be done with certainly... Unfortunately ATL does not require load cals for permitting but IMO it would be a great idea.

gonekuku,

Yea I will be using 2x4s for the framing. My GC is recommending the spider because it is a spray insulation and can get behind and into some cracks giving a proper R value over batt insulation which is meant to leave loads of gaps around electrical outlets and such making the R value void in areas... But we are tapping the house wrap to stop even more air flow which is why we were not going to use spray foam everywhere and try to save some money. I was also told that most heat (60%) is lost through the ceiling and floor, so that is where we focused the spray foam. I don;t think I will be able to get to that 1000 sq/ft rule of thumb, I just hope I am much better than the 500sq/ft rule of thumb... I'd be happy in the middle at 750sq/ft which would be about 4.5 tons... But I will do a load calc to be sure.

[hvacsmart]

This needs correct Manual J calculation. 95% pros do not know how to prepare correct unabridged Manual J. Make sure to get the best guy in the business at least for the calculation.

Do not forget about the ventilation and all the other variables here. Nice structure.

[BaldLoonie]

A friend of mine is a foamer and he recommends these people be consulted to size the equipment.

http://www.energywisestructures.com/

Too many heating dealers like to oversize not understanding just how tight a foamed house is. These people know and will guarantee your bills too.

I saw an e-mail from one of his foamees that a local company put in what they wanted not what these consultants said to do. Real bad humidity issues in the summer.

[pstu]

Looks like a real attractive design. I appreciate you put the major glass area facing South, that is what I would want to do if I ever build new construction. I am not a pro but a homeowner in S.Texas, we have some similarities between your climate and ours.

I would consider it cheap insurance to *pay* for the Manual J from the best guy you can find, even if he doesn't get the final job. Many techs will do Manual J calcs but withhold them until they are sure you won't take their work without compensation and use it somewhere else. And I would really worry the Manual D (and S) problem to make sure your duct system will not be undersized (as mine is), that can rob you of whatever capacity your AC has. I imagine you will be receptive to the idea of a high end, variable speed air handler too, if not a full two-stage AC system.

Best of luck -- Pstu

[Vivid]

We do have an non-profit in Atlanta called Southface.org who promotes green building. They offer a manual J test and a duct layout as part of their residential services... BUT I was warned agaist using them by a HVAC dealer who claimed that they always under size equipment and that the manual J is a minimum and one should always put more that what the manual J recommends.

any thoughts on that?

Also could some one tell me what the maual S and T are? I know the J is for load and D is for duct work but not sure about the others.

thanks Mark

[pstu]

Manual S is for the purpose of selecting equipment, after Manual J heat load has been done. You cannot just read mfr specs because they are tested under standard conditions which are not like Atlanta climate. I believe in your climate and mine, Manual S will specify a certain amount of de-rating of equipment btu/hr output, based on high outdoor temperature. That *might* legitimize what your HVAC dealer said, but I would be watching my baloney detector each time I spoke to him. I am kinda cynical and skeptical.

If you find you must negotiate with this kind of thinking, consider using a two-stage system. I happen to admire the Carrier Infinity control, you might too. However my system is Trane brand, with the 16i and 19i 2-stage systems the 1st stage is 50-67% of the total capacity and much of the time that will satisfy cooling needs. The consequence is long runtimes and better humidity control. They often adjust airflow downward on low stage I am told, in order to improve humidity removal.

Since Trane makes their 2-stage line only in whole-ton units (except a 2.5T model), that endorses the idea of "rounding up" capacity using that technology. Suppose your heat load using Manual J is calculated at 3.2 tons, with single stage models you would be advised to get 3.5T capacity (after doing the Manual S calcs), with two stage models you would be offered 4.0T capacity. Isn't that telling you something?

If you are having any kind of tug-of-war about sizing equipment, I suggest letting the duct design be for the larger size. Many HVAC guys worry about you having July 4th parties with the doors open, teenagers having long talks with boy/girl friends out the open window, etc., so that would make sense to those guys. It's a lot easier to downsize airflow with existing ductwork, than to upsize it and do it right. My own house had puny size ductwork for the airflow, the previous owner probably upsized and worsened the problem, and I am downsizing AC to get it more acceptable.

Thought about ventilation? A dehumidifier? A ventilating dehumidifier can do both jobs, and I challenge anyone to find a way to keep humidity down in mild weather when the AC is not needed much. This is doubly true with a tightly built house. Thermastor makes one mature model of this, Aprilaire another which is less expensive but also less energy efficient. I have a stand alone Thermastor Santa Fe model, rated 90 pint/day which knocks a good 5-10% off humidity in my 3430 sqft 1989 house, and does it with only a 35% duty cycle too.

Run all this stuff by a bonafide expert before you make a financial committment, but I believe he will support most if not all my ideas expressed here.

Best of luck -- Pstu

[hvacsmart]

Manual T is: Air Distribution
Manual S is: Residential Equipment Selection

[dash]

We do have an non-profit in Atlanta called Southface.org who promotes green building. They offer a manual J test and a duct layout as part of their residential services... BUT I was warned agaist using them by a HVAC dealer who claimed that they always under size equipment and that the manual J is a minimum and one should always put more that what the manual J recommends.

any thoughts on that?

Also could some one tell me what the maual S and T are? I know the J is for load and D is for duct work but not sure about the others.

thanks Mark

IMHO and some here will disagree.Contractors that insist Manual J undersizes,are either doing an incomplete calculation ,skiping Manual S,or undersizing the ducts.

Manual S adjusts for ,as some brands data does,the lower capacity of equipment when the indoor design is less then the standard ratings printed,of 80° F .Most people want it cooler then 80°.

Undersizing ducts results in lower then specified air flow,lower air flow,reduces equipments sensible capacity.


Go with all the calculations,by a Pro.

[dan sw fl]

I am in the process of building a new house and I am trying to be as green as my budget will allow. I wanted to check with you guys what you though this house would need in terms of tonnage?

I have heard rough rules of thumb that say an older house should get 1 ton per 500sq/ft but when reading about spray foam insulation the say you can use 1 ton per 1000 sq/ft, a very big difference. Let me point out a few important factors.

the windows are non-thermally broken storefront system with double pane, tinted, low-e and insulated glass.
the front of the house faces due south with minimal shade.

the house is 4000 sq/ft but 3300 sq/ft heated, the basement (1/2 basement really) is not going to be conditioned

located in Atlanta GA

Mark
No rule-of-thumb can apply to such a unique structure.

Need specific glass properties, area and orientation.
The overall glass area seems to be ~ 600 square feet.

U-value might be 0.3 and S.H.G.C. in the 0.35 range
based on your window description.
Cooling load due to the glass could be in 16,000 BTUh range.

A/C needs may be close to 5 tons ( > 39,000 BTUh Sensible)

[Vivid]

Dan,

I just got the window information.

U-value .30
SHGC .42

west 201 s.f.
south 398 s.f.
east 8 s.f.
north 324 s.f.
---------------
total 931 s.f.

total exterior material 5300 s.f

[seatonheating]

Dan,

I just got the window information.

U-value .30
SHGC .42

west 201 s.f.
south 398 s.f.
east 8 s.f.
north 324 s.f.
---------------
total 931 s.f.

total exterior material 5300 s.f

C'mon man, we can't give you a free load-calc on here. Save it for your HVAC contractor.

[Vivid]

Sorry Seaton,

I was just trying to ans questions people had about the house... I apologize and did not mean to cross a line. thanks for all your help guys.

[pstu]

Looks to me like a good design, probably very energy efficient. As a homeowner I say forget about sqft/ton until AFTER you see the Manual J model. The least educated techs see things in terms of sqft/ton, and that only applies if you always have the same tightness, the same insulation, the same compass orientation, etc. Everything of importance is assumed to be the same when you start with sqft/ton, that is the very opposite of what this site is about.

If you have cad files it may be easy to get someone from this site to do your load calc for a couple hundred dollars. I would consider it a wise investment. I did it for my own house using HVAC-CALC, cheaper but a couple hours labor, and would consider paying a pro to repeat my work just to be sure I didn't make some expensive mistake. My Manual J model comes out to around 4.5-5.0 tons depending on some assumptions. This is for 3430 sqft, 1989 house with lots of single pane window area but good shading and tightness at 5.3 ACH50. What I actually have is 6.5 tons and I am reasonably certain that can be downsized 0.5 tons, probably not 1.0 and still perform well (Manual S may account for the difference, or my ductwork in unconditioned attic).

One detail is that with a multi story design you may want or need either zoning with one system, or multiple systems for the different floors. Many posts on this site are complaints about temperature differences between floors. Using that setup you may find the two systems add up to more tonnage than a single one would -- in my house the two halves apparently have their peak loads at very different times. Absent a sophisticated zoning system, I need to size each half separately.

By far the best if you can get a pro in your area, familiar with the climate, ACCA methods, and your construction methods. Hope you don't have to settle for less than all three.

Best of luck -- Pstu

[dan sw fl]

Dan, U-value .30 SHGC .42
west 201 s.f.
south 398 s.f.
east 8 s.f.
north 324 s.f.
---------------
total 931 s.f. Total Exterior material 5300 s.f

You will need something like two 3-ton TRANE XL16i Heat Pumps
plus 5kW or 8 kW heat strips