Not as important as getting the equipment sized right. A spray foamed house shouldn't need 1 ton per 577 sq ft. get a load calc done by someone that is familiar with spray foamed houses.
I am certainly a novice when it comes to HVAC systems and certainly don't know the proper terminology. I have a few basic questions about the location of the inside units, duct work, and returns. I live in central Texas and am building a 2 story 5200 square foot home with low E windows/doors and spray-in foam insulation throughout, including the attic. The units (3 3-ton units) I'm having are: Carrier Heat Pumps, 16.00+ SEER, high efficiency with 2-stage variable speed air handlers. All of the units will be in the attic but the location in the attic is my main question. In a well insulated home with the above units, what is the maximum distance a room being supplied by one of the units should be (maximum length of duct work) before you start losing significant efficiency (decreased air flow to the room with loss in coolness)? Basically, how important is it to have the main unit central to the rooms it's providing cool air to so the duct work is shorter versus the unit being farther away with longer duct work? I'm sure there are a lot of variables but maybe some general estimates or recommendations would help. Thanks
Not as important as getting the equipment sized right. A spray foamed house shouldn't need 1 ton per 577 sq ft. get a load calc done by someone that is familiar with spray foamed houses.
If it were my home, I would try to keep the units in the conditioned space. I would put the 1st floor system on the first floor (somewhere) utility room or build a room in the garage. There are building techniques that will allow you to run the ductwork in the floor trusses (for the first floor system). The second floor system(s) ductwork can go in the attic if need be. I just like to be able to see the air handler for easier service access and prevent it from sweating.
I agree with B.T., 9 tons seems a little much. I know it gets warm in TX but I've seen new 2-story homes in IA with 2700 sq. ft. cooling with 3 tons without foam.
I would set Thermal Building Envelop Specs and building inspection to achieve < 5 tons Total capacity.
http://www.sprayfoam.org/news/index....le_view&id=556
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
I come up with two 2-tons for a 5,200 Sq feet 2-story residence
when using
104'F Outside Air
S.H.G.C. window < or = 0.35, U-value = 0.4
R-28 ceiling spray foam (on roof deck .:. ducts in conditioned space),
R-14 walls,
semi-tight (ACH ~ 0.25) infiltration,
ducts in conditioned space,
6 people,
2,400 BTU internal gain
per ABRIDGED Manual J8 worksheet - ATTACHMENTS
Duct in conditioned space saves about 14,000 BTU/Hr
Tinted Window Heat Gain for 700 Square Feet (14% of floor area) is 25,000 BTU/Hr less than Single, Clear Windows.
This assumed Window area may be more than what is going to be used in this residence.
Maybe someone can suggest where this heat gain analysis could possibly
be considered undersizing for a premier thermal building envelop.
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
3200sqft with 4 tons on my home... and if I had a single system it would be 3 tons with at least 6 zones. My home is 88 years old and not foamed. Its' all in the little details. Larger home have more mass and don't see the peak heating and cooling loads in extreme weather. High temrpatures only last part of a day. Larger homes take a long time to heat up and cool off. On a 40F day, 1st stage on a 2 ton heat pump will hold temperature downstairs including some heat going to a full unfinished basement. On a 85F sunny afternoon, 1st stage runs only about 1/4 of the time downstairs to cool it.
You build a house right, it will really amaze you.
No reason you can't get under 5 tons, if not down to 4 tons. Thinks like a sealed attic and cool roof go a long way.
That is another option, place the unit in the attic, but spray foam the roof deck and use steel roofing and radiant barrier and insulation on top of the roof deck to create a cool roof using steel roofing. Make sure a vapor barrier is used on the roof deck.
If possible, spend a little extra on a larger roof and create deepr overhangs. Many new homes only have barely 12" eaves. 2' should be a minimum, 3' even better. Ideally, thte upstairs is almost completely shaded for at least 8 hours a day in summer, but recieves more sun in winter.
Design Condition: 96 Days over 90'F per this link
http://www.climate-zone.com/climate/...as-fort-worth/
I believe recent years in TEXAS areas have seen > 60 days > 100'F.
ASHRAE 1% 92 to 102'F across th state.
ASHRAE 0.4% is 100.3'F in Dallas & 104.2'F at Laughlin AFB
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Even more reason to focus on things like cool roofs and radiant barriers. Its' really hot.
If costs are a concern, use fewer windows, but make each one larger to keep the same window area. It will also propotionally reduce air leakage along with using more fixed windows like picture windows, transomes and sidelights. But keep them all low E. Operable windows are a big source of air leaks, and in a hot humid climate in a moderns home, there's limited days in winter where you'll likely use natural ventilation, And honestly, it doesn't save you that much because 1) you let humidity indoors in that you'll have to remove later when it heats up, and 2) AC systems are pretty cheap to operate in cooler conditions anyway. <65F a high effceincy, 2 ton, 2 stage unit on 1st stage uses less than 1000Watts.
Thank you all for responding. Here's a little more information about the house, climate, and my concerns. The roof is going to be cement tiles (a mediterranean clay tile look). The temps where I live have stayed over 100 degrees for more than 100 days out of the year. My biggest concern is not adequately cooling the house. It sounds like the 9 tons may be a little much but I don't mind paying more for a comfortable home during our long summers. Is there any harm in having a "larger" system (9 ton total vs. 4-6)? Also, the house is already framed and the roof is going on now so there can be no alterations in placement of the units...they will all be in the foam insulated attic. However, one of my main question is how close to the rooms a unit is providing does the unit need to be or does it even matter (is having more/longer duct work worse)? Should they be in the attic area over the main portions of the house close to the rooms or over the attic in the garage with longer duct work traveling to the rooms? Thanks again
My Biggest concern is not defining and adhering to specs for the Thermal Building Envelop.
I cannot see Your Specific duct plans developed in accordance with ACCA Manuals J & D.
Flexible duct Length SHALL BE LIMITED to 10 feet ( or it ain't gonna work too well).
Of course, you may experiment and find out yourself.
Then you can have an experienced mechanical contractor not beholden to the builder
fix it for $$,$$$
after a couple years of frustration.
Now when & where did you say the layout and Manual D calculations would be
provided and independently reviewed?
Certainly with 9-tons
one has about 30 times the chance of not doing the ductwork adequately
than with one 2 ton and one 3 ton system.
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Thanks Dan.
Also, no basement in the house (most homes in this part of don't have a basement). So, since my home is being built by a builder who has his own subcontractor for the HVAC work, what would you suggest I ask to see from the subcontractor and/or builder to verify that what is being proposed is adequate/necessary. I've been told already that the recommendations are based on the house structure/components (# of windoes, door, insulation, etc.). Thanks
Last edited by dan sw fl; 05-03-2013 at 02:52 PM. Reason: trust BUT VERIFY USUALLY
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
http://en.wikipedia.org/wiki/Trust,_but_Verify
I WOULD TRUST THE BUILDER
just about
EQUAL TO ... the boston bomber.
He doesn't happen to be Russian, does he?
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Those 2 stage units will be operating at anywhere from 2.1 to 2.4 tons capacity in first stage. And probably won't dehumidify very well, so you will end up having to cool the house to a lower temp. Which means paying more yet to be comfortable. Which defeats the purpose of a foamed house(almost like going to a casino to save money).My biggest concern is not adequately cooling the house. It sounds like the 9 tons may be a little much but I don't mind paying more for a comfortable home during our long summers. Is there any harm in having a "larger" system (9 ton total vs. 4-6)?
If your going to let them put in 9 tons anyway. get a whole house dehumidifier installed right a way also. So you don't have to set your thermostats to 66 to be comfortable.
Here's a quote from a guy living in Texas:
"I heat & cool at least 4,000 sq. ft. with one 2 ton heat pump in north central Texas."
You can read more of Paul42's posting here:
http://hvac-talk.com/vbb/showthread....as-tight/page2
Of course having 3 separate units in 3 different zones throughout the house is not the same thing as having one huge 9 ton a/c blasting out all at once, but it's still something to consider.
An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............
http://www.mohomeenergyaudits.com
If the units are in the attic, then you should foam the roof deck and seal the attic... If you aren't planning to already. That will drop capacity requirement by at least 10%to start.
No reason 3 2ton units won't be adequate in a foam house that size. Larger homes have less surface area to volume and proportionally more mass too. They do not change temperatures quickly. It might take 3 hours when ion a100k day to cool the house of by 1f, but it will hold temp on single stage.
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POST # 6 - I believe "Ducts in conditioned space" reduce load by close to 30% based on ASHRAE Manual J8.
You are putting the air handler and SIGNIFICANT AMOUNT of duct in an ~ 80'F environment, instead of 140'F in TX.
IF not putting foam on the roof deck and using 3 units ( Only 2 actually needed ), the required duct work space may necessitate that the duct be stacked about 5 feet high.
The _extended plenums_ must be arranged so that they fit on specific horizontal planes unless the units are separated by about 30 feet.
FLEXIBLE DUCT LENGTH MUST BE < 12 FEET or one ends up with the usual 20+% reduction in air flow and capacity.
Last edited by dan sw fl; 05-04-2013 at 06:47 AM.
Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities
Thanks again for the advice Dan. I've asked my builder about the manual J calculations and duct design as you suggested and should hear back after the weekend. I'm pretty sure he told me at the beginning of construction the HVAC contractor did a load calculation for the house but as you suggested, it's worth verifying. The proposal by the contractor, assuming we're using 3 units, was for all of the units to be in the attic space above the 2nd story. 1 unit would supply the 2nd story (3 bedrooms, 2 bathrooms, hall and large game room),1 unit for 1st story below the 2nd story area (kitchen, breakfast area, laundry,2 halls, mud room, bedroom, and dining room), and 1 unit for rest of house (2 story family room, study, hall, master bedroom and bath). Would the unit supplying the family room, study, hall, and master be better off being centrally above these areas in the attic (above study between family room and bedroom) or is it ok for it to be with the other 2 units above the 2nd story game room? In other words, would it be possible for it to cool our master bedroom adequately if the unit is that far away from it (roughly 50 feet away) if proper duct work was used?