View Full Version : Load calc. Finished
02-28-2005, 09:26 PM
Installer called me back with load calc. 33000 btu
3ton unit...1950 sq.ft house with a total of 18 windows
(7) 3x5 windows on west side
(7) 3x6 windows on east side
(1) 3x5 northside
(3) 3x4 southside
All windows single pane with storm windows single pane
Built around 1980 31/2 studs with brick exterior r-19 insulation in walls r-38 ceiling I think
Existing unit is 31/2 ton unit..don't know model it replaced are how big.
Sound about right?
03-01-2005, 09:14 AM
Seems high for small house with great insulation but he ran the numbers.
My own house is 3400 sqft and HVAC-CALC models tell me 4.5-5.5 tons is called for, depending on variables like infiltration values and design temperatures. Your number is compatible and I think our two climates are fairly similar. I would be curious to know what value is used for infiltration in your house but don't expect your estimator has told you that. Your insulation is great, I would keep an eye on duct leakage as something with potential to manage your AC size.
Best of luck -- P.Student
03-01-2005, 09:35 AM
The insulation levels seem high to me. The chart I have says that cotton fiber batt insulation has a r value of 3.85 per inch. That would be 13.475 for a 3-1/2" cavity. Do you think you have some sort of special insulation in your walls. In Florida the minimum level for a typical ceiling is R-19. R-30 is considered a upgrade. What kind of attic insulation do you have? Perhaps two layers of r-19.
03-01-2005, 09:42 AM
Is it a ranch or two story?
Is there a basement or crawl or slab?
What's the infiltration?
How many bedrooms?
How many people?
What color is the roof?
What is the roof made out of?
Antyhing like tanning bed or nuclear powerplant hidden in a closet?
Is the attic ventilated?
Where is this home loacated?
What are your expectations?
03-01-2005, 10:15 AM
ranch, 1950 underoof + 400 sq. foot garage
slab on grade
3 bedroom,2bath,living and dining room,kitchen,den with fireplace,fla. room,laundry room
Wife and I and 2 or 3 grandbabys on occasion
Medium to dark brown
arch. fiberglass shingle
no, just high heat surround sound in the living room on occasion
Ridge and gable end vents
north west fla...lower alabama
Looks like I may have overstated my insulation value earlier as I have 31/2in walls with batt insulation and insulation in the roof currently is 6in blown original and I guess the previous home owner added 6in batts on top of that.
Once unit is installed, my plans are to buy enough blown insulation to cover all duct work after I check duct for leaks
want to end up with min 12 to 16in across roof, higher at duct areas, but I'm open to all suggestions
a compressor I don't have to go set the reset button on at 2am or like this morning, turn up the heat to take the chill off and no heat pump running..got to go outside to reset..42 degrees this morning
I would just like my home to be heated and cooled as efficient as we can given the age of house, lower humidity would be nice
The wife says we aren't moving any more, so I guess we are here for the long haul..I have gypsy blood and staying in one place is hard for me as moving is for her
03-01-2005, 10:36 AM
just conversed with my guy and infiltration average? and r-30 for roof.
The more I talk with him the more I think they will do a great job.
I worked with them a few years back on a commercial job and they did a great job then.
Hopefully the home side will be just as good. And if they do..
I will be the first to admit all my fears where unfounded and will tell you and them what a great job they did.
I will ask them if they mind if I post their name in case someone else is looking for a little peace of mind.
I did download calc. program and may see what I come up with.
03-01-2005, 10:38 AM
While, in a perfect world you may have a R-38 in the attic. It's probably less than that once you figure in compression of the loose fill and any areas that didn't get the extra layer of batt insulation in the confined areas of the attic.
03-01-2005, 06:14 PM
Where do you set your thermostat - cooling cycle
If you set at 75 or higher, 3 ton will probably be best choice. If you want cooler than 75, and plan to stay in the home for awile, I would go with 3.5 ton
I had 1950 with 2.5 ton ( by Manual j )would not go below 78 if outdoor was more than 85. A three ton would have gotten it to 75, running alot, I think.
Hope this helps
Oh yea, we're in lower alabama
03-01-2005, 08:49 PM
insulate the ducts with that type of insulatin -- which has a vapor wrap -- be sure to seal ducts first with mastic, seal vapor wrap with foil tape--
read about attic ventilation in other threads here, & BUILDINGSCIENCE.com
water must flow from the slab downward 6" in 10ft; extend downspouts 10ft. both will help hold moisture out --
A rough load calc., based on what you have told us is ,32,021 total,24032 sensible,7989 latent.Fat eddy could probably give you a better number.LOL
Check detailed cooling charts,most will have a nominal 3 ton that will handle it,but not if you want 72° in cooling,75° maybe.
03-02-2005, 12:26 PM
Load calc. by ac man came up to 33000..95 degrees outside and 73 degrees inside.
We do like to keep it at 70-72 in summer..maybe with the dehumidfy we won't need to go this low.
relative hunidity 84 day, 61 night
Take a typical ton,26,800 sensible and 7,200 btus latent,total 34,000 btus.
Read the fine print of mfrs rating,rated at 80°F indoor(return) air,deduct 835 btus ,per 1000 cfms ,for every degree below 80°.
So 1200 cfms,equals 1.2 X835 btus X 8° below 80°=8016 btus to deduct ,from rated capacity.
So deduct 8016 sensible btus,and a 3 ton is too small.
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