Ok, so based on the advice of several people from this board I decided to check my house for needed load. Bought the software from bullseye above. I will try to make it short.
House in Raleigh-NC built in 1925. Down Stairs is 1400 sq ft normal amount of original windows. Uninsulated wall and floors. We are finishing the attic upstairs. Will be adding 6 double paned windows. R-13 insulation in walls walls, and hopefully will get R-22 out of the roof (putting composite system with reflective barrier). Upstairs will add approx 700 sq ft. Existing unit on downstairs is 3 ton unit. Does fine in normal summer (90 F) but works all day when it is 100 F outside. The load calc came back at 66,000 btu heat gain for entire house (5.5 ton).
Does this pass the smell test? Do you think I did the calc wrong? I am very worried about the contractor putting an oversized unit upstairs. 5.5 tons for a 2,100 sq ft? Any comments appreciated?
recheck calculations,sounds a little much. What design temp are you trying to maintain? 75 degrees
I went with the default values on the program. 95 outside and 72 inside. We really only keep it at around 76 as long as it is dry. I will try adjusting that.
Raleigh-NC summer design is 92-F dry bulb, 75-F wet bulb, or around 45% relative humidity. (Not a high humidity.)
Well, you could put R-38 in the attic along with the other insulation it does not appear that you would need that many tons of cooling.
You need to realize that the 3-ton unit may not have been delivering 3-ton of cooling. Locate a real good contractor and have everything checked out including the supply and return duct system and CFM air delivery. (Dozens of causes!)
Learn all you can before making a final decision on the size of the A/C units. Read all you can to make a wise decision.
Sized correctly the units should run most of the time during the day on the highest heatload days to achieve the "human comfort zone." Save energy and utility costs.
Did you allow for 700 sq ft of the first floor ceiling to be conditioned space above.
5 tons sounds high, let alone 5.5 tons.
I did account for conditioned space below the new second floor. But I had a wrong setting for the first floor. Should be in an enclosed crawlspace. That knocked off about 6,000 btu's. Every contractor I talked to said either 1.5 ton or 2.0 ton. None of them did or would do a load calculation even the most expensive contractor (hence me buying it). One even said can't do it until they saw the framing even though the only framing to be added is interior framing.and I have drawings of what we are doing. I have settled on the contractor. They recommend a 2.0 ton, but I want to ake sure it is correct. I guess most units only go down to 1.5 for SS.
There are 1 ton units.
2 tons for a 700 sq ft second floor sounds like alot.(350 on the ton?)
But not being there to see it, maybe it does need it.
But it is the installing contractor that has to stand behind it, not me.