If you have a lot of hot days over 100, 5 tons may be enough, but I would examine a Man J for answers. Since you know that 4 is not enough, the question is how much more is needed? It would be advisable to seperate the second floor from lower floor, and add a smaller system sized for that floor. I would expect maybe $4000-8000 for a 2-3 ton new system, including furnace. This would include running power, seperating ducts, running a new return into a ceiling. Lineset on outside of house. In short, installing a second system in an attic. You may just block the old vents and install new ceiling vents. If the builder said 3.5 is enough, I would question the duct sizing for even 4-ton, much less 5-ton. If registers whisle, then the duct is not adequate.

The first floor of our house is smaller (due to 3 car garage)- maybe 1200+ sq ft. It is also totally open except one room (bedroom/computer room), and open to a large stairwell and all upstairs halls.

The problem I'm facing with the 2-unit-option is there is not room in the attic for a 2nd unit. There is a single platform for the current unit, and all the rest is a spaghetti mess of trusses, etc. Too many would have to be cut, making it undoubtedly a "permit nightmare" to prove modifications are still structurally sound.

Thus, even if I did 2 units (somehow), both air returns would have to be on the 2nd floor.

For what it is worth, the builder only put 5 ton AC's on the 4,000 sq ft houses...

Insulation is BARE-MINIMUM legal (did not even insulate garage with living space above it). They went to no extra effort. That's KB Home for you...

Mike,
Stick to the facts. The builder said 3.5 tons was enough. He used a 4-ton condenser and a 5-ton furnace. Check the specs. The 4-5 ton furnaces may only use a different motor size. If this is the case, the builder probably sized the ducts for 3.5 tons, and stuck the larger furnace in, because he questioned the accuracy of his installer. As I said, use a man J to verify btu sizing. I should have included a man D for duct sizing, but I expected the reader to read between the lines a little. This would tell the owner if the system was designed for a larger blower. In all probability it was not. If this is the case, then the few ducts from a floor could be closed to improve the cooling of the remaining areas. The remaining area could be a new system. In Texas, ducts are usually flex, or round metal duct. You can place them just about anywhere, after the fact. If you have to route the duct thru difficult areas, a flex duct is easily routed.

As to design, I would imagine that the system was thrown in place with hopes it would stick. Using a 5-ton furnace, with a 4-ton coil, does not sound like any planning was attempted. When the builder said the design required a 3.5 ton cooling system and installed a 5-ton furnace, tells me that he was not very accurate. This would mean that some work may be needed to reduce the furnace blower cfm. A 4-ton coil in a 5-ton furnace does not sound very efficient anyway.

To the owner, donÂt take no from the installer about a system separate for the second floor. You could maybe use a closet that can open from the hall or common area. Get a framer to give you an estimate about supporting the rafters where the HVAC will be placed. You may be surprised at how little the cost will be.

"Insulation is BARE-MINIMUM legal (did not even insulate garage with living space above it). They went to no extra effort. That's KB Home for you..."

- Sounds like adding a lot more insulation will go a long way in your case. This is the first thing that I would do especially because it is a known problem.

They did tell me the ducting is sized for 5 tons. They gave some strange excuse that heating efficiency is different, so oversizing the furnace is "better". This, from an area where it never freezes. Uh, hot California needs more cooling than heating!

As for the insulation, I can add more in the ceiling, and plan to - but I cannot help the vast stucco walls that absorb a lot of heat. I've planted LOTS of trees, but it will take many many years for them to remotely help. I am planning on installing radiant barrier foil in my attic (already did on my garage doors), but need to add much more vent area to the attic (6 or 8 soffet vents, 2 semi-circle roof vents - no gable vents).

I am just trying to understand about what sorts of prices to expect for AC install. Yes, I know California is more expensive, but 3x? That seems just too excessive :(

Had to run the AC last night, it was so hot (76 at midnight). Summer is here with a bang.

goglen: I kindve came to same conclusions about CA pricing. After considerable effort and research, I determined the equipment costs avg for a new split system for my 2000sqft hse in SantaBarbara to be about $4k.(that was for a fancy duel-fuel 3ton HP + 95% variable 70k-btu gas furnace + coil + thermostat, no new ducting neccessary) I dont know much about HVAC installation so was perfectly willing to pay up to $2k for a pro to install system and everyone I spooke to said it would take no more than 1-day. But every HVAC contractor I talked to came in at $8k-$15k!!!
I finally decided to have a general contractor buddy do the install for $1000 and am putting $2k into a mutual-fund earning interest for possible maintainence and repair costs down the line. I may be making a huge mistake but just cant justify the additional $3-5k. I asked my brother in Seattle how much his similar home system cost and it was half!!! And that was with the HVAC contractor warrantying it for 10years!
Not sure whats up here, maybe its just very expensive to run a residential HVAC company in CA. I'm not blaming the contractors, Its just that at $6k I can justify and afford to heat and cool my home, and above that, I,m better off not doing anything.

matt: you're absolutely right about heating days vs. cooling for us down in SoCal! There are far more days where its 40-60 and requires heat than over 90 and demands cooling. Although the few times I've been to the Sac its been a absolute furnace!! But thats always in summer. I guess it gets right cold most of the time, eh? I always wondered if it was just me or if others who live in CA noticed this. None of the contractors I have spoken with have even discussed a heat pump, but that should be more prevalent with our climate, no?

I did get quotes from 3 dealers - all were over $15k for a replacement 5-ton.

I may start looking a bit more "off the beaten track" and try to find local installers from the PennySaver, etc.

Interestingly, a 4th dealer did not even bother getting back to me (he was recommended by a friend who used him last year).

As for heating, the heater can change the temperature in my house by 10 degrees in an hour, easily. The AC cannot change it DOWN one single degree in 3-4 hours. It can only maintain (on a under-100-degree day) or warm up 3-10 degrees on 110-degree days. To me, it shows that the heater is more than adequately sized.

-Glen

did something happen to remodeler_matt's replies and some others maybe on this thread? I was reading his reply to a post yesterday and now it is gone? Oh well...