Get the installer to fix the problem and forget the diy route.

Whose idea was to increase the size to 3.5 tons?

Where are you located? Is this a one or two story house? What temperature is the thermostat setting?

You may be able to increase the temperature swing on the thermostat, but that has its drawbacks. Trying closing the vents in the room that has the thermostat. It may help but it is not a good solution. If you live in an area of high humidity you are not going to be comfortable.

To Answer the above questions:
1. Contractor
2. Texas
3. One story
4. 78

also I live in a 1600 sq ft home

"I installed a 3.5 ton unit in house that is 1614 square feet and it is not running for more than 10 min. My previous unit is a three ton. My question: did I oversize my new unit and if so how do fix it so it can cool my house properly. my company did do the heat load calc. it is possible something else might be the cause."

That is only 461-sf per/ton of cooling; that is usually crazy. What were the indoor & outdoor conditions when it was only running 10-mins then off?

Get others to help you & do a load-calc on your home:

http://www.loadcalc.net/

thanks but my old system was indeed a 3 ton system per contractor original to house and my company is well respected company that to the time to look at everything. it ran long cycles and I did so go well but stopped working after 14 years which is not premature if you ask me ! when my company did the load calculation they came up 3.5 ton and I was like couldn't it be to big but they said it was right and that if we add on to our home it would work as well as now .now it is runs 15-20 min stops for 5-6 min and repeats is this a good cycle time because if so than it cools well. I haven't run into to many humidity problems yet which is why I am not sure about this

If U live in a dry climate where humidity is not a problem or if it's a mild problem set the blower for 350-CFM per/ton of cooling.

The duct system may have been small for a 3-Ton A/C & it was probably not delivering near its nominal rated Btuh. You can't go by the old A/Cs sizing...

to be clear I dont feel clammy in my house which one of the signs of an oversized system. my understanding is an oversized system will have problems mantain 50 percent humidity. a system sized properly should be able to matain 50 percent humidity. is this correct logic. No I wouldn't have bought a new unit without the heat-load calc

thanks every one I am planning to have routine service done on my system. it is under a 10 year warranty so if I have to downsize it they shouldn't charge me.. I appreciate the help and I will make sure my system works and cools properly if it is the last thing I ever do

Do a whole house load-calc ten click on SIZING.

http://www.loadcalc.net/

The contractor calculated the size to be 3.5 tons. Most likely he fudged the numbers or made an error in the calculation. Be prepared for a fight if you asked for the condenser to be replaced with a smaller size.

Good luck and keep us posted on what happens.

What if the same scenario as above, but the unit in question is two speed 4-ton? Would that be too large, as part of the time it would be running at lower speed?

A 2-stage 4 ton unit would be a waste of money in my opinion if the cooling load is 3 tons. You have to realize that most 2-stage units use compressors which have a 75/100% split. That means the AC will have low and high capacities of 3 and 4 tons.

What will happen is the thermostat will start the condenser in the low stage when calling for cooling. If the the AC is cycling on and off to maintain a set temperature, then a large majority of the time (98%?) it will stay in the low stage. The only time the AC might go to the second stage is when the thermostat has been set back several degrees and the thermostat can't reach the temperature setting in a specific amount of time.

called the install company

When sizing air conditioners, be careful not to make the mistake of getting one that is too large. I think for 1614 square feet House 3 tons AC unit is a better option than 3.5 tons. The problem is that when you install larger one, It will shorter cycle, off more frequently, increasing operating costs and reducing the system life. Proper air conditioner sizing is critical to optimize performance. For me, it might be an insulation problem as well.

Weren't you advised on what size is appropriate? In such cases I would depend on professional opinion. I did that when choosing my trench heaters. I asked guys from Verano and got all the parametres I needed.

I wouldn't e surprised if a 2.5-Ton would cool your 1600-sf home.

I acually got a manual J load calc and it said i needed 37725 btus of cooling. per rules you round up so in this case a 3.5 ton system was installed. now the previous unit was 3 ton with NO SIZING ISSUES and was the orginal unit. All the issues I had with this old 10 seer unit were not attributed to an oversized system . as far as I am concerned nothing has change to our home since thre day it was built. call me crazy but I should be able to go up by 1/2 ton or 50 percent and still have it work well. now I am being told I should have gone with a two stage system due being right size in one of 2 stages

Well, I use a very small mere 6,000-Btuh Half-Ton window unit to cool the +650-sf first floor of a 1937 2-story home with a lot of windows & with a deep basement in SW WI.

Less than a half ton upstairs that cools, when it's real hot, one bedroom the hallway & bathroom; I use fans on both floors to move the air; a 'Wind Machine' on 1st floor.

That's around 1200-sf per ton of cooling. Takes care of both %relative humidity & sensible temp. We can have very high heat indexes that the little units handle perfectly; my summer electric bills are very low, with a dehumidifier in the basement being the energy hog.

Most A/Cs & heat pumps are oversized; too many have bad installs & poor engineering design of the project... !

so if a manual j said i needed to go bigger would redoing the faulty sized ductwork help me meet the load

Your measurement was barely over 3 tons. Minor adjustments to the values (windows, shade, insulation values, design temperatures, etc) could have easily brought it under 3 tons. Changing the duct work isn't going to change the heat load.

now can you give someways to reduce humidty with out the a/c or a dehumdifier

Move to Colorado?

so my installer came to do a duct test and claims "your system is powerful and it is cooling fine" so practically he is telling me that my system is powerful and cools quickly as if that is a good . I am pissed the most important part of his visit and he tells me my system cools fine with a short run. I am not that stupid. practically he is just saying that 10 min is how long a system should run to do both of its jobs(cool and dehumidify) is this for real. even not at design temps the system should run longer. He apparently claims he checked the duct work. so upset right now.I am basically crying. 10 min or less run= high humidity PERIOD ! This installer should just quit his job! so my day is wasted thanks to this installer. even my previous unit wasn't giving me this issue when it was installed. I was looking foward to comming to a cool house and now it still feels like a swamp in here

@Emre, in an earlier post you said it runs for 15-20 minutes and off for 5-6 minutes? I'm not an expert but that doesn't sound like short cycling to me; it's running 3/4 of the time, which to me sounds like a reasonable cycle. You also stated earlier no humidity problems, but now it's like a swamp?

Also, you have longer less frequent cycles if you set your thermostat for a wider tolerance, +/- 1.5 degrees instead of +/- 0.5 degrees, but your current cycling sounds adequate to me.