Need AC sizing help: Unit too small?

I just bought a new home about 1.75 years ago, and have been having AC problems ever since. My AC seems to run constantly, (set on auto).
My electric bills have been in the $400 range and I think my unit is too small for the house.
Quick Description:
3150 SqFt Supposedly "Energy Star" Certified, although you can see daylight through some doors, attic had to have insulation added 3 times, etc.
There is an "owners retreat" above the master, that is isolated from the rest of the house. This room is constantly in the 79-81 range, no matter what the AC is set on.
5 ton unit on house now, 6 SEER I believe.
I am in Houston, where it's hot as hell, but I keep the unit set on 77. At 10pm it goes to 75 until about 7am just so we can sleep.
The builder says the unit is the right size. Their AC company agrees. Everyone else I know does not experience the same problems I do. They keep their AC much lower, and their bills are much lower as well. All of my appliances are new. I work from home, and just about the only thing on is my computer/monitor/printer and ceiling fans. I work out of the home office or the "master's retreat".
The TSTAT is located upstairs. The temps listed are in the "Masters retreat/office".
I was logging the AC cycle the other day for about 4 hours and this is what happened:
It is now 10:22am and it is 81.5 degrees in this office, (masters retreat). It has been hovering at this temperature for over 30 minutes.
The AC has just kicked on. The thermostat is set on 77.
It's now 11:00am and the AC has just stopped. It has not stopped since 10:22am. The temperature in the office (master's retreat) is now 78.3.
It took the AC 38 minutes of continous blowing to drop the temperature (when set an at uncomfortable 77deg) in the house back to 77. The temp in the retreat went from 81.5 to 78.3, a difference of 3.2 degrees.
It's now 11:15am and the temperature in the master's retreat is once again 79.6 degrees. That's just 15 minutes after the AC stopped blowing.
It's now 11:36 the temperature is 80.8 in the retreat and the AC has just kicked on again. (NOTE: Today at 12:00 the TSTAT is set to go to 76 deg for a few hours, just so I can work without sweating.)
It's now 12:53 and the AC just shut off. The temperature is 77 in the retreat. The AC ran for 1 hour and 17 minutes STRAIGHT, just to get the the house to 76 from the moment it went over 77 and turned on.
So to sum up, the temperature at the TSTAT went above 77 and caused the AC to start at 11:36. The AC blew constantly until 12PM at which time it still had not lowered the house temp to 77 (24 minutes), at 12PM the TSTAT setting went to 76, causing the AC to continue to blow, to try and bring the house down one more degree. It took until 12:53pm to get to that extra degree of 76. That was a continuous cycle of 1 hour and 17 minutes to blow, to get the house to go from the moment it went over 77 (setting off the AC) to 76 degrees.
It's now 1:15pm and the temp is 78.6 in the retreat. It's now 1:20pm and the temp is 79.2 in the retreat. The AC has just kicked on.
It's now 2:15 pm and the temp is 77.2 in the retreat. The AC just turned off. It ran for 55 minutes straight.
So from 10:22am - 2:15pm - that's 3 hours and 53 minutes. In that 3hours and 53 minutes, the AC ran for 2 hours and 50 minutes. (2:38pm and it's on again) <<<<< END OF LOG>>>>>>>
Is this normal? The builder, in an effort to fix the hot owner retreat problem has done the following to no avail:
Repairing ac leak 03/28/05 Adding a straight run to master's retreat 05/26/05 Adding Insulation (I was told the insulation was WAY too low to begin with) 06/28/05 Changing out the thermostat 06/28/05 Checking the AC system 08/18/05 Engineers come out to check system 05/23/2006 Checking / Recharging AC (tech said it was a bit low) 07/05/06 Adding Insulation 07/12/06 Adding Insulation to attic door, (there was never any from day 1) 07/12/06
None of this has worked (it's 80.1 in the office now). When the unit kicks on, it always runs for a long, long time. The engineers have said the system is working at 100%. The AC company says the AC is working at 100%.
I say, if the house was run off of a single window unit, the window unit would run at 100% but it would run all the time. I think the AC is too small for this house.
We have window coverings on all windows. We have ceilling fans. Others with the same floorplan are experiencing this same problem. I am about ready to sue the builder under the DTPA.
Am I way off, or is this unit too small for the house?
THanks,
please copy any response to snipped-for-privacy@houstonpi.com so that I can be sure to see it.
Scott
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HoustonPi wrote:

How do they know? Did they do a room-by-room heat gain calculation?
Ask to see it and have them explain it to you. If they can't produce one, ask them to do one now.
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HoustonPi wrote:

FWIW I have a 5 ton AC (12 SEER) cooling 3000 square feet, all downstairs. I have a 1.5 ton unit cooling upstairs (about 600 square feet or so). I also live in South East Texas. The downstairs stays cool, and the unit does not run all that much. Cools down very quickly. The upstairs gets very warm if I do not keep its AC on a fairly low setting. And then it runs all of the time.
I think it is unrealistic to cool an upstairs area and a downstairs area in this part of the country with the same unit (unless you have multiple zones). My upstairs AC has been running about 2 hours a day lately and I have had the thermostat set on 90 all day long (and night). Don't use the upstairs much in the summer.
Brad
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After reading your post and seeing that you had to have it topped off every year, that may be a problem.
My advise to call another A/C company other that the company that installed the system in the first place.
Pay the money and have them come out and to check the duct work, the operation of the total system. You might be surprised on what they find.
Heck I have even found systems when a part of the strip heater staying on while the A/C was running and have found holes in the return duct in the attic so the system was sucking in hot air from the attic. There could be allot wrong with your system so like I said get another company out there to check.
--
Moe Jones
HVAC Service Technician
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Moe Jones wrote:

Moe, you said a mouthful! I am willing to bit that there is plenty wrong and that the ductwork integrity (leaks, lack of insulation, improper sizing, diffusers, etc.) and possibly a lack of airflow.
He needs someone that can run a valid manual D on every aspect of the duct system.
Also, it appears the heat-gain rate needs to be slowed down. Construction may have been sloppy allowing too much air infiltration, etc. Start with the ductwork and 400-CFM airflow per ton of cooling or 2,000-CFM for the 5-Ton system! Read about static pressures, etc. If the condenser is round with a top air discharge, measure the temperature rise of the air off the condenser coil outdoors. A 6-SEER condenser can have up to a 30 degree temp rise off the condenser. Let use know the indoor Supply Air and Return Air temperature too! - udarrell
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Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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Moe Jones wrote:

Moe, you said a mouthful! I am willing to bit that there is plenty wrong and that the ductwork integrity (leaks, lack of insulation, improper sizing, diffusers, etc.) and possibly a lack of airflow.
He needs someone that can run a valid manual D on every aspect of the duct system.
Also, it appears the heat-gain rate needs to be slowed down. Construction may have been sloppy allowing too much air infiltration, etc. Start with the ductwork and 400-CFM airflow per ton of cooling or 2,000-CFM for the 5-Ton system! Read about static pressures, etc. If the condenser is round with a top air discharge, measure the temperature rise of the air off the condenser coil outdoors. A 10-SEER 5-Ton condenser should have around a 23 to 25 degree temp rise off the condenser. Let use know the indoor Supply Air and Return Air temperature, & humidity if you can! - udarrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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Correction, Unit is a 10 SEER.
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Sorry to hear that your A/C isn't your biggest problem.
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HoustonPi posted for all of us... I don't top post - see either inline or at bottom.

Let me get this straight.. the office is where you retreat to masterbate?
--
Tekkie

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Tekkie wrote:Date: Thurs, Jul 20 2006 8:09 pm

Let me get this straight.. the office is where you retreat to masterbate? -- Tekkie
From: Tekkie - view profile Date: Thurs, Jul 20 2006 8:12 pm

You would think retreating to the office to materbate would have improved his unit. I know he certainly strengthened his forearm... >>

Wow, that's truely amazing. Two totally original cutdowns, in only a matter of 3 minutes. How do you do it? Funny though, how when talking about AC size, you go off on a masturbation tangent. TWICE. Are you dynamiting the pond looking for someone with your own mastabatory addictions? Maybe looking for a reach-around partner perhaps? Something you'd like to share? On the other hand, I think you've shared quite enough already.
No go churn your own butter or maybe go give some guy a PJ.
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HoustonPi wrote:

What you're describing sounds like an undersized unit. Based on some of the professionals in this forum, you'd understand how this occured.

Approx 292 m2.

approx 17.5kW

You expected different???????????

I agree its most likely it is at 100%.
You really need a heatload to correctly determine the required unit capacity. However a good rule of thumb for a domestic would be 130w/m2 for the areas conditioned (this obviously varies on differant projects). I you don't have zones, then this applies to the whole house.
You currently have around 60W/m2 (simple math). Units appears too small. You need around 40kW to do the whole house. If you zone it, and isolate those zones from the rest of the house, the unit size will come down.
Best of luck.
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