Is my a/c unit too small for my house?

stella_2006November 9, 2006

I moved into my newly constructed house in April 2006. I live in Houston, TX (hot and humid summers!) and the builder installed a 5 ton 14 seer Goodman air conditioner. I have a 2 story 2600sf home and the builder only put one unit in. The builder said that they don't start putting in 2 units until the square footage reaches 2700 sf. Despite keeping my thermostat at 78 and sometimes 79 degrees I had $500 electric bills 3 months in a row during the hot summer months. I expected our bills to be around $350 but $500 seemed outrageous to me. Do any of you have an opinion on if my a/c unit is not large enough for my house? Would 2 units make a difference in my electric bills? BTW, I have a roof tech radiant barrier on my roof, and low E windows so I was expecting to save money with that too. Thanks!

-Stella

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bob_brown

Hello,
By today's design standards, you have adequate cooling. It is too big to qualify for energy saving status by the state of Texas. (750 sqft per ton)

In days gone by, the bill would have been $350, but with rate increases beyond the average increases, you have an efficient system if it is only $500 per month.

If the unit cycles in less than 30 minutes, it is about as good as a air cooled system gets in Texas. If you were to change the condenser to a water cooled unit, the house would cool better.

Builders are normally cheap. 2 units cost more than 1 unit. The difference was probably less than $500. To change now to seperate systems will cost many dollars more. Probably $4000 if you add a seperate unit for the upstairs. Zoned HVAC, if not designed correctly, does not work. Very few know how to design zoned systems. I do not claim any expertise in zoned systems. The best way to design a 2-story HVAC system, is a system per floor, or a system per living area.

    Bookmark   November 9, 2006 at 11:58AM
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aemeeich_

Where is your ductwork located? If it is in an unconditioned space (attic or crawlspace) you very well could have a leak somewhere. If this is happening you are loosing alot of your cooled air, resulting in longer run times and higher bills. So the first thing to check is for leakage.

Michael

    Bookmark   November 9, 2006 at 2:39PM
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dadoes

Wow. $350 and $500/month is awful! ... but you may have a large family. I live about 60 miles west of Houston. My house is 1-story, 2550 sq ft, 2 years old. I have a 5 ton 12 SEER Carrier heat pump. True, I'm a single-person household, and run the thermostat at 82°F when I'm gone for much of the day. 77°F when I'm home, although I may drop it a bit lower during mild weather conditions for better humdity control. My highest bill ever of $178 was two months ago, and that's only because of a problem with the system that caused one or both of the auxiliary heat strips to energize whenever the blower cycled off (longer setback times actually caused a *higher* bill). The most recent bill was $102.

    Bookmark   November 9, 2006 at 4:27PM
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bama_dude

stella,your house will probably never cool properly with one system.One system in a two story house does not work in a hot humid climate.There will always be a balance issue from upstairs to downstairs and a temp difference.It would also be cheaper to operate(cool)your home with two systems.You have a cheap builder trying to put more money in his pocket and less in yours.

    Bookmark   November 9, 2006 at 6:33PM
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stella_2006

Thanks everyone for your answers! I do feel like I am getting ripped off by my builder. I am going to have them check the duckwork for leaks and if that isn't a problem then I am going to see about getting another a/c unit installed. I have no idea if they can even do that after the house is completed but something has to give. I am under warranty until the end of April so I have a few months left to squack about this. I have a neighbor that got her builder to install a bigger unit for her house so I have a little hope that I can get something done. Wish me luck...I think I'll need it!

    Bookmark   November 10, 2006 at 1:04PM
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jeffw_00

go to www.hvac-talk.com, and buy a copy of hvac-calc and do a heat-calc measurement - then you'll know

    Bookmark   November 11, 2006 at 12:41AM
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udarrell_2007

It would appear there are problems causing the high utility bills.

Check the temp drops of the Return Air & Supply Air & get a percent Relative Humidity gauge. Then also check the condenser discharge air temp against the outdoor temp. Post that info here.

It could be drawing hot air into the Return Air!
udarrell _ 2007

Here is a link that might be useful: udarrell.com

    Bookmark   August 12, 2007 at 12:59PM
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jmor

If you keep it in the lower 70's, & you probably have to in order that the upstairs be live able, then $500 isn't far off at todays rates. Upstairs is a killer! I've had single floor & 2 story with upper & lower units & no one should want 2 story when it comes to temperature control, especially so with todays 'open space' designs. Many homes do not have as many square feet up as they do down & often this means upstairs rooms have some walls exposed to attic temperatures! Builders almost always use the least duct insulation they can (R4) and in a big house with long runs, I have measured 50% loss in capacity, i.e., paying for 4 tons, getting 2 tons delivered to living space. A crime! Especially considering the very small incremental cost of installing R6 or R8 in the first place. It would pay for itself in one season. Of course to rip it out & re-install would be another matter at this point.

    Bookmark   August 12, 2007 at 5:14PM
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pieinsky

I live in Houston also, but in a 1976 constructed 2 story 3000 sqft home. My house has equal square footage on both floors----1500sqft each. My home stays nice and cool (I pulled it down to 74 yesterday and you know how warm it was---I believe 102 is what the weatherman said) with an old 1988 5 ton Trane and it does so only because this unit is a true 5 ton rather than the BS they sell you now days. From the info I've received on this forum, I'd never change out this unit unless I was ready to have a complete change of ducts and install 2 units rather than the old Trane that is now taking care of business. Any person in our region that has an older or newer 2 story home with an old 5 ton that will "just get the job done" will have to purchase 2 seprate units if there old 5 ton crashes-----because there is not a true 5 ton unit to be bought at this time. It would take a 6 ton unit to do what an old 5 ton 10 seer unit will do on a hot and humid day in Houston and I don't believe they make a 6 ton for residental usuage. I believe 100% what BOB BROWN has said about how all the high SEER numbers that are thrown out there nowdays are nothing more than 100% BS.

    Bookmark   August 13, 2007 at 10:48AM
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udarrell_2007

Pieinsky & Bob Brown are on the mark when it comes to high SEER units.

I ask any rational person, does it make sense to Rate season energy efficiencies based on an 82-F outside & 80-F inside temps which is a mere 2-F degree differential. A mere 2 degree temp drop virtually eliminates the latent load humidity problem.

I know they claim with a Variable Speed Blower, etc., etc., they can somehow get thwe humidity down. Well, maybe but prove it to me in those conditions.

The SEER of a system is determined by multiplying the steady state energy efficiency ratio (EER) measured at conditions of 82°F outdoor temperature, 80°F dB and 67°F wB indoor entering air temperature by the Part Load Factor (PLF) of the system. (The PLF is supplied by the government.)

Consider the criteria being used for their SEER formulas.
Air conditioner EER ratings, and BTUH Tons of Cooling Capacity ratings on Air Conditioning units are rated at an outdoor temperature of 95°F, and an indoor 80ºF dB 67ºF WB or, a 50% Relative Humidity.

The larger coils & smaller Btuh compressor cause a higher pressure/temperature resulting in increased Volumetric efficiency for the smaller compressor's capacity at 82-F ambvient & 80 indoor temp. At 82-F outdoors our homes in the Midwest are well into the comfort zone until late afternoon, etc., fans could keep us comfortable.

Rate the SEER at 95, 100 & 105-F ambient & 76-F indoor dry bulb with a 55% Relative Humidity & watch the numbers SHIFT!
Additionally, how about a 62% outdoor Relative humidity, with those outdoor indoor temp spreads & the high SEER's higher pres/temp E-Coil it would never control the humidity.

Is it against the law for contractors' to rmove the smaller compressors & install an oversized compressor like we used to have 40 years ago? udarrell _ 2007

Here is a link that might be useful: udarrell.com

    Bookmark   August 13, 2007 at 11:29AM
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pieinsky

I forgot--we keep the thermostat set at 77 during the day and 74-75 at night and our old Trane does just fine. Last month our bill was 425.00 and figure it'll be a little more than that this month but we expect that. We also have a pool that runs 12 hours a day. Our heater is a natural gas that runs us about 115.00 on an average to heat the house. Our average electric bill in the summer which includes april, may, june, july, august, sept and october is around 385.00 with July August and Sept being the biggest but to be expected.

    Bookmark   August 13, 2007 at 11:35AM
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amtaustin

I just don't understand why these new houses have such high electric bills. I am in central Texas, 3000 sq-ft single story, 30 year old home, 2 10 year old Goodman units (4ton and 1.5 ton, about 12 seer I think). We keep the temps at 75 during the day and 73 at night. So far this summer we have not had an electric bill above $160, and we also have 2 electric water heaters. In fact, we are remodeling the kitchen, and I still don't have insulation on that ceiling!

I wonder if 2 story homes are just fatally flawed when it comes to A/C efficiency.

    Bookmark   August 13, 2007 at 2:20PM
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saltcedar

I think our electric rates are lower than Houston's too.

Regards
Chris in austin

    Bookmark   August 13, 2007 at 2:30PM
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garyg

"I wonder if 2 story homes are just fatally flawed when it comes to A/C efficiency."

- It's the 3-story homes (including basement level) with only 1 unit that are the killer in a/c. The t-stat is on the 2nd floor and the bedrooms are on the 3rd floor. 3rd floor is 5 to 7 degrees warmer than the 2nd floor. T-stat set to 75 = bedrooms at >80 and uncomfortable.

    Bookmark   August 13, 2007 at 2:47PM
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