# Thread: AC won't cool below 79 degrees!

1. Regular Guest
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This post is a copy from a community form I host. I realize there are many factors in choosing the peak effeciency system - none of with which I am familiar. I've downloaded the trial version of the calc program, but was intimidated by the many options. My question is fairly simple... Should I expect to have a new system get my house to 78 degrees?

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My 1 year old, single-story, Florida spread, 2000 sqft Pulte home came equipped with a 3.5 ton Lennox AC. Since day one, it has struggled to get the temperature down to 78 degrees during the day.

In fact, I recently connected a data logger to the AC to monitor the temperature and the AC on/off cycle time. Turns out, when the AC turns off, the house rises about 3-4 degrees per hour. When it's on, it drops about 1 degree per hour - until about 79 degrees. Then, the AC simply doesn't turn off.

My questions to other homeowners (especially those in Pulte homes) are:

1) Are you able to get your home lower than 78 degrees during the hottest part of the day (with outside temps >90 degrees)?

2) If so, does your AC cycle at all, or does it stay on?

3) How long does it take for your AC to lower from, say 80F to 78F?

4) What is your ton to sqft ratio (i.e. how many sqft do you have in your home and how many tons is your ac)?

My thermostat has a runtime meter on it. On average (with temperatures above 90F) I will have between 12-14 hours of run time on my AC per day - with the house never getting below 79F until either the sun goes down, or the rains start.

Technical:

Supply vs. return differential = 22F
Temperature drop w/AC on = 1 to 1.5 degrees/hr
Temperature rise w/AC off = 3-4 degrees/hr
Sqft vs. Ton = 571:1
Stall point = 79F w/temps >90 outside.
Runtime = 12-14 hrs/day

Any help would be appreciated!!!

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The first thing I noticed is why does your unit stay off for an hour? Once the temp rises around 2 degrees over your set point it should cycle back on. Granted there are different parameters for different stats & systems, but there is more to this....
call a pro and have them evaluate the system. Is this an old stat?

3. Regular Guest
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No, I manually turned it off while logging the temperature rise/fall. It was intentional to see how quickly heat builds up. The thermostat is about a year old - Honeywell 3600 (I think). It's pretty close (within .5 degrees of my datalogger).

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The only way is to have a professional load calc done. You have put in a lot of time and energy, but only someone who is trained will be able to tell you why.

5. Member
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no.1 22 degree split indicates low airflow across evap coil.
should have 15 to 20 degrees.

no. 2 571 sq ft per ton means your system is to small, depending on type of construction, shade, window, etccc.,
and most important how well insulated. at 571 sq ft per ton tthen if your house is not highly efficient then eighther upgrade efficiency of home or invest in larger a/c or install a zone system. if you have a good system then i would zone it. many benefits. lower utilitys, more even temps through out home. if you go to a larger ac system then utilitys automatically go up.

6. If the A/C was designed & installed properly, you should be able to get a 20 degree drop indoors compared to outdoor temp. For example if the outdoor temp is 90 degrees your system should be able to maintain a 70 degree temp in your home. You may need to call someone to take a look at your duct size and thoroughly check the systems capacity.
Make sure you have enough return air. Common problem is NOT enough.

[Edited by 2hot2coolme on 06-22-2004 at 11:03 PM]

7. Senior Tech Guest
Calculating tonnage based on square footage...why not just use the three finger method...if you want the right size equipment and possibly ductwork call a pro...get a professional load calc done and be happy...not hot.

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All valid points. I've had two "professional" companies come out and inspect the system. However, since the house is only a year old, everything is done through the builder (Pulte) and their contractors. In fact, one of the AC guys came out of the attic saying that he'd found "something of interest". But, he wasn't allowed to tell us (the homeowner) anything without first consulting with the builder. I guess they don't want to bite the hand that feeds them. Clearly, I need to find an unbiased professional who is willing to go on record with facts - and neither a) fear builder retribution, or b) try to oversell me on something I don't need.

Ideally, I'd like to prove to Pulte that the system (whether the AC unit, insulation, or design) is inadequate. If it is as simple as saying a system should be able to reach/maintain a 20 degree differential (over outside temp), then it would be pretty cut and dry. Unfortunately, Pulte says 78 is where new systems "stall", and it is not unusual for AC's to run without shutting of at 79 degrees. Arg.

For the record, the first guy found that the plenum was too small - it was one for a heat-pump system (mine is straight cool with gas furnace). After consulting with Pulte, they deemed it acceptable, and no action was taken.

9. There's something wrong then. I live in Chicago where the summers got very hot & humid. We design our systems here to remove humidity and our extreme set design temp are 95-98 degrees. We have to be able to install a complete system that can maintain indoor temp (summer) at 75 degrees. This can be done. The only time it would probably be 78-79 inside is if the outdoor temp would get to 100 with a very high relative humidity.

10. Professional Member*
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Your system is in the attic.....It could be a problem with poorly sealed/insulated ducts.What type of duct do you have & insulation?I would pay for the heatloss calc at the top of the page & also pay an independant contractor to inspect the ductwork.

11. ## Get a 2nd opinion !

Don't mess around with the builder at this point get a proffessional opionin be prepared to spend some money for a complete system diagnosis and written evaluation. Shop around and find a shop that can provide you a written system evaluation. When you find one you'll have found someone that mos tliely will be prepared to tell you whats wrong and correct it. Typically these complaints are from inadequate sizing but other thigns may be invovled such as open return in the attic among some things... Can't see it form here...

Get someone out there to look at it and then go bear hunting for your buidler when you find the solution.

12. Senior Tech Guest
Originally posted by erkme73
All valid points. I've had two "professional" companies come out and inspect the system. However, since the house is only a year old, everything is done through the builder (Pulte) and their contractors. In fact, one of the AC guys came out of the attic saying that he'd found "something of interest". But, he wasn't allowed to tell us (the homeowner) anything without first consulting with the builder. I guess they don't want to bite the hand that feeds them. Clearly, I need to find an unbiased professional who is willing to go on record with facts - and neither a) fear builder retribution, or b) try to oversell me on something I don't need.

Ideally, I'd like to prove to Pulte that the system (whether the AC unit, insulation, or design) is inadequate. If it is as simple as saying a system should be able to reach/maintain a 20 degree differential (over outside temp), then it would be pretty cut and dry. Unfortunately, Pulte says 78 is where new systems "stall", and it is not unusual for AC's to run without shutting of at 79 degrees. Arg.

For the record, the first guy found that the plenum was too small - it was one for a heat-pump system (mine is straight cool with gas furnace). After consulting with Pulte, they deemed it acceptable, and no action was taken.
"new systems stall"...what a line of crap...tell them it's a ac unit, not a torque convertor, give them the business card of a lawyer and I'll bet they start singing your tune.

13. Professional Member
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I agree with my fellow contractors - you need to find a well qualified contractor to perform a load calc and survey
the current installation. I live in Tulsa and can tell you
that a 3.5 ton A/C wouldn't haul a 2000 sq ft house here and
I think you have much higher humidities there. Spend a few dollars, get the right answers, and get ready for a fight.
Good Luck

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