2+ Hour Run Times


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Old 07-02-17, 02:39 PM
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2+ Hour Run Times

I recently purchased a home and have noticed some longer than average run times on my air conditioner. I had the system inspected by a HVAC company prior to purchasing and they said that the unit was older, but in fair shape. They cleaned both the inside and outside coils.

The system is a 3T Goodman from 2003. It has all metal ductwork. The main supply/return plenum is uninsulated, (about 2ft of duct) but all of the branches are insulated with R2. The air handler is located in the attic. The liquid/suction lines are run about 20ft in the attic. The return is 18x24.

The home is 1600 sqft, 9-12ft ceilings, single level home with r38 above the ceiling, double pane windows, metal insulated doors. The current weather is 90 degrees with 80% humidity. My thermostat is set at 74. Indoor humidity is 55% according to my Nest.

I went up and did a basic check on the system myself today. I noticed a sizable gap where the liquid/suction lines enter the coil and I sealed that up with metal foil tape. I also noticed another hole on a section of duct running between the furance and the coil and sealed that up as well. Other than that, it seemed pretty well sealed up. Everything is sealed with foil tape or mastic. I didn't notice any other leaks when inspecting it.

I plan on adding in a 14x14 return to the master bedroom suite to get a more proper amount of return space.

My question is: Should I expect such long run times in the summer given the age/conditoin of the system? Would adding a layer of R6 over the existing duct be beneficial? Any other ideas to boost efficiency?

Thanks in advance!
 
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Old 07-02-17, 02:54 PM
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FWIW, I have slightly more square ft than you, less insulation and a 5tn AC and my experience is 90 w/ relative humidity of 80% and thermostat at 74 has my AC running for similarly long periods of time.

Maybe there is a scientific way to measure how long your system should run based on the stats you gave and if there is, someone here probably knows it. My lay opinion is that 90 & RH 80, which is a heat index of 114, is just really too darn hot.
 
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Old 07-02-17, 03:50 PM
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Check the temp at the supply register and at the return air. Let us know.
 
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Old 07-02-17, 04:15 PM
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If you just bought the house.... what are you comparing your run times to ?
Do you mean expected run times ?

There should be no un-insulated duct work in the attic.
 
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Old 07-02-17, 04:50 PM
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You're absolutely right. I have nothing to compare it to directly. It just seemed like a long run time. Plus, I figured with such long run times that the indoor humidity level would drop a good bit.

My concern was that if it was running so long to maintain a 74 temp at 90 degrees outside, what would it do at 95 or 100 degrees.

I'll get those temps to you guys as soon as I find a 9V for my meter.
 
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Old 07-02-17, 05:05 PM
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Supply temps range between 63 - 68.5 degrees.

Return temp is 75.5 degrees.
 
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Old 07-02-17, 06:41 PM
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^That's pretty bad.

Are you measuring at the supply vents or at the air handler?

Take measurements at the return at the furnace and right after the coil. ideally take a return air wetbulb temp as well; google how to do that. the wetbulb can be used to determine what the supply temp should be.

You can tell if you have a problem with the ducts or problem with the refrigeration side of things.

R2 insulation is minimal; i would want at least R8. Do you have hard pipe or flex? how thick insulation on pipes?

3 tons sounds like a lot for that size so it shouldn't be running for hours at a time at 90f. of course sq ft is not a good determinate of required capacity.

i'm in a much cooler climate than you are and houses where i am of your size, if two story and detached with ducts in the basement would have a 1.5 or 2 ton a/c; and at 90f you get operation for hours. but it hits 90+ 1 week or less per year, some summers, not a single day.

You have more capacity, but it's of no use of it gets lost to the attic.
 
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Old 07-02-17, 07:54 PM
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I took the measurements at the registers. The cooler measurements were closer to the air handler.
 
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Old 07-02-17, 08:14 PM
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try at the air handler or furnace. because you're probably losing a lot through the ducts.

 
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Old 07-03-17, 08:35 AM
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At 90 outside your system should run basically all day. satisfying at those higher temps would indicate the unit is over sized.
 
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Old 07-03-17, 12:01 PM
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oversized is a relative term. if the op's area hits 100f cycling at 90f doesn't mean it's oversized.

And being 3 tons, conditioning a little more than 500 sq ft per ton, it shouldn't be running hours on end at 90 unless there's something wrong with the house or system.

if it was sized how units are sized in the northeast, different story.
 
 

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