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My AC is set to 77F and the thermostat reaches that temperature but the AC keeps running. What could be the causes for this? This is in connection with this post How long should it take to a 1.5 tons AC to cool down a 2000sqf house from 80F to 77F? where you can read a lot of details regarding my AC

Edit: If I set the AC to 80F then it goes on and off every 10 minutes. It can possible be air leaking to this degree.

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    How much longer does it run? Most thermostats overshoot by at least 1 degree.
    – Tester101
    May 29, 2016 at 11:37
  • It keeps running and running and running-it is still running and it was indicating 77 when I walk up, 45 minuntes ago. It is still indicating the same. I checked the temperature and it is 77 (25C) -checkd that with a couple of thermometers
    – MiniMe
    May 29, 2016 at 11:43
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    Are you sure the Ac unit itself is actually running? Could it just be the blower is set to "continuous"?
    – Roberto
    May 29, 2016 at 11:50
  • What's the outdoor temperature?
    – Tester101
    May 29, 2016 at 11:52
  • The blower is on Auto.
    – MiniMe
    May 29, 2016 at 11:52

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It seems that the compressor lost the refrigerator and that is why it could not go below the temperature that would force the thermostat to say enough! It makes sense if you think about it... When I posted the question the AC was still working but at lower capacity due to the lost refrigerator. I did notice a strange smell (like cigarette ashes, but very light) coming out of the registries...

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When you say that your AC "keeps running", does it ever stop? Or does it keep going forever?

I know that many thermostats actually cool a few degrees below the target temperature and dont kick in till a few above said temp (usually a delta of 2F).

As a few people have said in the comments, it could be that your blower is set to "on" even though it indicates that it is set to auto. The only sure fire way to tell this is to actually let the system run for a while and hit 77F then go outside and look at your compressor to check if it's actually running. If it is than the problem is beyond my scope and if its off I'd be willing to bet the thermostat unit was busted and suggest getting a new one.

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  • I can hear the compressor and I can feel that the incoming air still has higher velocity. No need to go outside but just for curiosity I will do that to make sure that what I am hearing and feeling matches what is happening outside. BTW, the furnace does not seem to produce condense anymore. I have a dehumidifier running downstairs that is keeping the basement at RH 45%
    – MiniMe
    May 30, 2016 at 8:15
  • Okay cool. So when you're AC is blowing and the indoor temp is above 77 does the air feel any cooler than when it hits 77? Also, as a stopgap, try turning the fan from "auto" to "on" to help ensure that if there were some kind of ice build up on the coils that it would melt faster.
    – HMSCelestia
    May 30, 2016 at 8:21
  • Don't know, let my set it for that temperature to see what happens. The fan is on and if there is any ice build up I should continue feel colder air when the AC is switched off by the thermostat but that is not happening. This is a brand new unit installed in November. The house is 1900sqf (basment+main) and the unit is 1.5 ton (18000BTU) I should be OK by my calculations)
    – MiniMe
    May 30, 2016 at 8:28
  • This is a weird one to be sure. I'm definitely leaning towards a bad thermostat being the cause, but I also think that my reason for thinking that is a result of my bias toward electronics because of my more solid knowledge in that field.
    – HMSCelestia
    May 30, 2016 at 8:33
  • Do you have any handheld temperature sensors you can use to take the temperature at various places?
    – HMSCelestia
    May 30, 2016 at 8:34
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The problem is... your unit is too small for your house. My home is 2300sq ft, we have a 5 ton unit. In order to figure out how many tons you require this is the calculations you should use: Square foot of your home multiply by 30, then divide by 12,000. That will give you the tons required to cool or heat your home. It is also recommended that you subtract one from this number. So your calculations would require you to have a 4 to 5 ton unit. The unit you have is inadequate and will continue to run all day because it's not large enough to accommodate your home size.

2000 x 30= 60,000 60,000\ 12,000= 5 Subtract 1= 4

Depends on where you live if you subtract the one. If you're in a hot area like Florida or Arizona don't subtract 1.

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    Sizing is a lot more complex than this. Insulation, climate, design, other heat load all pay as large a part as plain old floor area. 400sqft per ton is a lazy worst-case estimate.
    – SomeoneSomewhereSupportsMonica
    Sep 16, 2017 at 11:03
  • Yeah, oversizing a HVAC system is just as bad if not worse than undersizing, as the system will then short-cycle and fail to pull the indoor coil down to the dew point, which leaves you with a cool but extremely muggy house as the humidity is no longer condensing out on the indoor coil, but staying in the air instead.
    – ThreePhaseEel
    Sep 16, 2017 at 14:43

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