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Old 07-18-2013, 04:40 PM   #1
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Central A/C Running Non Stop


Hi all,
I have the Nest thermostat and starting last night and going into today I am noticing something strange. To start the weather here has been in the mid 90s for the past few days with the heat index around 100 or above. Starting last night I noticed the temperature at thermostat was 74 when it was set at 72. It did not actually reach 72 for several hours. This morning it was still set at 72 but the temp in the room was 73 and it has not reached the 72 degree temp all day. It currently stands at 74 degrees so it has only gone up 1 degree throughout the day even though we are currently at the hottest part of the day. My question is, is this normal to be running for so long and not reach the temp. The A/c is about 14 yrs old so I imagine it might be time to replace?

Thanks for any advice!
Andrew

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Old 07-18-2013, 05:11 PM   #2
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Central A/C Running Non Stop


This ultra hot weather is great for showing if your a/c is not working perfectly. Get it serviced... a coil cleaning might be all you need... or maybe a little refrigerant. A tech could tell you pretty quick.

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Old 07-18-2013, 08:46 PM   #3
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Central A/C Running Non Stop


You could try cleaning the condenser coil yourself or at least flushing it out with a garden hose. That might be all you need or at least improve the system operation a bit. With the heat wave it I suspect most HVAC companies are really busy.
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Old 07-18-2013, 09:03 PM   #4
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Central A/C Running Non Stop


Check your filter.
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Old 07-19-2013, 10:11 AM   #5
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Central A/C Running Non Stop


I would agree with Hardway. Step #2 Call a tech.
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Old 07-19-2013, 01:42 PM   #6
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Central A/C Running Non Stop


You say you have your thermostat set at 72, actuall temp is 74, and it is in the mid 90's with heat index of around 100. It sounds to me like your AC is functioning properly, it just does not have the capacity to do what you want it to do. A 20-25 difference in temperature from outside to inside is pretty decent for a residential unit and a standard house.

How much heat gain the house absorbs is dependant on many things like insulation single vs double pane windows, shade etc. As the temperature outside goes up, the gain goes up, the AC has to remove more to maintain the temperature. Once the heat gain reaches the capacity of the AC, the AC will run continuously and not be able to maintain the set point. Don't forget removing humidity from the air also utilizes some of the capacity.

Example - I'll use my house and numbers to illustrate

Inside temp set for 75, outside it is 85 AC is a 3ton AC = 36000BTU. With a 10 difference, the house is gaining say 15000BTU, so to keep the house at 75 the AC just has to remove 15000BTU. No problem.

Now say the outside temperature is 95. That 20 difference is driving a higher heat gain of say 30000BTU, so now the AC must use most of it's capacity to maintain the temperature, The AC will be running a lot more.

Now say that it is 100 outside, causing a heat gain of 37000BTU. The AC is not able to remove 37000BTU. It will run continuously trying. The inside temperature will slowly rise until the heat gained by the house due to the temperature difference is the same as what the AC can remove. In this case the temperature would probably rise to 76.

The maximum inside outside temperature difference any AC can achieve is also dependant on the humidity. Some of the AC's capacity is used condensing water out of the air, in high humidity areas, it take more of the capacity and thus a smaller temperature difference is achievable.

Also, there is a design temperature for each area. The simplified explanation is this is the temperature most total hours are at or below. This temperature is considered the continuous run temperature for an AC for the thermostat set at 75. For my location it is 100 If it is low humidity (my area usually is) and 100 outside, the AC will run continuous to maintain it. If the temp goes to 105, the inside will gradually warm to 80 even with the AC running continuously. I personally set my thermostat to 78, and when it hits 103 the AC does not shut off, above that and the indoor temp begins to rise. Or if we have unusually high humidity, the indoor temp begins to rise at a lower outdoor temp, like 99.

If I want to maintain a larger temperature difference, say 72 when it is 100 out, I need to do a couple things.

1. Reduce the heat gain of the house by adding more insulation, double/tripple pane windows with UV blocking, white shingle roof, light color house paint, shade trees, etc

2. Increase the capacity of the AC to say 3.5 or even 4 ton. Problem with this is in high humidity areas, if the temperature difference is not much at the time say 5-7, the AC will not run long enough to remove the humidity for comfort. This is where 2 stage compressors really shine. Operate at lower capacity for longer period of time to remove humidity when the difference is not that great. Opperate at high capacity when the difference is greater. Since I live in the desert, this is not really an issue for me.

Hope this helps some
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Old 07-19-2013, 06:00 PM   #7
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Central A/C Running Non Stop


Hard to say without a location. Is the mid 90's way hotter than it gets in your area,normal or cooler than normal for mid-july?

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