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Summary

The question pretty much says it all. It's probably a ridiculously easy one to answer. It's cool outside but I know that my house is heating up because my AC kicks on periodically. I've had this experience in multiple houses, so I presume it's not just caused by some sort of one-off issue with an AC or thermostat. It seems strange and I'd like to understand why it happens so that perhaps in the future I can make my house more energy efficient.

More Details

I'm a life-long Florida resident and have had this experience in a few houses. I've noticed that even when it is cooler outside than in (say the AC is set to 80°F (27°C) and the weather is 76-78°F (24–26°C) outside), my AC still runs periodically throughout the day. Note that the obvious answer is "Just open the windows", but for those not familiar it is usually 80%-90% humidity outside, and even 76°F (24°C) isn't pleasant at 80% humidity (it's also a recipe for mold, which has been a problem for me). Basically, there are only 1 or 2 months of the year when it is both cool and dry enough to open your windows in Florida (IMO).

In that sense having the AC run periodically is not actually crazy, mainly just to keep the humidity down. However, it still seems strange to me that if the AC should kick on (aka the house is warming up) at all if it is 2-5°F (1-3°C) cooler outside than the temperature I have the AC set to.

I realize this is likely house-dependent, but I've lived in more or less the same "kind" of house for the past decade or so: wood frame, insulation-in-attic, single pane windows, and a few trees around providing shade for the house (although certainly not enough to shade the entire roof).

I presume this is a sign of some general inefficiencies in my house's thermal-insulation-design. Why might my house be heating up even when its cool outside?

  • 35
    Your thermostat (which controls the AC) doesn't know what the outside temperature is, it only knows what the inside temperature is at the specific location where it is installed. If the temperature it measures is higher than the setpoint you've selected, then it commands the AC to run to reduce the temperature inside. – brhans Aug 19 at 12:45
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    My personal suspicion is that direct solar flux on my roof is heating up the attic (reference: attics are hot) and therefore what is going on is that poor insulation in the attic is allowing the heat from my attic to warm up the house faster than cool air from outside leaks in though my windows/walls. – conman Aug 19 at 12:53
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    That may or may not be true - but either way it's irrelevant. Your thermostat does not 'know' what the outside temperature is.. All it 'knows' is that the temperature it's measuring is higher than the setpoint you've set it to, so it runs the AC. – brhans Aug 19 at 12:53
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    @brhans lol, yes, indeed - I am well aware of how an AC works. I'm not literally asking why my AC is turning on. I'm asking why my house is heating up even when its cool outside. – conman Aug 19 at 12:55
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    Easy: Because you are sitting in front of the computer all day, reading StackOverflow. – Peter - Reinstate Monica Aug 21 at 10:18
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Meet solar gain

For the most vivid example of solar gain, sit in your car with everything off - A/C off, blower off, windows rolled up tight, doors shut tight. You can't. Your body will force you to open a door or something because it will become unbearably hot within minutes.

If it were always night, or if your house were entirely in shade, this would not be an issue. However, your house is in sun, and that means it is being actively heated by about 100 watts (300 BTU/hr) per square foot (around 1 KW/m²).

That's really a square foot directly facing the sun, so it'll be less if you're not at the equator at noon... but walls count too, so that's kind of a wash. The reflectivity of the roof and walls help. So let's say all in all, 50% gets through.

This is why a car with 48 square feet (4.5 m²) of cabin gets so hot so fast - it's intaking 2400 watts/8000 BTU/hr of heat. That's the heating power of an oven. This is why leaving a baby in a car is so serious. I'm not equating a house to a car, just pointing out that solar gain is not to be armwaved or ignored, and is definitely your #1 load by a wide margin, maybe even by an order of magnitude.

So, your 1000sf (93 m²) house, is absorbing 50,000 watts of heat, or about 150,000 BTU/hr, from solar gain.

Yes, this is a stupidly large amount of energy. Too bad we can't harness it for something, eh? :)

Insulation slows it down. But only slows it. Given enough time, it still gets through.

Thermal "mass" works both for and against you. The thermal mass inside the insulation envelope helps your house resist changes in temperature. However the thermal mass outside the insulation envelope (roof, joists etc.) has been warmed by the sun all day, has reached 120-140F (50–60°C), and holds that heat for a few hours after sunset. That means its high heat is still trying to push through the insulation even after the sun has gone down. Conversely in the morning, even with full sun, you get some relief before the roof and joists heat up.

At least it's more efficient...

Your A/C unit interchanges with air, and in your case, you are saying the ultimate heat sink is cooler than the thing it's cooling. That gives a heat pump an advantage; it is pushing heat "downhill". You can check it with a load monitor (if the cycle length doesn't make it obvious), but you probably aren't using near as much power as on a hot-air day.

It's even better, of course, to interchange heat with groundwater. But that is a more specialty unit.

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    This is very misleading. Sure, every square foot of your house is receiving a ton of heat from the sun — so is every other square foot of the daylit side of the earth. They don't heat up to oven-like temperatures because they're also radiating heat away at pretty close to the same rate, and the net heating power is very much smaller (or negative, in the late afternoon). In the absence of other effects that capture heat, the sun would heat the inside of the house to the same exact temperature as the outside. – hobbs Aug 19 at 23:45
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    @hobbs so according to your theory, earth sheltering a home shouldn't do much since roofs and earth should be equivalent. That's not true. Also many soils do get very hot. Examples are the desert, roadway surfaces, etc. What helps most other surfaces is deep mass: nobody cares if the road and 2 feet of mass under the road gets hot. Also, where soil is vegetation covered and watered, the dirt can do two things roofs cannot: absorb energy via photosynthesis, and use latent heat of vaporization of water. Your comparison of "roofs" to "every other square foot of earth" doesn't hold up. – Harper - Reinstate Monica Aug 20 at 0:03
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    I've often wondered if having a 'double-roof' would be worth it in places like Florida. By that I mean having a second layer of sheathing separated from the home by a an inch or so with a ridge vent and soffit vents. The solar gain on the top roof should then cause air to flow between the two layers and help keep it away from the structure. – JimmyJames Aug 20 at 13:32
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    @user3067860 Not misleading at all. Most houses have windows and any light rays that enter the house are captured and mostly converted to heat. This is in addition to the heat radiating off the structure. Many homes are not designed with the passive heating from the sun in mind: the developer doesn't pay the AC bill and many/most home buyers don't understand the impact. – JimmyJames Aug 20 at 13:42
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    @conman Designing things so you avoid solar gain in the summer is the main goal in Florida e.g. large overhangs on the roof to shade windows. The second image in this article shows a home somewhere where palm trees grow with an example design using plants. Solar energy can be used to create a chimney effect or stack effect which can draw cool air up through the home but with the humidity in FL, solar panels driving AC might be the way to go. – JimmyJames Aug 20 at 14:30
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Because you have indoor heat sources

There's a lot of reasons your house will get warmer than the outside temp, but this is the single largest one.

  • The human body produces as much as 400 BTU (422 kJ) per hour at any given time, equivalent to 117 W.
  • Your refrigerator can give off close to 500 BTUs (528 kJ) per hour (147 W).
  • A TV can use about 20-30 watts, or about 70-100 BTUs per hour
  • Stoves and ovens add varying BTUs, but don't constantly run either

Add all these up and you'll find your house reaches the necessary temp to kick on, even if it is cooler outside

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    I'm not sure why neither the fridge nor myself occurred to me, lol! I work from home and we homeschool, so there are literally 7 people home all day long... in retrospect it may not even be my house's fault... – conman Aug 19 at 13:10
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    I have a hard time with the notion that a few bodies and appliances contributes anything near the heat that solar gain does. Surely it's a factor, but a minor one. – isherwood Aug 19 at 15:38
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    BTUs per what ? – user253751 Aug 20 at 2:29
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    I looked up this strange BTU unit and it doesn't make sense here. – Nobody Aug 20 at 13:53
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    That can't be done without considering how long it took to produce that much energy. I suspect air conditioners are abusing notation, and instead mean BTU per some unspecified standard unit of time. Since humans and appliances are continuously making heat, you'd have to compare units of power, like watts or BTU per h. – Vaelus Aug 20 at 14:18
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In addition to the other answers that address your question for the most part, there is one more possibility I can think of. It is possible that your AC is not only trying to get the inside temperature to the target,but also the humidity.

Some high-end thermostats will have a humidistat built in, while it is a separate unit in other cases. Regardless, some AC systems are setup to also run when the humidity inside is too high. Some systems have special modes for this, that somehow don't cool as much, but pull more humidity out of the air they handle (probably at a loss in efficiency, but I don't know for sure how this works), while others just run like normal, causing it to get colder than your target temp inside. In US brands, I think one that I've seen the built in de-humidifcation feature a lot is Trane, but others may have it also.

  • That's helpful to know. I own my current house (which needs a new AC soon), but my previous houses were rentals and definitely did not have high-end thermostats. That may be of interest to me in the future though! – conman Aug 19 at 14:02
  • Do you know keywords to search for to buy such a thermostat? I've considered rigging one up just to avoid having to constantly adjust the setting to ensure that the AC runs sufficiently often to eliminate humidity without wasting lots of energy midday on hot days. – R.. Aug 20 at 1:44
  • There are stand-alone units you could probably wire in parallel with your regular thermostat (search dehumidistat). I've seen thermostats with this function that are usually made to work with a specific air handler/furnace/AC unit, that has the aforementioned functionality. This would supposedly avoid overcooling, while still maintaining the desired humidity. But there maybe also regular thermostats that just run the AC with lower fan speed or something, maybe search for thermostat with dehumidification control ( I found a Honeywell one with that term, for example). – PhilippNagel Aug 20 at 2:23
  • @R.. Indeed, during cool whether I adjust my thermostat up and down during the day, not because the temperature is too hot or too cold, but simply because the humidity is too high and I need the AC to run for that reason alone. I basically have to overcool my home, just a little. – conman Aug 20 at 12:50
7

Because an Air Conditioner doesn't give any airflow between inside and outside. An AC is essentially a refrigerator. Inside is the conditioned space, and outside is the heat dump (i.e. the back of the fridge where it's hot) The heat is transferred from inside to outside via the liquid coolant - it evaporates inside, collecting heat due to the latent heat of vapourization, and compresses outside, giving off this heat. This is a closed system, contained almost entirely in sealed copper pipe.

There is no air transfer between outside and inside. So, if your inside temperature is higher than the thermostat set point, the AC will run -- regardless of the outside temperature. If you want to take advantage of the lower outside temperatures, then you need to bring outside air in. i.e. Open a window, which you have good reason NOT to do given the humidity.

As a simple thought experiment, we could remove all insulation from the house, and then your internal temperature would be a better match to the outside, as the purpose of insulation is to significantly reduce heat transfer through the walls and roof. However, this plan would be advantageous for only a few days of the year.

In addition to Machavity's answer, I'd also like to comment on solar warming. The sun hits your roof, and adds heat. This heat transfer is mitigated by the roof insulation, but not eliminated. Another source is the sun hitting the windows. Even if you have curtains or internal blinds, a lot of heat enters the house this way. The best way to mitigate this is with external blinds or shutters. These stop the sun from hitting the glass and causing a greenhouse effect.

  • "it evaporates inside, collecting heat due to the latent heat of vapourization, and compresses inside, giving off this heat" - You have 'inside' twice. – Adonalsium Aug 20 at 13:27
  • Good Catch. Fixed – Chris Cudmore Aug 20 at 13:28
5

There's nothing wrong with your thermal insulation design. Insulation is designed to isolate your inside temperature from the outside temperature. So your a.c. is set to 80°F (27°C), then at 3am the temp outside slowly drops to 76°F (24°C)... it will take hours for your inside temp to see the difference because you attic is still going to be hot. By the time it could see the difference, the outside weather is heating up. The same happens in the winter. Your inside temp is at 78°F (26°C), we have a cold snap (yes, I live in Fl too) and the temp drops down to 40°F (4°C), your inside pretty much stays the same. Hope this helps.

Last year I installed a whole house fan similar to the one below. It vents into the attic so when it does get cool out, I open windows and turn on the fan. It sucks air into the house and cools off the attic. I had to install 13 additional soffits in the eves but it was well worth it.

enter image description here

  • While I mostly agree, we can't say that there's nothing wrong with the insulation because we know nothing about it. There may be gains to be had by modernizing, especially in the attic. – isherwood Aug 19 at 12:54
  • This largely conforms to my suspicions. In other words, this is likely a sign that my house heats up a lot due to the heat in my attic. i.e. better something in the attic may help with my overall AC usage. – conman Aug 19 at 12:54
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There are a lot of answers that address why it can be hotter inside than outside. But to your point, "it still seems strange to me that if the AC should kick on at all if it is 2-5 degrees cooler outside than the temperature I have the AC set to." The thermostat is inside and has no idea what the outside temperature is. It turns the AC on when the inside temperature is above its set point.

The outside temperature has very little instantaneous effect on the inside temperature. As Jack describes, the house insulation creates a long delay between changes in outside temperature affecting the inside temperature. So in terms of the thermostat turning the AC on and off, the outside temperature at the time is basically irrelevant.

  • The question is "Why might my house be heating up even when its cool outside?". Do you answer that? – axsvl77 Aug 21 at 12:19
  • @axsvl77, I state in the answer that that was not the aspect I was answering. The question is now displaying the 7th revision. I answered before revision 5, when the relevant wording was changed. At that time, the wording suggested an additional question that was not being addressed. I quoted the exact wording I was responding to. It appears that the OP didn't realize that the original wording was ambiguous and could be understood in different ways, and added clarification after seeing this answer. – fixer1234 Aug 21 at 15:26
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Check your thermostat is in 'cold only' mode in summer and 'heat only' mode in winter (not in AUTO mode). If in AUTO it could be starting the furnace if temperature goes under the setpoint then kicking in the AC when it goes over

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