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Recently, we've started using the AC less for cost and environmental reasons. When it gets below 82 degrees (F) at night, we open the windows. My understanding is that the cool air from outside should eventually replace the warm air in the house.

This generally has worked ok the past few months, but recently, it seems to be less effective. Last night, it got down to 76 degrees around 9 PM, 73 degrees when I woke up at 7 AM, but the temperature in the house didn't drop at all from 80 degrees according to our thermostat, which was set to circ with the AC completely turned off. All windows in the house were open. The thermostat is in the front part of the house, a 1,200 sq foot ranch. (This part of the house is an open kitchen/living area with eight windows, so there should be plenty of airflow.) We do not have an attic above that part of the house, but we do have an attic through the rest of the house.

We live in a suburb of Philadelphia, for climate information. It has been pretty hot lately. Is this due to hotter weather recently, a thermal mass problem, airflow problem, inaccurate thermostat, something else? I'm new to passive cooling, but I feel like with a 7 degree difference between outside and inside, the temp should have dropped at least a degree or two. Not sure what to make of this exactly. Any insight would be great.

  • Home cooling depends almost entirely on wind. If there's very little breeze, my well-insulated home doesn't cool off much at all at night. Unless you've been tracking average air speed, there's not much to say. All you can do is run fans directly in windows to force air movement. – isherwood Aug 8 '18 at 13:31
  • And for that reason we don't "open up" on calm nights unless the temperature gets below 70. – isherwood Aug 8 '18 at 13:32
  • Simple enough answer, thanks! I'll invest in some window fans and see if that helps. My understanding of airflow is limited mostly to computer and server hardware, but if it's anything like that, I could exhaust out one window, intake through the other and leave the other windows closed? – Robert Miller Aug 8 '18 at 13:37
  • Not an answer so much as a feeler for more information. Please take the tour so you know how this site works. – isherwood Aug 8 '18 at 13:44
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    Circ doesn't magically pull air from the coldest of { basement | outside }. Circ simply pulls air already in the home, adds the fan's wattage to it as heat, and puts it back. If you want to pull cold from outside (or to be more precise, dump the home's heat there), you need to force air in from outside... or use some sort of, I don't know, "heat pump" type tech that runs on freon. Those run quite efficiently "pumping heat downhill", i.e. moving heat from a hot place to a cool place. – Harper - Reinstate Monica Aug 8 '18 at 18:12
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Here's what may be happening...

At night, heat stored in the roof and walls due to solar gain soaks into the home's interior. This is balanced to some extent by cooling from outside air flowing through the home. Three outcomes can result:

  • The home cools somewhat if the inside/outside air temperature differential is substantial, and if there's a breeze to move air at a significant rate.

  • The home stays at the same temperature if there's not enough breeze and/or the temperature differential is small. Heat soak and cooling are essentially balanced.

  • The home actually warms, especially on the upper levels of a multi-level home, due to heat soak. This occurs if there's little or no breeze and the temperature differential is small.

The change you've noticed is probably related to lighter overnight winds and slightly reduced differential. If you don't have a differential of at least 10 degrees, and the wind is calm, you'll probably want to bite the bullet and keep the a/c on. Run circulating fans and/or the furnace fan to keep all areas of the home comfortable.

If you do open the home up in questionable conditions, run box or dedicated window fans to exhaust the home overnight. Run fans inward in sleeping rooms for maximum comfort.

  • Ok, sounds good. So the AC is running because the conditions are basically just not good for passive cooling. The house is hotter than the outside temp. The next question I'd ask for this scenario is: is it more energy efficient to bring the interior temp down to the outside temp or let it sit at the existing interior temperature and have it run when it needs to? – Robert Miller Aug 8 '18 at 14:32
  • It's generally not good to ask new questions in the comments thread. I'm not sure I understand the question anyway. What is "when it needs to"? – isherwood Aug 8 '18 at 14:33
  • Oh ok. Sorry, new here. When it needs to refers to when it needs to come on to reach a certain temp. Like if it's 80 inside, it will need to come on when the house reaches 81. Concrete example: interior: 80 degrees, outside: 77 degrees. Is the more efficient setting for the AC 77 or 80? Feel free not to answer if I'm using the site wrong. I will take no offense! I'm simply clarifying since you asked. – Robert Miller Aug 8 '18 at 14:38
  • I think you have the makings of a good question there. Go ahead and post another. – isherwood Aug 8 '18 at 14:55
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    Temp controlled window fans will help move the air and cool the home. Attic vents can really make a big difference here. One house I had in the 80's you could feel the heat radiating from the ceiling and although a small home a 3 ton unit couldn't keep up, I added a small temp activated fan to draw excess heat out that made a huge change and it was much cheaper than trying to go to r60, they make solar powered fans that work quite well because on the hot days the solar power has the fan going like crazy when the heat of the day and light is gone the fan stops a bit sooner than a powered fan – Ed Beal Aug 8 '18 at 16:32
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I have read all the answers and suggestions from the other guys so I won't add any new advice. By the way, they are all right about air flow, temps , the house insulation giving off heat, etc. My only addition is that you have to consider the relative humidity of the outside ambient air. As the air temp cools as it does at night the relative humidity rises, maybe to 100%, so you feel uncomfortable at any temp. I live north of Pittsburgh, pa. (go steelers) so we both have approximately the same temps however the wet weather has been relentless. If it is humid outside opening the windows will make it worse inside at almost any temp.

  • I agree, but only to a point. When we drop to say 55 degrees in Minnesota at night, RH is often at 100% and dew is forming rapidly. It still feels pretty good after an 80 degree day, though. – isherwood Aug 9 '18 at 13:40
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Natural cooling will be very limited. A large fan will cool fine ( depending on out side temp). I have a 24" fan and it cools ( to the outside temp ) in 15 minutes. I run it 30 minutes typically, to pull heat out of walls and house contents, then leave windows open for the night . Maybe another 15 minute run in the morning.

  • Can you clarify on the size of your house and the placement of the fan? Also what type of climate are you living in? – Robert Miller Aug 10 '18 at 16:05
  • 2400 ft; TX coast so The fan is only used a couple months in spring and fall. You can put the fan anywhere then open windows according to where you want airflow. Ours happens to be in a utility room in the corner of the house. This is an attic fan; blows from the house into the attic , so it even cools the attic a little. The belt drive fans turn slower so are not as noisy. – blacksmith37 Aug 12 '18 at 14:06

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