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I have central air in my house(20 year old house). In my basement it gets cool, and cold air is coming from a vent in the ceiling. My upstairs gets really warm and humid even though central air is on. What should I do to resolve this? I was suggested to replace windows, fill attic/crawl space. I heard there could be a tool to help even out the air but the technician wouldn't share the secret.

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    If the vent has a way to restrict air flow, use that. If not think about taping a piece of cardboard or material over the vent for a temporary test. If it helps, block some or all of the airflow with a more permanent solution, like plexiglass screwed to the ceiling over the vent or other creative and attractive measure. – George Anderson Jul 14 '20 at 0:25
  • Duct tape works very well also. – blacksmith37 Jul 14 '20 at 1:12
  • Unless it was built badly (too often the case) a 20 year old house should not need windows replaced, and should have been insulated when built. What is the insulation situation? Do your ducts leak, or are the upstairs ones poorly insulated in a hot attic? – Ecnerwal Jul 14 '20 at 1:18
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George Anderson is right. Balancing air flow is the easiest and most cost effective solution. Obviously, cold air is heavier than warm air. In the summer you need to direct AC air flow to higher floors and restrict it on lower floors. In the winter you reverse this directing heat to the downstairs and letting it rise via convection to upper floors.
Ceiling fans also help if used properly. To cool use mid to high speeds blowing down in the summer. In winter use slow speeds blowing up in order to bring warm sir from the ceiling down the walls.

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    Dang, you turned my comment into an answer! Stupid me! LOL NP!!!! Upvote my comment to make up for it! – George Anderson Jul 14 '20 at 0:53
  • Well actually...making the answer "community wiki" is the traditional approach to turning someone else's comments into an answer. – Ecnerwal Jul 14 '20 at 1:15
  • Sorry. Well I did expand on it a little. Put it in as an answer and I'll give you an upvote. BTW - what is "community wiki"? – HoneyDo Jul 14 '20 at 19:19
  • I already upvoted and would have answering with a similar answer as I have in the past, simple dampers can easily help winter summer heating / cooling. Close or restrict lower floors open upper floors in summer in winter close upper floors and open lower floors ant take advantage of heat rising and cool air dropping many don’t add the trunk dampers a very small expense that can do this in 1 minute if properly set up in the trunk.+ – Ed Beal Jul 14 '20 at 20:45
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Is there sun shining on the upstairs moreso than the basement? It sounds like part of your problem could be solar gain. The sun applies 1000 watts per square metre (about 100 watts per square foot, or 300 BTU/sf), almost all of it turns into heat. A light colored wall or roof will reject some of that, the lighter the better - my preferred white paint has 91% albedo (reflection of solar energy).

So most of your A/C capacity is going to reject that “solar gain”. However, solar gain falls only on parts of the house, and this changes through the day. In the morning, your east-facing rooms take the brunt (but they’re usually cool from the night before, so not much of a factor). In the afternoon, south and west facing rooms take the brunt. If clouds roll over, solar gain is reduced, and now the house is being heated evenly by the outside air which is warm.

Let me guess, you’re trying to counter that with a “1-zone” system. That is, the system cools all rooms equally, and is not mechanically able to redirect the cooling to where it is needed most. Further, I bet that same system is used to distribute heating. Heat turns the formula upside-down, because now, “solar gain” is your friend! The solar-gain rooms require less heating than the rest of your house, and may get too much heat.

So how do you want your system balanced?

  • Afternoon sun during A/C
  • Evenly, ignoring all solar load
  • Afternoon sun during heating

Whatever choice you make, you’ll have to adjust registers to accommodate that choice, and then, you have to live with it. That’s what a 1-zone system means. This is why multi-zones are becoming popular, particularly with “mini-split heat pumps” which both air condition and cool.

It’s also possible you could use any adjustment features on the registers to adjust it “on the fly” hour by hour, but then you’ll a) need to have a strong grasp of the problem, and b) go around and do it all the time.

Another, simpler option is to install a window A/C on the room in question.

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  • Thank you for the explanation. I am very certain the heat is primarily caused by solar gains. Which is why I wanted to find out would more insulation be a proper solution to help reduce the amount of heat coming in. We rent out half the basement, so covering registers aren't an option. – Master Jul 14 '20 at 18:06

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