Over this weekend, I have made a breakthrough scientific discovery that will likely change the world - hot air rises compared to cooler air! :)

My Central AC system has its intake at the ground level, and the ceilings are 8' - it's an older but remodeled house.

I have the space to reroute intake from the floor to the ceiling. My idea: why have the AC unit pulling in the coolest air when it could be working on the warmest air instead? And, additionally, that would be the logical flow direction, so this must be more efficient. With its location it would be easy to construct, too.

My question is, just how much more efficient is this? And do you have any tips on the routing? I will be able to keep a wide radius on the new ducting and it will have at least 20x20 cross-section (equal to the intake). Thanks!

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    Not only that -- humid air rises compared to dry air too – ThreePhaseEel Jul 1 '17 at 23:31
  • "why have the AC unit pulling in the coolest air when it could be working on the warmest air instead?" You seem to be assuming that, given the same humidity level and the same outdoor conditions, the intake temperature somehow impacts "efficiency" in an counterintuitive relationship. A system that is appropriately sized and currently cools 72°F intake air down to 56°F... would reasonably be expected to only cool 76°F intake air down to ~60°F. – Michael - sqlbot Jul 2 '17 at 3:55
  • Good point. I was thinking more abstractly that perhaps a) the relationship is not linear for all ranges (not an engineer though) and b) I'm assuming that there is overall greater relative efficiency working a greater load. There must be an analog to a power curve there. So that yes you're not cooling as much but you're cooling "more" - does that make sense? – Oliver Williams Jul 2 '17 at 4:29
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    Yes, it does make sense, conceptually -- getting more work out of the system by offering it more work to do. However, much about refrigeration is counter-intuitive when the physics aren't taken into account. The formula is linear: BTU of sensible heat = airflow-cfm × temperature-drop-°F × (constant) 1.08, so over normal operating temp ranges, this relationship should also be pretty linear. The system is already removing as much heat as it is able to. Offering it more heat to remove (within normal operating range) shouldn't increase the amount of heat it is able to remove. – Michael - sqlbot Jul 3 '17 at 17:06

Most forced-air HVAC systems in Residential are both heating and cooling.

What might be efficient in the summer will not be efficient in the winter.

So, where do you live and which do you use more?

If you live in Arizona you optimize for cooling but if you live in Minnesota you optimize for heating. Then you just deal with a less efficient system in the opposite season.

Re-routing your ductwork may be much less cost efficient than simply buying a newer AC unit with a higher SEER rating.

Make sure you think this all the way through before pulling the trigger on anything.

Good luck!

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    +1 for the comparison between heating and cooling. Definitely cooling in Texas! – Oliver Williams Jul 2 '17 at 3:51
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    You can also install a second intake but keep the original one there. Then, during the summer you cover up the lower one and during the winter you cover the upper one. I know some people who do this with intakes on multiple levels of their houses. – Moshe Katz Jul 2 '17 at 5:29

I am not quite sure what you are trying to accomplish but let me add my "2 cents". Many years ago heating and cooling systems were installed properly with both high and low supply registers and return grills in every room. Today it is too expensive to do a "good job", so everyone is now doing a cheaper, "less expensive" job. If almost every room has only low return grills and you want to add high return grills which will add to your homes comfort, you or an HVAC company could replace the "low only" grills with high and low returns. "GOOGLE" The Hart& Cooley register company for an idea of the registers you will need. You will need ones similar to the #651 wall registers and #672 grills, size 6" X 14". These 2 registers used together will allow for a high open summer and low open winter room return system. As I said this combination was used years ago, I have them in my house. You use the same stud space to mount the register/grill combination. The single flap design allows the register to be open, blocking the stud space. When the flap is open, only that register is used and when the flap is closed only the other register/grill is working. You would now have a cheap way to have high summer / low winter returns by only having to buy the single flap registers, no expensive duct changes would be needed. HVAC companies used to do this all the time. If this is not what you have in mind, rewrite your question with more detail. Hope this helps.

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