Tuesday, September 28, 2010

Seriously hacked central air conditioning

In my old rental house, I had a large window air conditioner mounted in a window in my living room. It did a great job blowing cold air, but was ugly and noisy (as are all window air conditioners). When I moved to my new house, the need for air conditioning was a little lower since the new house has double-pane windows and the house is also shaded by trees. However, I play music with my band every week in one of the small bedrooms, and with six people in one room in the summer, the need for air conditioning is easy to grasp. I didn't want to mount the large window air conditioner in a window, since it is so big and ugly, so I mounted it in the house's crawl space and installed flexible ducting to connect it to the house.

I know it is very difficult to see anything in this picture; the crawlspace is very dark and cramped. The air conditioner is an LG 15,000 BTU/hr model. It's the largest LG model that runs on 120V. I modified it by adding standard register boots to its cold-side inlet and outlet. I also used a plastic storage bin to couple the inlet register boot to the evaporator coil. Everything is sealed with rubber foam gaskets. Currently, the hot-side inlet and outlet are not connected to anything and just vent into the crawlspace under the house. I know this is pretty dumb, so my plan is to connect the hot-side outlet (the back of the air conditioner) to some nearby crawlspace vents. I can use uninsulated flex duct for this.

I removed the unit's control panel and built some custom circuitry that allows me to control the device via the standard wall-mount thermostat in the house.

The inlet and outlet flex duct are 8" and travel up from the crawlspace in a utility closet that is open to the house's attic. In the attic, the return duct connects to a 14"x6" ceiling vent:

14"x6" return vent

The supply duct connects to an 8" inline duct fan that is mounted in the attic:
$130 shipped, new on eBay. This particular fan is very powerful and also very quiet. I was able to mount it to the ceiling joists, and it cannot be heard over the general air noise when the system is running. I originally tested the system without the duct fan, but the air conditioner's stock fan did not move enough air through the ducts to be useful.

10"x6" supply register

Another 10"x6" supply register

Yesterday, it was over 92*F according to this thermometer. It was 81*F inside my garage.

Ahhh, 70*F in the house!

I had just installed the new duct fan and was curious to see how well it worked, so I turned the thermostat way down and was pleased to see the system had plenty of cooling power. The house is only 1100 ft^2, so the 15,000 BTU/hr air conditioner is sized well. The supply air is just under 20*F cooler than the return air, which indicates a properly-sized system.

Today, I set the thermostat for 75*, and the system was able to hold that temperature while running between %25 and %50 (the thermostat uses "four cycles per hour"). It was about 92* today as well.


  1. Be careful when running flex ducts from crawl space to attics. When you cut holes in sheet rock you are breaching a fire wall most flexible ducting is not fire resistant and becomes a path for fire to accelerate through a structure. It was not a pleasant experience.

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  2. Wow, you're good at that. But yeah, do be careful since you're running it up the hotspots in your house. But your work is ingenious - you must have saved a lot from that.

  3. I did a similar install last summer in my single level ranch.. mounted a 28,500btu unit in my attic, and ran ducts for supply and return.. works great!!!! i need to figure out the electronics though so i can connect it to my hall mounted t-stat... saved my self thousands of dollars...

  4. I agree with Trey and the anonymous commenter, Ben. You have to be wary of scratching holes in sheet rock for you might break the fire wall. Indeed, majority of flexible ducting is not fire resistant and can lead to fire and haze through the structure. Just be careful.

    Harold Rhoads

  5. Hi. from australia.
    I'm intrigued by the installation. I have a 2 HP window mount AC unit which I've decided to put under the house and duct into 2 bedrooms. It'll mean removing the front mounted controls and lengthening the wiring so I can mount the controls in the bedroom.
    The controls have a thermostat poking out about 10" which sits directly in front of the return air coil. To me this is just measuring ambient air temp so does it matter if it gets hidden in the wall cavity - if you follow what I mean.
    Will this matter ?

  6. i have a GE 24000 unit that had digital control panel. unit worked but controls kept failing. would like to hook up to remote thermostat so i can use the ac in my shop. any help with the wiring details would be much appreciated

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  8. which meant running the wires behind walls. In older buildings this was a difficult process. room air conditioner

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