Which is more energy efficient: central AC or in-window air conditioners?


Which is more energy efficient: central AC or in-window air conditioners?

Asked by Laura Bianchi Payne

My home is 25 years old in the new addition, and the rest of the home is 70 years old. I have a forced-air heating system in the new addition. I am considering adding a central AC unit to the newer system. I live near San Jose, CA. The main living area of my home is an open floor plan with 13' ceilings in the living room. Even without an energy audit, I know my home is very leaky. We currently don't use the forced heat because the vents are 10' up on the walls and it is very expensive to run. We use individual space heaters in the bedrooms and a wood-burning stove in the living area with ceiling fans to circulate the heat. We are debating removing the forced heat system to free up some closet and attic space. I don't like the look of window ACs, but if it is more cost-effective I am willing to put them in the kitchen and master bedroom.

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Randy Potter's picture


From what you have explained about the history of your house, it sounds like you have a very inefficient and likely leaky forced-air duct system.

  • Most traditional duct systems can have upwards of 40% leakage.
  • That's right, almost half the heat that your furnace creates never reaches the living space!
  • In addition to this, you likely have an old and inefficient furnace unit and your high ceilings and vents make it even harder and more inefficient to heat and create any feeling of warmth for occupants.

Electric space heaters are highly inefficient

Running electric space heaters is absolutely the least efficient way to heat your home. These units typically draw 1500 watts continually, so running several of these all night can quickly become an energy use nightmare.

Regarding your wood stove, I also live in Silicon Valley so I know that there are more and more frequent "Spare the Air" days during the winter when you are not allowed by law to use your wood stove, and these also happen to be the coldest, so counting on the wood stove can be tough.

Consider a mini split system

It sounds like abandoning the forced-air system would be a good idea if indeed you are also looking to solve your cooling needs, since then you can go with a highly efficient heating and cooling unit called an air-to-air heat pump "mini split" system.

  • Mini splits are relatively inexpensive units that use the temperature of the air to create either heating or cooling and are extremely efficient at doing both.
  • They are small units that are perfect for space-conditioning a single room, so you would likely need a couple of systems.
  • They sit in an external wall and have their operating "guts" in a separate unit which sits outside, just like a small wall- or window-mounted air conditioning unit.

Please read my GreenHomeGuide article on these units --"I can't afford geothermal. Is an air-to-air heat pump my best option for heating?" --which includes manufacturer information and cost.

Also, if you would like any further information or would like some personal feedback on your project, we are located in Santa Clara. You can find more information on mini-split systems as well as our contact information on our website at www.myearthboundhome.com.