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The central heat and air in the house I'm renting is unable to keep the house below 80 °F during the hot summer here (100 °F days). We've had 3 HVAC techs out to look at it and they all say there's "nothing wrong with the system" and that it's "sized appropriately." The latest said perhaps it's just a combination of small factors (old house, leaky windows, etc.).

The property owner is going to provide me with two window units to use - one in the bedroom and one in the living room. Would it be more efficient (in electricity cost) to turn off the central AC entirely and rely on the window units, or better to run all three systems?

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  • $\begingroup$ Before anyone can attempt to give you answer the amount of electricity each unit uses will be need to be known & how you intend to use each unit. By that I mean, do you need to cool the entire house when you're sleeping, are you happy to just cool the bedroom & living room when you need cooling? Also, will the window units be sufficiently sized to cool each room by themselves or will they be boosters for the central AC unit?. $\endgroup$ – Fred Jul 14 '16 at 1:19
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Test it.

Any answer without a test is really just a fancy guess. And a test will answer your question definitively and quantitatively.

Your test needs to be as controlled as possible. Which means you will need to turn off all the other major power draws (or keep them on and constant — like the refrigerator — during the test.) You also need to make sure you have the same starting temperature AND ambient temperature and humidity when you run your test trials.

After you account for all the control factors, just record your starting meter reading. Cool to your desired temperature. Then record your finishing meter reading. Then repeat for the other configurations you wish to test.

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Many new air-conditioners have a SEER efficiency rating. SEER, Some cities have rewards for higher efficiency units. Generally every thing else being equal having more units means better control and lower electricity bill.

In average households the ducting system is not efficient and balanced, Partially due to the architectural layout of the rooms, partitions and spaces and the fact that the farther downstream of ducting the more loss of pressure and warmer outflow.

The other advantage of multi-unit system is you don't waste energy keeping spaces not in use cool. In modern homes they split the central air-conditioner into several units depending on the layout of the floors and utility of spaces!

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