Anybody know about AC? Electricity? I have a question...

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Rann's picture
Joined: 2012-01-04

Yes, I know it's hot in Georgia in the summertime; I've lived here since 1966...but last summer the temps were particularly horrid here. I think we skipped "highs in the 80s" all together and went straight to consecutive highs in the humid, though rainless 90s, May-September. This winter may have been the mildest I can recall, but somehow I doubt that a correspondingly "mild summer" will be in store for us in I'm planning my annual home improvement efforts along these lines.

My 1946 house has central heat, but no central AC. Each summer my very life has rested in the capable hands of a lone & stalwart 15K BTU window unit. It has cooled my 1000sqft house adequately, reliably & very date. However, I've decided to install a second, "backup" window unit...just in case my primary AC breaks down at the worst possible, least convenient AC tends to do.

Here is my question. I've owned several window units through the years, but they've all been 110v models. The ONLY other window in my house where I could hang an additional AC unit has BOTH 110v & 220v outlets nearby. So I have an option...but I have no recent experiences with 220v AC units...

A 220v AC unit would cost me a bit more initially than a 110v model, and would be heavier. My poor recollection of high school electricity is that a 220v appliance technically draws twice the current, but only has to work half as hard to get the same work done...might run cooler, quieter, last longer...actually cost less to operate somehow...all other things being equal.

Would you opt to buy a 110v or a 220v window unit, and why?

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Rob's picture
Joined: 2009-10-14

Current draw will be determined by the machine, not the voltage it uses. You need to look at the actual draw or power rating for the machine to figure out the power use. The 220V unit likely uses more power overall, so it would increase your electric bill even more than a 110V unit. However, it will cool better as it should have a larger compressor and be able to move and cool more air than a 100V unit.

So, the 220V unit (probably a higher BTU rating) would probably run less and I would guess be slightly more efficient. You should compare the energy ratings of them to determine what your actual cost/savings would be long term. You may also want to consider central air and compare it with your window unit costs to see if there are advantages (and it would likely increase your re-sale value if you ever sell your home).


Rann's picture
Joined: 2012-01-04

Thanks, Rob. I suspected as much.

When/if I need to replace my furnace, I might see about including centrai air. I should have mentioned that my house is over 65 years old, built for returning WW2 vets...and except for some rewiring back in the 70s, it is very...authentic. Just try to find an outlet in my dinky kitchen. I have no dish washer, sister thinks I live like a cave man! I prefer to think that I still live like a grad student, but truth to be told, most nearby students live higher on the hog than I do! Their SUVs might cost as much as my house & lot combined.

I could spend many thousand remodeling this or that, but my efforts aren't likely to pay off. When I (am possibly ever able to) sell this place I suspect the "highest & best use" at that time will either be college student rental or, more likely, the bulldozer.

I don't want to spend a penny more than I have to on the upkeep (much less on the cosmetics) of this old shed, but spending to remedy structural issues or planning for emergency contingencies is another matter.

John of the Dead's picture
Joined: 2012-01-04

(A little background - I'm a mechanical engineer for a major HVAC manufacturer. We mostly work in central AC split-systems, not window units, so my expertise is a little bit off here in the specifics, but the generalities should apply.)

Something else to keep in mind is that cooling faster is not necessarily more efficient. An AC compressor runs most efficiently when it's in steady-state operation. It is least efficient at start-up and shut-down. Think of it like your car on the highway versus city stop-and-go driving. When your car is cruising along steadily, it's burning less gas than accelerate/stop, accelerate/stop. A larger unit might paradoxically cool your home *too* quickly, spending most of its time in start-up and shut-down mode. I would see if it has SEER and energy usage ratings and compare those against your current system.

Also check the tonnage rating for the unit. If the home is moderately well insulated, a 1000 square foot home in the deep South should be rated at about 1.5 to 2 tons of cooling*. One ton of cooling is roughly equal to 12k BTU, so it sounds like your current window unit is right on the borderline for size, maybe a little undersized for your cooling needs. 1.5 tons would be 18k BTU, 2 tons would be 24k BTU. Also, depending on the age and state of repair, the current unit is likely seeing a 10 - 20% degradation in efficiency, so it's probably pulling more like 10k BTU, or under a ton of cooling. When's the last time you cleaned the coils? That right there can boost efficiency 5 - 10%.

So, a properly-size unit, new and in top condition, could run even cheaper than what you have. I'd look for something pulling on the order of 20k BTU (but not more than 24k), give or take, and wouldn't worry too much about the voltage. That makes a lot less of an impact than the sizing and the rating. Of course, a larger unit might require higher voltage, but that doesn't necessarily mean it would be drawing more current. It's complicated once you start getting under the hood.

* If you care about the HVAC professional jargon, a ton of cooling is equal to the removal of the amount of heat required to melt one ton of ice. Old Imperial units of measure are pretty funky, especially in terms of power and energy. Smile


When there's no room left in Hell,
the Dead shall walk the earth.