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Is my 5000 BTU LG 10 year old window unit significantly inefficient ?

burkecabaniss | Posted in General Questions on

My wife says so, in comparison with similar modern window units. She wants it replaced. The unit runs on all work days, during summer work hours, in my home office in San Antonio, Texas. San Antonio summers are hot, but not as hot as north Texas, as an example comparison. My question is whether efficiency advances in 10 years have made a big difference for small window units.

I tried to search, but every search returned hundreds of irrelevant topics.

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  1. jackofalltrades777 | | #1

    Is a 10-year old A/C unit inefficient compared to modern units? The short answer is YES.

    If you choose to replace, I would replace it with a 30-SEER wall mounted ductless mini-split by Mitsubishi. They sell a 9,000 BTU unit and the cost of the unit plus install will range between $2k - $3k.

  2. GBA Editor
    Martin Holladay | | #2

    If your existing air conditioner is working OK, I doubt if you can justify the purchase of a new unit based on future energy savings.

    The current federal minimum requirement for room air conditioners is a minimum EER of 9.7. Units that have a minimum EER of 10.7 are eligible for the Energy Star label. When it's time to buy a new air conditioner, it makes sense to look for an Energy Star label, or (if you want the improved performance and can afford to upgrade), to consider installing a ductless minisplit system. In the meantime, if your existing equipment is working, I would continue using it if I were you.

    For more information on this issue, see Window-Mounted Air Conditioners Save Energy.

  3. user-4053553 | | #3

    Tell us the wattage printed on it and the BTU (which you already mentioned).
    Also keep in mind most units today come with a 1 year warranty and are designed to die in year two, they probably spend more on the planned obsolescence engineering then on the actual parts costs.
    If you tell us the wattage i can calculate the efficiency, compare it to the current energy star standard and tell you the payback in hours.
    I can say with fair certainty the payback will be longer then the expected lifetime of a new unit (ie you will never make the money back in energy savings before it dies)

  4. Expert Member
    Dana Dorsett | | #4

    You'll NEVER make a financial rationale for a new mini-split over a working decade old half-ton window shaker at any efficiency, given the much high up-front cost. But that's only looking at the net-present value of future energy use savings. But from a noise & comfort point of view there may be a case.

    In office settings the noise factor of window units can sometimes make the case for whisper-quiet mini-splits, but that's not measured strictly in dollars. If the thing just rattles and shakes too much and she wants to move it along, a PTAC with a scroll compressor would be quite a bit cheaper than a mini-split, and a lot quieter than a window unit (though louder than best-in-class mini-splits.)

    If you're biting the bullet on a high-efficiency modulating mini-split, a heat pump version isn't a huge upcharge from cooling-only. Since the loads are quite small relative to the max output of a 3/4 ton mini-split, pay close attention to the minimum-modulated output numbers for any units you're looking at. eg: Most 3/4 ton Fujitsu units only modulates down to 3100 BTU/hr, in cooling or heating mode, which is probably higher than your average load, whereas some of the Mitsubishi 3/4 units modulate down to 1700 BTU/hr in cooling mode and would modulate. But it varies from unit-to-unit, vendor to vendor. ANY mini-split is going to be oversized for a single office room, so it's the minimum modulation number that is key to getting the most comfort out of it.

  5. burkecabaniss | | #5

    Thank all of you for your comments. It is a small upstairs home office, 120 square feet. I work alone now - an Internet software maintenance business. I would not consider wall mounting or an increase in capacity - I have no reason to do so. My home has central A/C, upstairs and downstairs units. I use the window unit instead of the upstairs A/C for savings. I should have explained this at first.

    In reply to a wattage question, I looked at the tag. The LG unit is 8000 not 5000 BTU, 9.8 EER, 820 W input. It runs on and off. I now see this makes my question much more specific. I am surprised by the 9.8 EER which is higher than I would have expected.

    Thank you again.

  6. Expert Member
    Dana Dorsett | | #6

    There are even smaller (and quieter) window units out there. An 8000 BTU/hr air conditioner is a LOT of cooling for a 120 square foot office space. The single room probably has a peak cooling load of less than 2000 BTU/hr, maybe even less than 1000 BTU/hr.

    If the oversized LG isn't annoyingly loud there isn't an energy-use reason to swap it out.

  7. mackstann | | #7

    The minimum EER from 2000-2014 was 9.7. The minimum EER from 2014 onward is 11.0.

    So basically you'll spend about 12% less on electricity for AC with a new unit over the old one.

  8. user-4053553 | | #8

    So a new energy star rated unit at 11.3EER would use 708W for 8000 btu/h. So your saving 112W of power, nothing to shake a stick at, at 25% load (averaged through the day, i'm assuming 50% during the day and none at night) you would save about 0.672kW/h per day. I'm going to assume you use the A/C every day for 9 months a year, and your electricity costs 10c/kWh

    This yields 6.72 cents a day of savings, times 275 days a year of use, making about $18.48 a year in electricity savings..
    I'll assume you can replace it with this model, i have had good luck with frigidiare products and they typically have good warranties (though i am used to seeing 5 years and the page says one year, could be misprint)

    It costs $216 plus your local taxes which i don't know, your looking at a payback period of 11.68 years before tax which may add another year. This is longer then the age of your current very old window shaker.

    Hence its unjustified based on electricity savings. When your current one dies then replace it with an Energy star unit, if you get a cheaply made one it may die before your current unit would have if you had not replaced it unjustifiably.

  9. user-4053553 | | #9

    So what has the OP decided to do?

  10. Expert Member
    Dana Dorsett | | #10

    Methinks the average duty cycle is going to be a lot lower than that Nate. 25% of the 8K output is probably the PEAK load of that room on design day, not the average over any given day, and certainly not the average over a 275 day season.

    It's a only 120 square foot room, and unless it has a 10x12' window on one side sucking up huge solar gains, it's a truly pipsqueak average load. Typical peak loads on new construction averaged over a whole house is on the order of a ton per 1000'. If the room load is scaled to that sort of average (it won't if it has that full-height "sunset view" window-wall) we're talking 1500 BTU/hr for the peak hour of the hottest days of the year (about a 19% duty cycle) and less than 1/4 that for the 275 day average.

  11. user-4053553 | | #11

    I agree Dana, but it can be hard to calculate, i figured i would present an optimistic scenario and since i live in southern ontario and not texas i assumed the load would be higher then ours on average, so at 20% load it would be $14.78 a year in savings, making a payback of almost 15 years before taxes (which may make 17 years)!

  12. Expert Member
    Dana Dorsett | | #12

    It's only a ~20% duty cycle at the 1% outside design condition (1% of the time, or 88 hours per year), not the average duty cycle over the cooling season. The average duty cycle will be much much less.

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