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Posted by Brian Steele on September 14, 2006, 9:33 am
Hi Everyone:
I've got a Miller NHX-018 / MDX-01 split system A/C that serves the master
bedroom in my house (2,400 cu.ft.). I live in the tropics, and ambient
temperature outside during the day can hover around 29~31 degrees celsius
for most of the year, except when it's raining. Electricity here costs
approximately US$0.33/kWh.
My electricity bills are horrendous. My daily consumption hovers around
39kWh, and most of that seems to be caused by the A/C unit.
Are there more efficient A/C systems available, and will it be worth my
while (e.g. payback in 3~5 years) if I swap out the Miller unit for one of
those more efficient systems? A bit more about the Miller A/C unit I have
can be seen at http://www.ims-millerac.com/millerdlxproducts.asp, but as
they don't mention EER or SEER ratings, perhaps it's something they don't
want to advertise :-).
Also, is there anything that I can do to increase the efficiency of my
current system? I read somewhere that for best efficiency, the compressor
unit should not be located where it will hit by direct sunlight - is this
correct? The location of my compressor unit can be seen at the following
URL: http://www.spiceisle.com/brian/personal/2006/solar/. From 10:30 in the
morning to about 4:30 in the afternoon, it's exposed to the sun.
Regards,
Brian Steele
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Posted by Travis Jordan on September 14, 2006, 11:26 am
Brian Steele wrote:
> they don't mention EER or SEER ratings, perhaps it's something they
don't want
> to advertise :-).
SEER = BTU OUTPUT / WATTS INPUT
Your system 18,000 / (2190 + 50) = 8.0 SEER.
There are higher SEER systems available but you'd have to do the math to
see if they would save enough money over their expected lifetime to pay
for themselves.
> I read somewhere that for best efficiency, the
> compressor unit should not be located where it will hit by direct
> sunlight - is this correct?
Research shows that shading of the condenser from direct sunlight
doesn't cause significant energy reduction - unless you shade it enough
so that you can change the ambient inlet air temperature.
http://www.fsec.ucf.edu/bldg/pubs/pf302/index.htm
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Posted by Brian Steele on September 14, 2006, 1:02 pm
"Travis Jordan" wrote:
> Brian Steele wrote:
> > they don't mention EER or SEER ratings, perhaps it's something they
> don't want
>> to advertise :-).
>
> SEER = BTU OUTPUT / WATTS INPUT
> Your system 18,000 / (2190 + 50) = 8.0 SEER.
Thanks. So it looks like my unit is a fairly low-efficiency one, as they've
got units with much higher SEER ratings available at the moment.
> There are higher SEER systems available but you'd have to do the math to
> see if they would save enough money over their expected lifetime to pay
> for themselves.
Based on some of my meter readings, my current A/C consumes about 15kWh/day
at least.
At my current utilitity rates (US$0.33/kWh), that works out to US$4.95 a
day, or roughly US$1,800 / year.
Given the SEER definition, for comparison purposes a SEER 13 system with the
same BTU rating would likely consume 15*8/13 = 9.2 kWh/day under the same
conditions. That works out to US$3.04 / day, or roughly US$1100 / year.
So, I should save about US$700/year or US$3,500 over 5 years if I go with a
SEER 13 A/C.
Likewise, if I go with a SEER 16 system, I should save about US$4,500 over 5
years if I go with a SEER 16 system.
Am I on the right track here?
>> I read somewhere that for best efficiency, the
>> compressor unit should not be located where it will hit by direct
>> sunlight - is this correct?
>
> Research shows that shading of the condenser from direct sunlight
> doesn't cause significant energy reduction - unless you shade it enough
> so that you can change the ambient inlet air temperature.
>
> http://www.fsec.ucf.edu/bldg/pubs/pf302/index.htm
Great - thanks. This seems to make sense, as the ability of the unit to
dissipate heat will depend a lot on the temperature of the heat "sink" (air
in this case).
Hmm... maybe I should look into water-cooled A/Cs, LOL. I've got this huge
water tank below the house...
Thanks for the feedback,
Brian
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Posted by Travis Jordan on September 14, 2006, 4:16 pm
Brian Steele wrote:
> Based on some of my meter readings, my current A/C consumes about
> 15kWh/day at least.
Your system's total power input is approximately 2240W, so 15KW would be
the equivalent of roughly 7 hours of run time. Sound right?
> At my current utilitity rates (US$0.33/kWh), that works out to
> US$4.95 a day, or roughly US$1,800 / year.
Do you run it 7 hours every day, even during the winter? If not,
recalculate using your expected annual run hours.
> Given the SEER definition, for comparison purposes a SEER 13 system
> with the same BTU rating would likely consume 15*8/13 = 9.2 kWh/day
> under the same conditions. That works out to US$3.04 / day, or
> roughly US$1100 / year.
A 13 SEER system would use approximately 18000/13 = 1385W input.
Another way of getting to the same number (9.3kWh/day).
> So, I should save about US$700/year or US$3,500 over 5 years if I go
> with a SEER 13 A/C.
I come up with $687.
> Likewise, if I go with a SEER 16 system, I should save about US$4,500
> over 5 years if I go with a SEER 16 system.
>
> Am I on the right track here?
Yes you are.
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Posted by [SPICEISLE.COM] Brian Steele on September 14, 2006, 8:01 pm
"Travis Jordan" wrote:
> Your system's total power input is approximately 2240W, so 15KW would be
> the equivalent of roughly 7 hours of run time. Sound right?
Well, I leave the A/C on almost 24 hours a day, but the condenser does cycle
off for much longer times at night...
>> At my current utilitity rates (US$0.33/kWh), that works out to
>> US$4.95 a day, or roughly US$1,800 / year.
>
> Do you run it 7 hours every day, even during the winter? If not,
> recalculate using your expected annual run hours.
No winter here. I live on an island in the tropics (11 degrees North).
Ambient temperature typically varies from 26 to 30 degrees Celsius (79 to 86
degrees Farenheit). I've got the A/C usually set at either 24 or 25 degrees
Celsius (75 or 77 degrees Farenheit).
Thanks for your help,
Brian
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