Should I replace my A/C

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

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

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

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.

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

Page 1 of 3 1 2 3 > last >>

Similar Threads	Posted
R22 TO 410: HAVE TO REPLACE LINE??	July 6, 2007, 8:46 am
R22 TO 410: HAVE TO REPLACE LINE?? again	July 6, 2007, 8:53 am
Repair it or replace it	July 12, 2007, 6:55 pm
time to replace the bryant	October 30, 2006, 4:35 pm
Igniter on furnace - how to replace	February 16, 2008, 1:02 pm
replace hvac system question	July 10, 2006, 6:22 pm
11 year old Goodman - should I repair or replace?	August 18, 2006, 10:29 pm
heat pump repair/replace?	December 2, 2006, 8:55 pm
Help: replace Trane heat pump	December 5, 2006, 10:00 pm
Is it worth to replace my roof fans?	February 21, 2007, 11:50 am