[Greenbuilding] Split AC vs. Window AC

[Gennaro Brooks-Church - Eco Brooklyn]

An intern of mine just wrote this blog post, which concludes that it makes
no sense to use a SEER 26 split AC and you are better off using a SEER 10
window unit. It would take something like 40 years to pay off the split AC
IF his calcs are correct....

Would some of you number people be able to verify his calcs??

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Last week we installed a split AC in a yoga studio in Carroll Gardens. I
was interested to see the price differential between the two in energy
costs per year and how long the pay-back period for the split AC/heat pump
would be. The split AC/heat pump
<http://www.nationalairwarehouse.com/featured-products/9-000-btu-mitsubishi-26-seer-r-410a-heat-pump-mini-split-system.html>cost
$2100 to cover the cost of the unit and installation. It has a 9000 Btu/h
output and a 26 SEER rating. For this exercise I used the most energy
efficient,Energy Star rated, and most purchased window AC unit at
Sears<http://www.sears.com/kenmore-8-000-btu-room-air-conditioner/p-04279081000P?prdNo=2&blockNo=2&blockType=G2>
that
costs around $210. The unit has an output of 8000 Btu/h output and a 10.8
EER rating. I converted EER to SEER in the exercise below. I found out
there was a $47/year price differential in energy costs. That would be a 40
year payback period for the split AC.

The model I used tries to find the cooling load using the cooling degree
days. Here is the formula:
Cooling Load = (Btu/h x 24 x CDD) / change in temperature in the day

To compute the change in temperature I used monthly
averages<http://www.weather.com/weather/wxclimatology/monthly/graph/11204?from=month_bottomnav_undeclared>
from
The Weather Channel for Brooklyn.

Here are the results:

*Split AC*

May-               (9000 x 24 x 40) / 16  = 540,000 Btu/h
June-              (9000 x 24 x 120) / 16 = 1,620,000 Btu/h
July-               (9000 x 24 x 322) / 15 = 4,636,800 Btu/h
August-         (9000 x 24 x 207) / 14 = 3,195,714 Btu/h
September- (9000 x 24 x 88) /14      = 1,357,714 Btu/h
Total = 11,350,228 Btu/h

(11,350,228 Btu/h) / (SEER= 26 BTU/ Wh) = 436,547 Wh/year
(436,547 Wh/year) / (1000) = 436.547 kW/year x .18 kW/hour = $78.57 a year


*Window AC*
May-             (8000 x 24 x 40) / 16     = 480,000 Btu/h
June-             (8000 x 24 x 120) / 16   = 1,440,000 Btu/h
July-              (8000 x 24 x 322) / 15   = 4,121,600 Btu/h
August-         (8000 x 24 x 207) / 14  = 2,838,857 Btu/h
September- (8000 x 24 x 88) /14      = 1,206,857 Btu/h
Total = 10,087,314 Btu/h

Calculation from EER to SEER = -0.02 × 10.8² + 1.12 × 10.8 = 14.4

(10,087,314 Btu/h) / (SEER= 14.4 BTU/Wh) = 700,508 Wh/year

(700,508 Wh/year) / (1000) = 700.508 kW/year x .18 kW/hour = $126 a year
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