[Greenbuilding] Central vs window AC

[nick pine]

Gennaro Brooks-Church - Eco Brooklyn <info at ecobrooklyn.com> wrote:

> An intern of mine... 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....

>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.

Here's a $98.94 9.7 EER 
http://www.walmart.com/catalog/allReviews.do?product_id=20449871 and a 
$19.97 occupancy sensor with a 30 minute delay 
http://www.lowes.com/pd_355640-1571-RW600BTCCCV4_0__?productId=3455972&Ntt=600+watt+occupancy+sensor&pl=1&currentURL=%2Fpl__0__s%3FNtt%3D600%2Bwatt%2Boccupancy%2Bsensor&facetInfo=

>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

Why divide by the change in temperature? Each CDD is 24 Btu/day of cooling. 
How big is the house? How much insulation? How much air leakage and internal 
heat gain and solar heat gain?

> 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

NREL says an average year in NYC has 1096 CDDs with a 65 F base (too cold, 
IMO), so a house with no internal heat gains or solar gain and (say) 400 
Btu/h-F of thermal conductance needs 24h/dayx1096F/dayx400Btu/h-F = 10.52 
million Btu (note the units) per year of cooling, or 10.52M/26 = 405 kWh 
worth $72.84 at 0.18/kWh with a 26 SEER, or 10.52M/10 = 1052 kWh worth 
$189.39 with a 10 SEER. The simple split-system payback would be 
($2100-$210)/($189.39-$72.84) = 16.2 years, without the use of room 
occupancy sensors.

"Nick Pyner" <npyner at tig.com.au> wrote:

>After forty years, the split system might still be soldiering on bug very 
>likely to have seen a few window rattlers come and go.

I just got a guy to set up and charge a walk-in cooler, which involved 
replacing 6' of 1/2" copper pipe. He gave me an $800 estimate, then sent me 
a bill for $3125, including 26 hours of "miscellaneous labor." When he went 
away, it still didn't work right, so we cut a hole in the side of the box 
and installed a window AC, which works fine. You can buy lots of window ACs 
for the cost of one central AC service call. If one breaks, throw it away 
and buy another. Meanwhile, you still have N-1 that still work.

David Bergman <bergman at cyberg.com> wrote:

>The split unit will be quieter...

>You won't need to remove it in the winter.

Why remove it?

> see this recent article in GBA: Window-Mounted Air Conditioners Save 
> Energy Compared to homes with central air conditioning, homes with window 
> units have lower cooling costs 
> http://www.greenbuildingadvisor.com/blogs/dept/musings/window-mounted-air-conditioners-save-energy?utm_source=email&utm_medium=eletter&utm_term=cooling-options&utm_content=20120530-wall-mounted-air-conditioner&utm_campaign=green-building-advisor-eletter

Partly because people only AC occupied rooms. How would the comparison above 
change for a house with 8 rooms each occupied 2 hours per day, with doors 
and lots of wall insulation between rooms and 30-minute occupancy sensors?

NREL also says an average July day is 76.8 F in NYC, with a 68.2 min and a 
0.0123 humidity ratio, which is fairly comfortable. Smart night ventilation 
on comfortable nights can help.

Nick