[Greenbuilding] Central vs window AC

Thu Jun 7 20:37:01 CDT 2012

So...It seems window units are cheaper for the consumer.
Over the 16 years the window unit will consume 2.6 times more electricity,
or 16,832 kWh.
So is this a situation where it may be cheaper for the consumer but is more
costly for society and the planet?
What is the right thing to do (out of these two options) for the planet?

 These calcs are if CDD's and electricity costs remain constant, but odds
are they will rise, which would be in favor of the mini-split, although I
don't know how much those constants would have to rise before the minisplit
tips the scales. I suspect a lot. Temperatures have been in the 80's a lot
and we are only in June. And NY temperature is not forgiving at night
(compared to say NM). If it is sweltering and humid in the day odds are it
will extend into the night. I'm comfortable up to 80 F. After that I have
troubles sleeping. A noisy AC blowing cold air on me does not help.

It seems the trick (given these two options) is to reduce the install cost
of the split system. They go for $1,400 and that is pretty fixed. Install
costs can vary but $700 is reasonable. That doesn't include drywall etc.

All this does not consider the split system also heats, which is pretty big
if you don't wait think of the split system after you have installed the
heating system (like I did).

If you open yourself up to other options, night ventilation is obviously
part of the equation. Ceiling fans. External shades, good
insulation.....but I tell you there are some NY nights that are just
unbearable no matter how much of a green building genius you are.

Gennaro Brooks-Church
Director, Eco Brooklyn Inc.
Cell: 1 347 244 3016 USA
www.EcoBrooklyn.com
22 2nd St; Brooklyn, NY 11231

On Thu, Jun 7, 2012 at 6:12 PM, nick pine <nick at early.com> wrote:

> 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<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<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<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=<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<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<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
>
> ______________________________**_________________
> Greenbuilding mailing list
> to Send a Message to the list, use the email address
> Greenbuilding at bioenergylists.**org <Greenbuilding at bioenergylists.org>
>
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.**org/mailman/listinfo/**greenbuilding_lists.*
> *bioenergylists.org<http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioenergylists.org>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20120607/6004256f/attachment.html>