[Greenbuilding] Central vs window AC

[nick pine]

John Straube <jfstraube at gmail.com> writes:

> URL: 
> <http://lists.bioenergylists.org/pipermail/greenbuilding_lists.bioenergylists.org/attachments/20120608/8f8f77e1/attachment-0001.html>

>Almost all air conditioning systems have a sensible heat ratio of about 
>0.70 to 0.75.  This means that 3/4 of the energy goes into cooling the air 
>and 1/4 goes into removing moisture.

I wonder why. Something to do with government regulations? Why not cool 70 F 
air to 40 for dehumidification and reheat it to 70 and exhaust the latent 
heat outdoors?

> In Philly or New York, if you control solar gain (shade) and have decent 
> insulation and airtightness, the ratio required is more along the lines of 
> 0.30 to 0.60.  So, normal systems cause problems with high humidity. 
> ASHRAE has spent a ton of money in the last while on research to try and 
> identify and solve this problem.

Why not just reduce the airflow? When I put duct tape over some of the air 
over some of the air outlets of a window AC, the output temp dropped and it 
became a better dehumidifier.

>In weather that is dry enough (eg lets say that air would have less 
>moisture in it than it would at 75F and say 40%RH) it is so cool outside 
>that night ventilation works

Air at 75 F and 40% RH has vapor pressure Pa = 0.4e^(17.863-9621/(460+75)) = 
0.355 "Hg, approximately, and humidity ratio w = 0.62198/(29.921/Pa-1) = 
0.00746, which seems a lot more than "dry enough" for human comfort. The 
object is comfort, no? ASHRAE says still air at 80 F with humidity ratio w = 
0.0120 is comfortable. Higher humidity can be comfortable with a ceiling 
fan.

>This is Gennaro's New Mexico example. Works great. Does not work in NYC, 
>Philly, even Chicago.

I'd agree, if you were to say it does not work 100% of the time. Let's 
quantify this. My simulations say smart ventilation can work for all but 2 
weeks in August in an airtight brick row house in Phila, with significant 
savings, compared to AC without night ventilation. Big buildings often have 
night ventilation with enthalpy economization. Who makes one of these 
http://www.youtube.com/watch?v=wexdNx_StRc in a nicer package?

"Eli Talking" <elitalking at rockbridge.net> writes:

> We have some people who choose to live without ac.  Generally, I conclude 
> that they acclimate to the higher humidity.  I have one house I designed 
> with suspended concrete slab in the middle.

Sounds disquieting, like Paolo Soleri's concrete slab swimming pool shade 
perched on rotten telephone poles in Phoenix.

> If a house is very tight where night flushing can keep cool enough for 
> comfort temp, which is better for dehumidification: small ac or 
> dehumidifier?

The small AC pushes the latent heat outdoors. In mild weather, you might run 
both, depending on the house temp, using a humidistat 
http://www.grainger.com/Grainger/HONEYWELL-Humidity-Control-2E574?Pid=search 
and an SPDT thermostat 
http://www.grainger.com/Grainger/COLUMBUS-ELECTRIC-Thermostat-2E158 to make 
a constant indoor temp.

arch59 at aol.com writes:

> We are currently building a residential multifamily building that is using 
> a Liquid Desicant unit from Advantix to pretreat and Dehumidify the 
> outdoor air we send to the apartments.

I wonder what's in it. Lithium chloride is toxic, and calcium chloride 
doesn't dehumidify well.

Nick