Island Life 3

[Mike48099]

Here I am on a small south pacific Island w/out access to Ashrae...what are the best practices for HVAC design here? I see a lot of sweating walls and rusting duct. Whats best: Alum duct, flex duct, sstl, corrosion restiant coatings on equipment. Its 89 deg and 80RH here most of the time.

 

[AbbyNormal]

Mike I am on Caribbean Island.

You have internet so you have ASHRAE access.

Small island means your dewpoints will be very close to the sea surface temperatures. Your condition of 89F at 80% Rh is about an 82 dewpoint, I will get them that high every now and then but more like a consistent 78 to 80 here.

So if the coast is not far away, galvanized fresh air ducts will rot. The thicker the galvanizing, the longer it lasts, but it will fail. Need aluminum or stainless fresh air ducts.

If you run the fresh air duct through conditioned space you need to insulate it so that the air inside the duct does not condensate. The condensation speeds corrosion and there is a good chance that there will be organic matter in the duct- pollen, insects. You do not want wet organic matter at room temperature laying around in ducts.It will be a spore generation factory.

You want to keep your buildings under a positive pressure to keep that humidity out. You bring in fresh air directly to a cooling coil, cool it off and then blow it into the space.

http://www.masongrant.com/html/samples.html

Order "The ASHRAE Guide for Buildings in Hot and Humid Climates" and the ASHRAE "Humidty Control Design Guide"

here is a free chapter from the latter

http://www.masongrant.com/pdf_2008/ASHRAE_HCDG_C7_Mold.pdf

You live somewhere, where the ambient dewpoint is higher than the temperature people will set their thermostats at so you need to be careful.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Here is a photo I call "But it worked great in Arizona"

It was an Arizona design recycled here-- it was a negative pressure ventilation scheme, as you can see it is disasterous. The decking is the underside of a floor slab, it was 78F, the ceiling space was demonstrating the rain cycle like a primary school lesson. There was over 1/2 inch of water in the sheet metal channels holding up the ceiling

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

nothing last longer than the trane spine fin all aluminum condensing coils

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[imok2]

Mike This may be of some value to you

http://www.baileyeng.com/paston_effect.htm

 

[AbbyNormal]

paston effect is good, I would say seal the attics and avoid most of the paston problems. Bailey's article even notes how older houses with out attic ventialtion did fine

Mike will be a lot more humid than Florida though

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Mike48099]

Thanks for the feeds, I get about 5min on-line time before the island is cut off again or DoD takes over the system. my architect wants to know where to put the vapor barrior on a cmu/concrete wall system with metal studs/insulation and gyp on the inside...ideas?

 

[AbbyNormal]

don't use a vapour barrier at all

never ever on the inside in a hot humid climate

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

concrete is permeable, water is going to wick through all the time, if it can dry to the room air it is a very insignificant moisture load.

If you place a vapour barrier inside the water will build up and be a real problem

In my experience I would insulate on the outside with styrofoam

If you insualte on the inside between those studs, use something that breathes

For a commercial building, ASHRAE 90.1 would not require those concrete walls to be insulated in your climate the thermal mass cuts down on the heat coming in during the day, it releases the heat at night

If you insulate on the outside, you cut down on the heat that gets into that concrete in the first place.

If you seal the attic and insulate the roof deck and any gable walls, the building will tend to be very dry, and I doubt the attic gets much hotter than 80F.

A white metal roof is a no brainer, keeps the heat out, reflects it away.

Driest condos in the caribbean (hurricane panels up for Hurricane Dean)

sealed unconditioned attic averaged 80.5F and 47% RH in Aug 2007

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

if anyone cares, the attic temperature peaked after sunset. Basically some thermal mass releasing heat and ceiling mounted lights were turned on. Attic insualted at roof deck tends to trap heat

The roof is about R7, I figure it is more effective then R30 at the ceiling plane with a vented attic.

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install