If you are sized correctly for new construction, then I don't think a dehumidifier is necessary. Because most new home construction has very good insulation qualities and a tight building envelope , you might need a fresh air ventilator for air exchange.

IMO

In my South Louisiana home, there are several weeks each year when I want to run my dehumidifier. The time I need to do that is when there is little heating or cooling to be done, but it is humid outdoors. I do, however, have a leaky, old house. If I were in your shoes, I would install two-stage and make sure that the dehumidifier can be added later if I need it. You might find that adding one is sufficient. In South FL, it is likely to be beneficial to run any intentional fresh air source through a dehumidifier. Here are some starting points for you:

(Added note, I fixed some of the links. The Building Science site has a lot of good information that you can find pretty easily.)

http://dnr.louisiana.gov/assets/docs/energy/buildersguide/Builder's Guide.pdf

http://www.buildingscience.com/
Search for "READ THIS: Before You Ventilate"

http://www.buildingscience.com

Search for "conditioning air in the humid south creating comfort and controlling cost"

This post was edited by ionized on Tue, Apr 16, 13 at 19:33

If not a dehumidifier, then definitely an AC system that has dehumidify on demand feature. This requires a var speed blower either on furnace or air handler plus the correct thermostat like HW VP IAQ.

IMO

Dehumidification does not work well with a conventional, variable-speed system, only with re-heat capability. Check the third link in my previous post.

"I am worried that the two stage unit will require me to turn the thermostat really low to keep the humidity under control"

This statement does not make sense. Why would you have to decrease the temperature with a 2-stage AC? I think you will find that if the humidity is kept in the upper 30s or low 40s you can raise the temperature and still feel comfortable.

Put the money towards a good 2-stage AC with a dehumidify on demand thermostat and you should not have a need for a dehumidifier.

I agree with above post.

Seems to be some confusion on "dehumidify on demand" feature. It works independently of temperature thermostat setting. And you don't even need a two stage condenser. Just a var speed blower and the correct thermostat.

IMO

The OP is probably expressing that they will need to overcool to keep the humidity low. It is a common problem when thermal load is low, but the environment is humid. Even with humidity control settings on central cooling systems, significant cooling occurs. Humidity control is poor compared to a dedicated dehumidifier.

Although variable-speed blowers and more advanced refrigerant flow control systems help a great deal, it is impossible to get away from the fact that some kind of reheat is needed for good humidity control in some situations.

Here is another case study that mentions the need for stand-alone dehumidification in hot-humid climates. Ironically, as the thermal performance of a home in these climates is improved, the cooling system can't dehumidify enough on mild days.

http://www.buildingscience.com/
Search for "HOT-HUMID CLIMATE CASE STUDY
for Lake Charles, Louisiana"

Respectfully disagree....

IMO

I believe you can set a whole house dehumidifier tied into each supply plenum, so that one dehumidifier would take care of the whole house.

visit www.hvac-talk.com with your questions.
Teddy Bear is the resident dehumidifier guy there.
Helped me a lot with the install of my Aprilaire.

I can easily achieve 55% RH inside in heat of
summer with variable speed air handler, but to
get colder air and lower RH to 45%, need to
use whole house dehumidifier..usually august
september.

dehumidification is difficult on days with little
cooling load. this is another time when whole
house dehumidifier use is recommended.

what type a/c system are you installing?
a/c with gas furnace
a/c with elec strip heat
heat pump

best of luck

I would definitely add the two stage air conditioner first before adding a dehumidifier, simply because new houses are built with excellent insulation and it may not be necessary. I would try out the two stage air conditioner and if you find that it is too humid, then invest in the dehumidifier! Good luck!

" I would try out the two stage air conditioner and if you find that it is too humid, then invest in the dehumidifier"

That and make sure it is designed to add a dehumidifier easily is all I am saying.

The forecast for my neighborhood tonight is

time/temp/dew point
1:00/72/70
4:00/70/70
7:00/72/70

Insulation makes no difference as there is no temp delta. Air sealing helps. I will be running the cooling in dehumidify mode and fire up the energy-star dehumidifier.

Thanks for all the good info.

I will check out Hvac-talk and see what Teddy says.

Right now I am leaning towards the dedicated dehumidifers as the cost to add dedicated dehumidifers to the single stage AC units is the same price delta as going form single stage to 2 stage AC.

I am planning on using Aprilaire 8800 communicating tstats and will be on the look out for this dehumidify feature some people are referring to.

It still doesn't make sense to me that the dehumidify feature will allow the AC to dehumidify without cooling. My understanding is that the AC derives it's dehumidifying capability from its cooling function.

Obviously there is a real learning curve and lack of technical understanding here on the dehumidify on demand feature that is not only available but will accomplish what you are looking for without the need for a dedicated dehumidifier. As mentioned you would require a var speed blower which you would have anyway plus a thermostat that allows this feature like HW VisionPro IAQ. No smoke and mirrors. This feature works independently of a regulator thermostat. Discuss with dealer the advantages.

A little learning goes a long way.

IMO

Tigerdunes, you could do me and other a great favor by pointing me, and others, toward some resorces that can enlighten us, or explain how it works. Some case studies would be great. Anecdotal observations, while obviously dependent on the skill and partiality of the observers, might be useful. Everything that I have seen indicates that dedicated dehumidifiction is necesssary to achieve good comfort in some climactic condions.

OP, don't look at a later installation of a dehumdifier as a waste on top of a more sophisticated system in the build. A variable-speed central blower, the proper controls, and possibly other additions will control the humidity much better than a more stone-hatchet system. That means that the dehuidifier will run less. You may need less efficiency in the dehumidificaion system since it will run less. Again, if it were me in South FL, I'd be installing a cooling system with great humidity control and making sure that some sort of dedicated dehumidification system could be easily installed later.

This dehumidfy on demand feature really sounds like a gimmick to me.

I am going to go with the dehumidifers to ensure that I can get adequate moisture removal during periods of no cooling load. I also just read somewhere that a dehumidifer removes more moisture per kW than an AC does.

" I also just read somewhere that a dehumidifer removes more moisture per kW than an AC does."

Perhaps, but they do produce waste heat. That really cuts into the efficiency if you are In a cooling situation.

"This dehumidfy on demand feature really sounds like a gimmick to me."

It is not a gimmick. The Carrier Infinity 2-stage AC condensers work very well. The Trane 20i dual compressor condenser would also do a great job. The key is a doing an accurate load calculation and proper sizing.

Agree with above. Single stage or two stage condenser. Must have a var speed blower plus the correct thermostat like Infinity controller for Infinity systems, Edge thermidistat, T900 series Trane stat, or HW VisionPro IAQ, etc...

It's a gimmick like regular AC is a gimmick for cooling.

It's a gimmick like regular AC is a gimmick for cooling.

what does that even mean?
dehumidification is a gimmick
and a/c is a gimmick??

doesn't make sense to me.

dehumidification is a necessary thing
in my hot humid climate.
there are different ways to achieve it
each with its own pros & cons

stand alone dehumidifiers
pro is low cost
cons are where to locate centrally
& how to drain
operation cost
noise of dehumidifier

whole house dehumdiification systems
pro is dehumidification of the whole house
cons are cost
issues with proper install
operating costs
introduction of heat back into a home
in a cooling climate

vs & two stage equipment
pro is dehumidification if system is
sized, designed & correctly installed
low operating cost
cons are hvac company installing 5 ton
set at 3 ton speed, and cost to homeowner
to purchase oversized equipment
hvac company having little to no experience
with higher end/tech equipment.

when operation isn't understood saying it
doesn't work is a common response.
much easier than saying that is isn't understood.

hvac companies often do this when asked for
equipment installs they know little about.

to me it is a good thing. at least they admit it
so you are then freed up to go to a company that
invests in education instead of just doing the
same old same old as taught to them originally.

in my house I planned to dehumidify..kinda need
that in hot humid Louisiana..my dehumidification
plans were to use vs ahu. it works well.
but in August/September when temps & humidity
are the highest...the whole house dehumidifier
is a huge bonus.
the vs ahu simply can't get RH below 55%, but
the aprilaire can.

if you are building new..you have every chance
to get it done right. why not take advantage of it?

best of luck.

Energy rater,

I believe Weed was being facetious....

IMO

Energy_rater, i am afraid that you missed the irony this time in the gimmick vs. gimmick post.

I like your explanation, but I am surprised that you find high humidity levels when temps are highest. I have the most trouble when temps are low enough to need little or no cooling and it is too warm to heat (heat thus lowering RH). It depends on radiant load somewhat too so it is hard to pin down the exact conditions. The heavy dehumidification zone for me has to be somewhere between 60 and 80.

It seems like the OP would find similar conditions to ours in S. Florida.

I find it interesting to think about how insulation just pushes that zone around, but does not fix anything with respect to the need to dehumidify. In fact, good insulation makes it even more necessary to have dehumidification. In a poorly-insulated home, the AC will run more in hot-humid weather than in a well-insulated one so constant dehumidification is available. In a well-insulated house, it has to get a lot hotter for the cooling to be used significantly.

my bad. sorry weedmiester.

.

My gimmick comment was referring to the notion that the dehumidify on demand feature can dehumidify without cooling the home.....

The idea is that the system runs at a reduced fan speed so that not that much cold air is delivered to the home. Hopefully less than the current heat load. Having a 2 stage system running at the low stage also helps.