You first order of business is to have a Manual J calculation done by someone who know how do it properly. A goal of 900 sq.feet per ton of cooling seems ambitious for Florida. I suppose it is possible if you installed spray foam insulation on all exterior walls and the attic roof deck.

The biggest down side of a whole house dehumidifier is the amount of energy they use. The model you looked at uses 920W of power. That's a lot of energy. I suggest you look into 2-stage variable speed AC equipment as a way of reducing humidity level. The Carrier Infinity/Bryant Evolution does a great job of humidity control.

If the house is tight you should not need a whole house dehumidifier. You could have the HVAC installed with the intention that the dehumidifier could be added easily at a later date, or install it at the beginning as a back up if the AC does do an adequate job. One XT155H should be good for your house.

Which is why, Pensacola we are going with a variable speed AC.

Your intentions are good but in no way practical. Put your efforts and money toward a well designed cooling system. Go with tried and proven and leave experiments to those with more money than they can find use for. If you would like to be "schooled" to the point of understanding the technical application,sign up for hvac classes at a jr college.

Thanks, this begs the question on a tight house. With the house being well insulated and tight, how does one remove the excess humidity at times when the temps don't warrant running the AC? Example, at 5 this morning we had an outside dew point of 99%. Temps were in upper 60's and it was very cool yet "clammy" in the house.

Outside temperatures are measured in the shade. If there is little cloud cover and the house has exposure to the sun, the temperature indoors can be hotter than what is being reported for the outdoors. This may cause the AC to operate for periods of time.

Some thermostats have a cool to dehumidify feature. If the house is at the set temperature (ex: 75 degrees F), but the indoor humidity is too high (greater than 50%) then the AC will turn on at the lowest speed in order to lower the humidity. The AC may over cool up to 2 degrees (this limit may be set) in an attempt to lower the humidity. This feature works well in my house, but it may not be enough in areas of very high humidity. This is why I said you may still need a dehumidifier.

Thanks Mike. Based upon our experience in this rental house for a minimal investment of around $1500 it's well worth it to us. Others mileage may vary but better safe than sorry. Shoulda woulda but didn't is not what we want to say.

Now that the pro crowd has weighed in, let me have a turn to tell you how we dehumidify a 2 story 2800 sf house 30 minutes south of Birmingham, AL. The house was built in 1974. It is in the woods, as in the lot is old enough that the trees have grown to the point you cannot see the thing on google maps. Ambient heat and forest level humidy are our norm. It is moist enough that moss can grow on the sides of buildings, as well as mold etc. And let me say I agree with you on 900 sf per ton. I would rather run a unit a little more than less, because it results in a more condition environment. I learned this over 20 years in this home, which came with a 2.5 ton ac, which we switched to a 3 ton heat pump, and then to a 4 ton heat pump at the guidance of a professional HVAC company. I now have design hate for it with a vehement passion due to its increase in average humidity in my home, due to shorter run time cycling. In the mix enter a 70 pint dehumidifier that lives in the 1 car downstairs garage. Because water will move from the wet to the dry areas through air at a speed of roughly 400 cubic feet of migration per hour, the little rig will dry the whole interior of the house by the second day. I empty it once per day onto plants. This is my personal experience, and nobody can tell me I am wrong. I use the little guy in the summer and sometimes in the winter when it rains a lot. Its not as nice as a quiet unit, so its in the garage, it breaks after a number of years and I replace it.

"Tightness of home doesn't do anything to remove humidity."

No, but won't a leaky home add to interior humidity by allowing in humid outside air?

Did somebody ask for me? Actually it is a sophmoric question. Let me extend it. And dont be offended, you have to be old to have experienced this one...

If my house is leaky will not putting it under water make it wetter inside?

No, but won't a leaky home add to interior humidity by allowing in humid outside air?

Well you have to realize humidity can be often times be generated inside the home... so a tight house in a humid climate can work against you just as much as it 'could' work for you.

After you take a shower the mirrors are fogged up as an example.

You have to think about the 'whole' picture, not just one attribute.

A leaky house lets humid air enter in the summer, and dry air enter in the winter. The "tightness" of the house, or better put the number if air exchanges per hour can't remove humidity but it does contribute in making the controlling the humidity level more difficult.

I own two houses built before 1980 and I live in a high humid climate (Katy, Texas)

Where do you live?

Just because you read it on the internet doesn't make it automatically true.

EXPERIENCE FOLKS --- 22 years and counting.

Tick Tock.

I live in New Jersey.

Yeah I know where you live... that was my point. I've said it once I'll say it again... providing misinformation is not helping people.

Sick building syndrome didn't show up until the buildings of homes and businesses were made tighter. Humidity is only one problem. If you focus on only ONE problem what does that do to the other problems? I shouldn't have to tell you this as much as you post here, you should already know it.

Can you please point out what misinformation I posted?

A leaky house lets humid air enter in the summer, and dry air enter in the winter. The "tightness" of the house, or better put the number if air exchanges per hour can't remove humidity but it does contribute in making the controlling the humidity level more difficult.

Before you respond to this think about it long and hard please.

I have thought about it. In your expert opinion how have I misinformed the original poster?

IMO, fresh air is important and making it tight is desirable for thermal performance and I agree with trickyputt on the supplemental portable dehumidifier. I use one, have it in the laundry room. Dont have to empty it maybe once every couple weeks. I also hang some laundry in there to dry during winter as a supplement to running dryer and it aides in removing the humidity caused by wet clothes. (I dont like to dry heavier items like jeans and towels in the dryer, seems a waste of energy) I have it set at 50 and it doesnt run all the time. I dont feel I want it set lower or the air is too dry, irritates lungs. Now where I live is not super humid all the time so yours may run more often.

All that being said, perhaps go with the AC that controls humidity with a fresh air intake and HRV and a portable dehumidifer near where the fresh air comes in. There are quieter and energy star dehumidifers as well, just have to look. Next one I get I will go for a quieter one, for now I just close the door to the laundry room. I also agree with trickyputt that if the portable unit ever craps out you just get a new one, it is a small investment but not near as bad as a new AC unit. Does not require much maintenance either.

www.hvac-talk.com

residential forum, teddy bear the pro member

you want to address your questions to.

pros answering homeowner questions.

This thread is a little confusing to me. When talking about a whole house dehumidifier, is this a portable unit that is inside the house and can dehumidify the whole interior of a house regardless of the number of floors ?

We live in Charlotte, NC, hot and humid during the summer. We have flexible ducts in the crawlspace and attic and metal ducts inside the house within the lower floor. We have lots of mold in both places, and the ceilings under the metal ducts fell down due to excessive condensation. We have been told that we need to insulate the metal ducts or replace them with flexible ducts, install a dehumidifier in the crawlspace and insulate the metal ducts, and, of course, mold remediation. Our mold remediation company recommended we go with an Atmox system.

So basically we have different trains of thoughts and are totally confused. Any insight you can provide would be most appreciated.

A whole house dehumidifier can be either a free standing unit, or one that is integrated into the duct work. Here is a web site that has several whole house dehumidifiers.

If the indoor the humidity is below 50% then should not have to insulated any duct work inside the conditioned space.

You seem to have a severe humidity and mold problem. Start a new posting if you have more questions.

This is just our experience. A different climate, but........ Rapid City, S D. In 2004 my mom bought a new house from a reputable builder. He was known for tight envelope homes and guaranteed that a 6 month electric bill would be less than $200 total or he would pay the bills. That was in writing. The house was built on a 4' dirt crawl space. The water heater was a 80 gal Marathon. I do not remember the vac system brand. Yes, the house was extremely airtight. So tight that I could not breath decent (asthma) due to the stale, humid, smelly air from the crawl space. The thermostat was set at 55 degrees in winter (bought house in beginning of winter 2004). Nobody ever got to live there because my mother had a stroke the same day we closed on prior house/day before she was to move into new house. I went to the house about every 4-6 weeks and stayed about a week each time. Had the house for a year thinking mom might get there... I hated the air quality. I asked neighbors what they did (builder built several homes in neighborhood). They all had large dehumidifier in the crawl space that they had to empty every 3+ days. No thanks. Rapid City is much, much drier than Pensecola. This is just my experience. I will be building near future and want a tight house, but it has to be done correctly. This one for the climate, was not. Thx

I wouldn't worry so much about a tight home. I would worry more about the efficiency of the systems and design of them.

A tight home isn't necessarily what it's cracked up to be. A HVAC system can be extremely economical when designed and installed properly.

I used to live in one and in the video below I show my power bill for the whole year and go thru it to show you what I pay for utilities. Realize this house was built in 1977.

Maybe a shade tighter than Swiss Cheese.

My current home is 1979 and is in much better condition than when originally bought but I still have work yet to do. (It's a tad larger than the other house so the bills are currently a bit more.)

Ed, sounds like they build it tight on 3 sides of the house and forget the floor. The house needs crawlspace humidity controlled and a heat recovery air exchanger installed for fresh air.

I don't think I would have made a big deal about tightness. IMO it's better to not have wind blowing through the doors and windows.

Also IMO, if you need a whole house dehumidifier, then you got your air conditioner sized wrong. After all an HVAC unit is nothing but a whole house dehumidifier. The portable ones work by first heating the air and then cooling it. Why would you want to heat the air on a hot day? Just get the AC unit designed right, and let that one machine do the job for you.

I had interior humidity issues in San Antonio to the point of mold growing in the closets. This was after I had a Carrier 2-stage system installed. Clearly it was sized wrong (2/3 ton stages) even after the Manual J calcs by registered professional engineers. The old house was leaky, partially insulated, and in the shade of big trees, so there was lots of room for mistakes to be made. Several years later we added a 400 square foot addition and suddenly the AC got the humidity down. Still later we replaced the furnace with a Carrier Infinity and thermostat. After that we had absolutely no problems with humidity. I set the humidity set point for 50% and let the temp fluctuate as needed to maintain the humidity. Generally we set the temp for 75 degrees, but sometimes we'd see temps a few degrees lower as it continued condensing humidity out.

Good if it works for you, but overcooling does carry a cost in comfort as well as electricity. As a first approximation, the extra electricity would be around the same as running a dehumidifier to remove the moisture.

It is nice to hear the Carrier Infinity AC does a good job in controlling humidity in San Antonio. I wonder if it would work that well in other parts of Texas?

From OP's first post:

"We hope to build a fairly tight house. On to dehumidifiers. What, if
any are the downsides to whole house dehumidifiers?

cost to purchase, install and cost of use.

I would "infer" mind
you that in theory we may be able to raise the thermostat on the AC up
to say 78 vs 72 and with the "drier air", feel just as if not more
comfortable by using a whole house dehumidifier. Ultimately, the HVAC
would possibly run less thereby reducing energy consumption. The unknown
to me at this point, is what the energy consumption levels are of the
dehumidifiers?

honestly I"m not sure, but Teddy Bear could answer this one! I find my

90 pt ultra air costs me about-$30 more per month to use., so for me the added cost is $1.00 per day.

as my utility bill rarely is more than $50 a month, my house is tight, my slightly oversized single stage heat pump with variable speed air handler maintains about 52% RH. I run dehumidifier in shoulder seasons...and sometimes when we have several days of rain.

other than that...the humidity is comfortable without whole house dehumidifier. note that this is an existing house, in full sun . I have shading mid july that shades the west facing windows from the brunt of the afternoon sun,and my duct work is all inside the living space.

One model I looked at was the Ultra Air XT155H. As a side
note, is adding the fresh air intake a wise option?

fresh air necessity is determined by the actual measured tightness of the home. build tight, ventilate right. plan for the house being tight enough to require fresh air. I'd have to check ASHRAE but off the top of my head 6 cubic feet per minute (cfm) per occupant, per gas appliance. the most fresh air I've added to a home was 40 cfm.

the fresh air intake needs to be taken from outside the house, I usually put mine in a soffit. filter back grill and filter, duct with barometric damper if you have whole house dehumidifiers you can tie directly to the unit. if not then duct goes to return air, so clean ...filtered...measured amount of air is dehumidified before entering the house. it all depends on the design of the house/duct system.

On the same note as
the HVAC equipment since we have TWO HVAC units, would we need TWO
whole house dehumidifier systems? As in one for each floor?"

Again it would depend upon the layout of the duct systems. now, on paper is the time to design a duct system to accommodate this type of install. but do understand that the best plans on paper don't always equal to what gets installed. someone has to oversee that design & install are the same.

and fwiw, the whole house dehumidifier with fresh air intake takes place of erv.

( not hrv which is for cooling climates)

best of luck.

Here's the thing you have to remember about relative humidity: when you cool air, relative humidity goes up. Yeah, air conditioners reduce humidity, but they do it by dropping the air to below its dew point, at which time the humidity condenses out of the air and becomes that condensation that drains away. The air emerging from the vents on your a/c unit is darn close to 100 percent relative humidity for its temperature, maybe 55-60 F or whatever. The lowering of relative humidity comes as a result of this newly-cooled air mixing with warmer room air and having its temperature go up.

The other way to dehumidify air is simply to heat it. That's why the relative humidity goes down in the afternoon and you see those 90 - 100 percent relative humidity readings early in the morning when the temperature is at the day's lowest. This is also why a dehumidifier first cools the air to below its dew point and then warms it back up by routing the air first through the evaporator then its condenser. If you only cool the air (and then throw the heat away outdoors, as an a/c unit does) you're only able to dehumidify to the extent that you can continue to cool the air. You take a cool damp basement that's only maybe in the mid-60s to 70 temperature-wise, start running an air conditioner there that's getting rid of heat, and pretty soon you can't remove anymore moisture because your air temperature is getting colder (naturally increasing the relative humidity). And, the air emerging from the a/c is now only a few degrees cooler than the air going in, so there isn't much moisture being removed there either. This same thing occurs if it's a damp day and you run a/c to dehumidify the house. Because you're only cooling the air, it can only be dehumidified to the extent that it, a) needs to be cooled, and b) can increase in temperature after emerging from the a/c system.

The bottom line is that precise control of indoor relative humidity is hard with normal mechanical equipment that's available to consumers. But here are a few things to keep in mind: First, a dehumidifier is good only where you can use some heat. They're going to add to the air an amount of heat exactly equal to the power they use, so if you buy a 900-way dehumidifier it's going to add about 3,000 btu of heat to your space. Do you want that heat? If you're also running air conditioning probably not. In cool, weather, or in a place that's normally damp such as a basement, yes - the heat is wanted and it also helps dehumidify. Second, the best way to control humidity with air conditioning systems is with low-capacity equipment running a high percent of the time. You can do this with a variable-capacity compressor, or if you want to do it on the cheap you can supplement your central a/c with a 5,000 btu window unit running constantly. (Combined with raising the thermostat setting of the main system so the window unit does more of the cooling, especially at high-humidity, lower-temperature times such as nights and rainy days.) The key is, keep removing moisture while cooling less.

I was surprised by some of the results when I read this case study cited below. I It is dated, but still valid and conceptually useful. Maybe you can search and find something more up to date. I am reasonably sure that a more modern, fully speed-variable system will work better at controlling humidity than the two-stage if the two-stage cannot drop its output as low as the variable speed system. No cooling system, however is going to keep the home at low humidity when the heat load is very low and the humidity is high. It just can't happen. I am thinking about daytime in the cooler parts of the year and at night time. The study includes data concerning target humidity levels.

Conditioning Air in the

Humid South—Creating

Comfort and Controlling

Cost

Research Report - 0214

November-2002

Building Science Corporation

Several years ago, the last I looked at mechanical, refrigeration-based dehumidifier efficiency, there were two tiers and about level in each tier. The DOE (Energy Star) maintained a list. I hope that they still do. There are the very common freestanding units on the one hand. On the other hand, there were the whole-house units and freestanding units based on them.

I used to be very interested in desiccant wheel dehumidifiers. I am familiar with them for use in keeping laboratory cold rooms (~ 40 F) dry. I know that they are used a lot in industrial and commercial human-occupied working space applications. I'll bet that they are becoming practical for residential use. A gas-fired model or perhaps one using heat supplied by an hydronic solar collector would cut operating costs drastically compared to electrical ones*. (You'd have to allow for super insulated storage for the solar hydronic.) If you find a clever HVAC pro, and you are lucky with the layout, you might be able to toggle the dehumidifier between the two zones getting double duty out of it. It might be worth installing even if the cost is high if you are going to be in the home for a long time.

VRF systems would also cut operating costs. Proving re-heat with hot gas from the compressor that is supplying your cooling that is normally "wasted" outdoors can cut costs.. Those systems have historically been very expensive to install on the residential scale and not familiar to residential contractors. They won't work, either, if cooling is not called for unless the system has the capacity to put the refrigerant cycle completely inside the house not using the outside coil much or at all.

*Recently a cold room was removed from the building that I work in. I was tempted to get my hands on the "Munters unit", as the HVAC guys that I know call them, until I remembered that it is electrically-heated.