Frugal dehumidification

Ventilate when the outdoor vapor pressure P(Ta,RHa) is less than the indoor vapor pressure P(Ti,RHi) in the table below, and try to do this with warmer outdoor air when the house needs heat and cooler outdoor air when it needs coolth.
20 FOR R=0 TO 5 30 PRINT TAB(6+10*R);50+5*R; 40 NEXT 50 PRINT TAB(66);80 60 PRINT 70 FOR TAP TO 80 STEP 2'temperature (F) 80 PRINT TA; 90 FOR R=0 TO 5 100 RHAP+5*R'RH (%) 110 PA=RHA/100*EXP(17.863-9621/(460+TA))'vapor pressure ("Hg) 120 PRINT TAB(5+10*R);PA; 130 NEXT 140 RHA… 150 PA=RHA/100*EXP(17.863-9621/(460+TA)) 160 PRINT TAB(5+10*6);PA 170 NEXT
50% 55 60 65 70 75 80%
50F .1836264 .201989 .2203516 .2387143 .2570769 .2754395 .3121648 52 .1976689 .2174358 .2372027 .2569696 .2767365 .2965033 .3360371 54 .2126632 .2339295 .2551959 .2764622 .2977285 .3189948 .3615275 56 .2286653 .2515319 .2743984 .2972649 .3201315 .342998 .3887311 58 .2457342 .2703076 .2948811 .3194544 .3440279 .3686013 .4177481 60 .2639307 .2903237 .3167168 .3431098 .3695029 .395896 .4486822 62 .2833194 .3116514 .3399833 .3683152 .3966472 .4249791 ..481643 64 .3039678 .3343646 .3647614 .3951581 .4255549 .4559517 .5167452 66 .3259469 .3585417 .3911363 .423731 .4563257 .4889204 .5541098 68 .3493307 .3842637 .4191968 .4541299 .4890629 .523996 .5938621 70 .3741957 .4116153 .4490348 .4864544 .523874 .5612935 .6361327 72 .4006234 .4406858 .4807481 .5208105 .5608728 .6009351 .6810598 74 .4286993 .4715692 .5144391 .5573091 .600179 .6430489 .7287888 76 .45851 .504361 .550212 .596063 .641914 .687765 .779467 78 .4901487 .5391635 .5881784 .6371932 .6862081 .735223 .8332528 80F .5237118 .576083 .6284541 .6808253 .7331965 .7855676 .89031 "Hg
We need a little computer to do this automatically, eg a Smart Vent!
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
snipped-for-privacy@ece.villanova.edu wrote:

Doesn't do a thing for life on the Gulf Coast.
The number of hours where we are off the end of this table (i.e. greater than 80F AND greater than 80% RH) is over 6000 hours a year. Travel away from here in almost any direction and lips will chap and crack.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Robert Gammon wrote:

Right now at 8:36am, the temp outside is 75 degrees and the RH is 83%. High temp today will be in the mid-90s with RH staying in the 80 to 90% range.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

You might enjoy an extended table (above the ASHRAE comfort zone):
70 75 80 85 90 95 100%
80F .7331965 .7855676 .8379388 .89031 .9426811 .9950522 1.047424 "Hg 82 .7830198 .8389498 .8948798 .9508098 1.00674 1.06267 1.1186 84 .8358255 .8955274 .9552292 1.014931 1.074633 1.134335 1.194037 86 .8917653 .9554628 1.01916 1.082858 1.146555 1.210253 1.273951 88 .9509972 1.018926 1.086854 1.154782 1.222711 1.290639 1.358568 90 1.013691 1.086098 1.158505 1.230911 1.303318 1.375724 1.448131 92 1.080018 1.157162 1.234307 1.311451 1.388595 1.465739 1.542883 94 1.150158 1.232312 1.314466 1.396621 1.478775 1.560929 1.643083 96 1.2243 1.31175 1.3992 1.48665 1.574099 1.661549 1.748999 98 1.302636 1.395682 1.488727 1.581773 1.674818 1.767864 1.860909 100 1.385372 1.484328 1.583283 1.682238 1.781193 1.880148 1.979103

Hard to believe, with only 8760 hours per year. Then again, YOU are often hard to believe. Perhaps your town has lots of 10 day wintertime months :-)
NREL says the average daily MAX temp is less than 80 F from October through April in New Orleans and Port Arthur, less than 80 from November through March in Corpus Christi, and so on.
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
snipped-for-privacy@ece.villanova.edu wrote:

as one local guy put it, we have 11 months of summer, and then there's August.
NREL does not accurately depict what Houston weather is like. RH is almost always above 50% and in the summertime, the air has WEIGHT when stepping outside on a sunshiny day with temps in the 90s and RH nearly 90.
Year round apparel is short, tshirts and sandals. The number of days where the temperature at any time of the day drops into the 30s can be counted on your fingers and still have fingers left over. In the nearly 30 years we have been here, it has snowed twice and we've had one or two ice storms.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Well, They've only been measuring it every hour for the last 30 years in Houston... They say the average daily max in Houston is less than 80 F from November through April. That's 6 months, ie 50% of the year, ie 4380 hours. The 24-hour average temps in May through October are 74.5, 80.4, 82.6, 82.3, 78.2, and 69.6, which eliminates 6 months of nights, another 2190 hours. There are only 2190 hours left in a year, on this planet. How many hours per year on your planet? :-)

Oh. That's different. The air has WEIGHT :-)

That statement has little to do with your claim:

How can you combine such a huge ignorance with such a huge arrogance? :-)
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Looking at weather.com, average high temps are above 80 from April thru October Average low temps are above 60 for the same time period.
Average precipitation per month ranges from just about 3 inches to almost 6 inches.
Record Highs are All above 80 for the ENTIRE year.
Record lows are all above the freezing point from April thru September.
wikipedia.com says average annual precip is 48 inches here with prevailing winds from the south and southeast for most of the year bringing heat from Mexico deserts and moisture from Gulf Of Mexico. "The air tends to feel still and the humidity (often 90 to 100 percent relative humidity, while average afternoon relative humidity is between 57 and 60 percent in the summer) results in a heat index higher than actual temperature. To cope with the heat, people use air conditioning in nearly every car and building in the city."
My thermostat is set to 80F, we have two ceiling fans and a box fan to keep the air moving in here and we still feel quite warm
The number of weeks in the year where the outside air temp is in an acceptable range and the RH is also in an accceptable range to allow one to open the windows and doors and let the outside air in, is a very very small number of weeks, my estimate is less than 3 weeks of the year.
We visited Palm Springs in the heart of the summer several years ago. Outside air temps were well above 100, approaching 110. It was a VERY pleasant environment for us as the temps were only a little above what we were used to, but the humidity was so much lower (course we paid with severely chapped lips as a result)
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

That means the outdoor air temp was above 80 once every 30 years or so.
How can you combine such huge ignorance with such huge arrogance? :-)
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
snipped-for-privacy@ece.villanova.edu wrote:

Nick, You deliberately misinterpret what I said. The data shows that the high temp for ANY given month in the history of record keeping for Houston TX, even in January and February, there have been days where the high temp was above 80F. The AVERAGE high temp is above 80F from April thru October.
Arrogance, hardly. The shoe clearly is on your foot not mine.
I invite you to spend a year on the southeast Texas Gulf Coast and then tell me that I am full of it.
OK, 6000 hours was way too high. I got interested after you made your posting.
BTW way do you continue to use BASIC for these simulations???? C or C++ will do the job as well and most anyone with a technical background can understand
I stand my my claim after living here since 1982, there are only about 3 weeks of the year when the temperature is in the right range and the humidity is in the right range that we want to throw open the windows and doors to let the nice outside air in.
Right now, we are in a low humidity state. Outside air temp is in the low 90s (TV says 90, my local in the shade thermometer is 92). RH is 43%, lower than average for afternoon temps. NO WAY DOES ANYONE WANT THIS AIR INSIDE!! Forecast temps for the rest of the week are highs of 94 to 97 and lows of 71 to 72.
\$75 for heating for the ENTIRE winter is NO joke. Thats all I spend TOTAL for the ENTIRE YEAR!!
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Well?
No.
So?

So?
As in "I was wrong." That's progress :-) Congratulations!
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Robert Gammon wrote:

Humid air is LIGHTER than dry air at the same temperature and pressure, NOT heavier.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Ether Jones wrote:

On what basis is this argument???
Seems to me that the more water vapor per cubic meter, the heavier the volume of enclosed air is.
Hot air will rise over the top of cold air, yes, but hot is not necessarily humid.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Well, things are not always what they seem. Mercury, at room temp, is a liquid, yet is nearly twice as dense as solid iron. It is true that moisture laden air is LESS dense than dry air at the same temperature. John
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Robert Gammon wrote:

Physics.
Your intuition is wrong in this case.

I did not say that hot air is necessarily humid... I said that humid air is lighter than dry air at the same temperature and pressure.
So for example one cubic foot of dry air at 90 degrees Fahrenheit and one atmosphere pressure is HEAVIER than one cubic foot of humid air at the same temperature and pressure.
The reason is simple: air is mostly nitrogen. Nitrogen occurs naturally as a diatomic molecule, N2. Water is H2O. Use a periodic table to figure the molecular weights of the nitrogen molecule and the water molecule. You will see that the water molecule is much lighter than the nitrogen molecule.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Ether Jones wrote:

It might also help in explaining to point out that, at a given temperature and pressure, a given volume of gas contains the same number of molecules no matter what those molecules are. This isn't exactly the most intuitive thing: after all, it's not how solids behave.
- Logan
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Logan Shaw wrote:

OK, OK, Its not weight we feel.
Still air with high humidity prevents our skin from effectively dissipating heat thru evaporation of sweat. The brain interprets this as weight, somewhat akin to the feeling of suffocation, but very very mild.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Most air has weight, about 0.075 lb/ft^3, on this planet.

This is progress :-) But your claim had nothing to do with feelings...
Now why not check your "facts" before posting them? :-)
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
snipped-for-privacy@ece.villanova.edu wrote:

This will be the LAST time I EVER respond to ANYTHING that Nick at Villanova posts to the alt.home.repair newsgroup.
For those of us who live in the region that NREL classifies as Humid-SemiTropical, lots of things that work elsewhere do not work here. For Instance.
Vapor barriers go on the OUTSIDE of the house, insulation is ALWAYS unfaced, and vinyl wallpapers should NEVER be applied.
Vegetation should never be closer than 3 feet from the house, and an appropriate ground cover should be maintained over than area. Ground should slope away from the foundation at least 2 inches per linear foot.
Windows on Southern exposure sides of the house should be eliminated, or scaled back in size dramatically if elimination is not possible.
geothermal heat pumps will either require open loop cooling, or will require multiple large diameter wells, 400 feet or more, likely two of these per ton of installed capacity to make a closed loop system.
RH levels outdoors much of the year make outdoor activities uncomfortable.
Construction workers all wear long sleeves, hats and scarves to keep the sun off their skins. Even the very low paid yard care crews do this.
Natural fiber clothing is preferred in most regions as polyester and rayon are poor at wicking perspiration off of the skin.
I wear sunglasses ALL the time when out of doors.
Relative Humidity levels and temperatures for most of the year are such that the struggle is almost always to keep moisture out, as we are NEVER too dry inside.
In the new house I will build, I MAY decide to include a ventilation damper to allow the ventilation system to inject some fresh air from time to time AprilAire makes one that is reasonably priced, it wont open the vent trigger the fan unless outside air temp is under 100F and over 0F, or if allowing the ventilation would cause indoor RH to rise above 55% However these requirements mean that the damper will not operate for most of the summer. The plus side of this is that it runs the fan in the hvac while fresh air is introduced.
Nick, your system should closely model what the AprilAire Ventilaion Control System Model 8126 does. The more deluxe ERV as documented at www.ourcoolhouse.com works ALL the time, exchanging heat and moisture with outside air. This one works better in climates that have 4 seasons, and ourcoolhouse is located in Colorado.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Nickie is a math and theory lover. That can be great if you want to know how much heat a standard ASHRE mouse puts out, but Nick sometimes gets confused when confronted with the fact that not all mice are ASHRE compliant.
Weather station instruments are commonly placed at airports or other open areas. That may not be representative for an area that otherwise has dense vegetation and/or standing water. Airports have to be level and dry enough to land planes, and they also usually have a lot of paved or concrete areas and low cut grass. That means more wind can sweep the ground, less groundwater can be transferred by vegetation into the immediate air, and temperatures and humidity can differ from areas nearby.
If Nick builds his constructions on or near a runway, he may be right. If he builds on a typical wooded or swampy southern lot, he may find the real world intruding.
In other areas, the representative weather stats can be off because the weather instruments are NOT at an airport. I used to take measurements at a radio station where my father worked. That station was (logically) on a hill, and the town below might be lost in fog while I was recording clear skies, low humidity, and a mild wind for the town. Fast foreword a few years. Now, another weather reporting site in that community is located about 50 feet above a river, at a spot where it enters a small canyon. It also has different measurements than if you set up a station in the middle of the small town. On a practical basis, weather is an extremely local phenomenon.
Here in south Florida, I have become accustomed to seeing rain-soaked streets that are totally dry 500 feet up the road. I've had my pool fill with four inches of water, while the airport reported no rain at all. The heavy rainstorms here can be that localized. Depending on the winds and the season, the coast may get strong breezes and a cool temperature, while five to fifteen miles inland, clouds billow up and create a line of storms that never touch the coast. You can't always go by reported weather from a selected set of sources
Having been through southern Georgia many times, I know there are large differences in temperature and humidity around the peanut and cotton farms compared to the pine forests and towns. That is common sense and experiential knowledge.
However, Nick takes it as an article of faith that discrete samples from professionals always represent a greater norm. :-)
FWIW, I've used economizer dampers on theatre HVAC units in Vermont and in Alabama. The greatest use was over Christmas vacation, when crowds would overheat the theatre and the outside air was less than 40 degrees F. Much of the rest of the time in Vermont they had to be sealed with plastic and duct tape to prevent drafts and heat loss. The integrated ones also had more electro-mechanical problems than other systems. That may have been design related - unsupported lever arms, corrosion, ice, etc. but they were only marginally cost effective overall. In Alabama, the vents were straightforward exhaust vents, and were used only if the AC was overwhelmed or there were smokers (which were still allowed in certain sections of some theatres).
Whole house fans were common in the south before AC, and venting is just a variation on this. The success of those whole house fans often depended on measures such as opening certain windows after 10 PM and closing the house up tight before 8 AM, drawing all the curtains, and sweating out the early evening on a porch. Once AC became affordable, life changed for the better. Getting people to go back is nearly impossible, and Nick should know this, but his stubbornness lack of experiential knowledge prevents it.
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Houston has two measuring stations, the airport, with a 96' elevation and a 96 F 1% summer design temp, and "Houston County," with a 108' elevation and a 97 F 1% summer design temp. Not vastly different :-)

Automatic controls can turn on a fan when outdoor air is cooler and drier than indoor air, in the absolute sense. This is quite recent, with existing unexpired patents for drying out houses after floods and so on. See http://smartvent.net .
Nick
âœ–
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>

Site Timeline

• Kitchen faucet chatter

• - next thread in Home Repair
• SPA -- replacing a 120V heater with 240V heater

• - previous thread in Home Repair

• Need help INTERPRETING these test results police cruiser SAE J866a Chase Test

• - last updated thread in Home Repair
• All This Talk of Gluing Edges: Practical Illustration

• Share To

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.