Frugal dehumidification

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Not necessarily best handled with AC tho in Jan and Feb.

Not a very frugal approach.

Doesnt really require the use of the AC.

Unlikely since most of that doesnt happen when the outside temps peak.

Mad.
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Rod Speed wrote:

mid 80s in January here and indoor RH above 75%, YOU COME SWEAT IT OUT HERE.
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I didnt say I didnt believe that YOU choose to use an AC like that.
I JUST said that its not a very frugal approach and it isnt the only possibility.

Bet thats rare there. And is mostly the result of having the AC set point too high.

And AC aint the only way to deal with that. Most obviously with the showers.

Thats not sweating it out and running the AC for hours aint the only way to deal with that in those months anyway.
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show us...come down here and use your idea.
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RH) and heat it up (to say 95F), the RH will drop from 90% to 41%, but the absolute humidity ratio of the moisture does *not* change (nor does the dew point). As we all 'know', warm air can hold more moisture than cool air, so the amount of moisture *relative* to how much it could hold goes down. That's one reason why warm air dries things out better than cool air.
So, *IF* the dew point of the outside air is low at night, then the absolute humidity is also low and one could get some benefits from ventilation. But if the dew point is upwards of 70F (as it often is in the summer along the Gulf Coast), then ventilation hurts, not helps. I don't think this idea would work well along the Gulf Coast (spent several years in MS), but it can work in other areas. I think the key is to track the dew point (a direct measure of absolute humidity).
My brother lives in Spring Tx (near Houston) and he mentions that they often report the RH as >100% in summer in late afternoon. At first this seems impossible (how can air be holding more moisture than the total amount of moisture air can hold??). But the answer lies in haze/ hot-fog. Moisture levels are so high during the day that as the air cools it actually becomes 'super-saturated' and as moisture condenses it doesn't 'fall' out of the air and remains suspended as haze.
It may be interesting to note that the 'overnight low' predicted by meteorologists is often tied closely with dew point. If the dew point is 78F on a sweltering evening in MS, you could bet the overnight low would not drop more than a degree or two below 78F. Not unless a cooler/dryer air mass was predicted to move into the area. Once the air cools to the dew point, the latent heat that must be rejected to condense moisture out of the air in cooling below the dew point pretty well stops any further cooling.
daestrom
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the numbers.You do not have a clue about actual local conditions.You try to dazzle with numbers ..then ask "where is your proof?"well....where is your proof...what house in middle georgia..or new orleans...or lower texas...have you used this frugal dehumid on???? there is a house for sale right down the road...I will get the realtors number and you can come try your idea.

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snipped-for-privacy@ece.villanova.edu wrote:

Yes, what I'm trying to say is this: in the climate where I live, unless you like it being 85F or even 90F indoors, you *will* run the air conditioning in a typical house for many hours a day. So, you have to take that fact into account when you try to understand whether, in actual practice, the outdoor air will have less moisture than the indoor air.
To put it another way, if you left the A/C and heat turned off indefinitely, eventually the indoor air and outdoor air would reach a point something like an equilibrium where the indoor and outdoor air have the same moisture content. That's because there is leakage and the two bodies of air are actually mixing all the time. So, with no climate control intervention taking place, they have the same average moisture content. Now, add in the assumption that you are running the A/C at probably a 50% duty cycle during the day and that the nights are hot enough that it continues to run all night. This tends to make the indoor air less humid, right? That is, after all, what air conditioners do.
So, to summarize, if you start with the assumption that the A/C runs several hours every day, the likelihood that outdoor air has less moisture in it than indoor air does is very low, because the indoor air is having its moisture level artificially lowered on a continuous basis.
To illustrate the difference, here's an analogy. Imagine that you were crazy enough to build a 5-story parking garage below ground right by the beach just a hundred feet away from the ocean. You encase the whole thing in concrete and anchor it into bedrock so it doesn't float due to buoyancy, but you find that there is a leak and the seawater is seeping in somewhere.
The water fills up 4 of the 5 levels of the garage. Because of tides, the water table changes; sometimes it's high enough that if it stayed at that level, it would fill up all 5 levels, and sometimes it's low enough that it would only fill up 3 levels if it stayed. But because it varies and sometimes water is flowing in and sometimes flowing out, the garage stays full up to about the 4th level.
What do you do? You put in a bilge pump. The seawater seeps in at a good rate during high tide, but you discover if you run the bilge pump at the right duty cycle, you can keep the water out of the top 4 floors even then. So you run the bilge pump, with nearly a 100% duty cycle during high tide and with a much lower duty cycle during low tide, and you manage to keep the top 4 floors free of water, which makes the garage useful, and you can charge people to park in it and make money.
Now you start looking at ways to save money. You suddenly hit upon an idea: instead of a pump, you can put in a valve that is controlled by a water level sensor. When the water table is lower than the level in your parking garage, the valve will open and water will flow out by gravity. When it's higher, the valve will close. Sounds good. But then you do the math and you realize that the lowest level that water table is ever at is 3 floors up in your garage, and since you are running the bilge pump to keep the water down to only the first floor of the garage, the water level in your garage is never actually going to BE higher than the water table. So you realize that the valve will never open and you will never save any money by installing it.

What I'm saying is that, in my climate, there IS no cooler air in the summertime. The indoor air is always cooler and has less moisture than the outdoor air, for months in a row. The only time when outdoor air is relatively cool is when we have a thunderstorm. Then the temperature drops, but the humidity goes way up.
If you go to http://www.weatherbase.com/ and look at the average low temperature for Austin, TX, you'll see that during the months of June, July, and August, the average lows are 72, 74, and 74, respectively. Also, go to
http://weather.noaa.gov/weather/current/KATT.html
and look at the 24-hour weather history for Austin to see what time that lower-70's weather occurs. It's usually only for 1-3 hours, right between 5:00am and 7:00am. Now go back to weatherbase.com and look at "All Data" instead of "Summary", and look at the "Average Morning Relative Humidity" data. You can see that in the mornings in June, July, and August, it's nearly 90% relative humidity. Meanwhile, with the thermostat set at 75F and with the air conditioning having run a moderate amount all night since it was probably still 90F at 10:00pm, would I want to ventilate the house and get some of that outside air? The indoor air is about the same temperature but the indoor moisture is low since the A/C has been taking moisture out of the air all night.
Now go back and look at the average high temperature for June, July, and August in Austin. It's 90-95F average for all three months. So, when do you think people in Austin spend the most on electricity for A/C? It's June, July, and August. If you want to make a dent in the cost of A/C and be frugal, you've got to attack the part where most of the money is going. A smart vent might be able to lower the bills for the months when the weather is relatively mild, but that isn't going to help much since those months are a small part of the total expenditures.
- Logan
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Logan Shaw wrote:

above sea level and I am about 50 ft above, Alvin is about the same distance from me as the 100ft measuring point for Houston, and MUCH closer than the airport measuring point.
COOLING DEGREE DAYS EXIST IN EVERY MONTH OF THE YEAR!! Nick may not believe, but yes A/C runs some days even in January and February if for no other reason than to remove humidity and the heat buildup from cooking, TVs, ironing, people showers, etc.
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wrote:

You explained that very well..I wish I could have made the point as simply and straightforward as you.
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Scheme II: Ventilate a house with some internal thermal mass at night when outdoor air is cooler and dry enough to avoid condensation inside the house, and AC the house during the day, for a net sensible cooling gain, compared to AC alone.

OK.
Then again, we have scheme II...

Airsealing houses with blower door testing can help a lot in these regions, but I suppose that's rarely done. They need dehumidification, but they don't need lots of cooling compared to Southwest houses, so airsealing can save a lot more energy than insulation.

Agreed...
The numbers below belie the statement above.

Hmmm.
NREL's long-term average Austin lows are 71.5, 73.9, and 73.9, but these are averages, so half the nights are cooler. The 30-year record hourly lows are 53, 64, and 61. These might be 4 or 5 sigma tails of a Gaussian distribution. We could do a simple TMY2 simulation.

We can do a lot with 1-3 hours of ventilation. If it's 80 F indoors and 70 outdoors, 2 hours at 5000 cfm (eg 2 90 watt Lasko window fans) can remove about 2(80-70)5000 = 100,000 Btu, like a 5,000 Btu/h 500W window AC running for 20 hours. At 65, we can move 150K Btu, like a 3-ton AC running 4 hours.

Sounds like scheme II...

Perhaps, if you want to save energy and money.

We might ask "How much does it save?" and "How much does it cost?"
Nick
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http://www.synapse9.com/airnets.htm http://www.motherearthnews.com/library/1982_May_June/My_Mother_s_House_Part_V
look for rockbed
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snipped-for-privacy@ece.villanova.edu wrote:

temps, the moisture content of the outside air is FAR higher than inside air. Ventilate a house that has 78F air in it at RH of 50% with 72F air at RH of 88% is a recipe for EXTREME discomfort. For instance, tight now in Houston at 7:15am, it is a morning that is close to the averages, 71F outside, 93% RH, 69F dewpoint. Introducing this air to my 79F will cool it down, BUT will introduce LOTS of added MOISTURE. And cause me to trigger the AC JUST to get rid of the added moisture. OK, get the temp of the house down to 74F with dewpoint of 70F is progress of a sort, but we had house temp of 78F with dewpoint in the upper 50s. Driving the dewpoint inside the house UP is the WRONG direction.
Now in more arid climes, YES this is a good idea, take 75F outside air at 40%RH (current conditions in Midland TX), and pump that into a 78F house, and we have a CLEAR solution. Midland is actually a VERY good example of this as it has average low temps that NEVER get up into the 70s, and the air is generally quite dry there. So ventilation of these houses is a GOOD idea.
OTOH, Midland has another problem, severe dust. Attics can get 1-3 inches of airborne dust deposited on top of the insulation over a 10-20 year period.

to have cool air in the morning at a low enough absolute humidity that we will want to introduce that moisture laden air to our homes.

can be carried to EXTREMES. It is possible to pinch a penny TOO hard.

grandfathers, big wide porches to screen the sun off of the windows and exterior awnings over the windows that can't be under a porch (built 4-6 ft wide porches ALL the way around the house). I have seen a house near here that got built in the last 2 years with EXACTLY those features. We'll sweat alot more, smell bad to others who come in contact with us, BUT WE WILL SAVE A TON OF MONEY BY NOT USING AC.
For Austin, Houston, and the other cities we have been discussing, it seems foolish to ventilate a house with lower temp outside air at the expense of RAISING interior absolute humidity to uncomfortable levels. Lowering temp is not the ultimate goal for us, dehumidification is.
It is ALOT more comfortable to be in Palm Springs, Palm Desert CA in the summertime, be out of doors in the sun, than to do the same thing in any of the other cities mentioned. Temps there often exceed 100F even over 110F, but RH is under 30%, dew points stay VERY low there. Mist cooling is a feature that WORKS there!!
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This may be like Honeywell's differential enthalpy economizer, which measures the RH and temp of outdoor and duct return air and calculates which contains more heat and uses the air with less heat for cooling.
Air has a specific heat of 0.24 Btu/F per pound, and 1000 Btu evaporates a pound of water, so a pound of air at temp T (F) containing w pounds of water vapor has about 0.24xT+1000w Btu, where w = 0.62198/(29.921/P-1) is the humidity ratio and P = e^(17.863-9621/(460+T)) is the saturated vapor pressure in mercury column inches at temp T.
We might use the air with more heat for warming a house, within a comfort zone, with an additional dew point calc to avoid condensation, unless it can harmlessly drain away.
It's 65.1 F with 63% RH in my house now, and it's 53.6 at 86% outdoors, so Pi = 0.63e^(17.863-9621/(460+65.1)) = 0.398 "Hg and wi = 0.00839 and hi = 0.24x65.1+1000x0.00839 = 24.0 Btu/lb. Po = 0.360 and wo = 0.00758, ho = 20.4, so outdoor air would work for cooling, if that were needed.
Nick
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if the air outside the home is drier than inside.Your number crunching is foolish.
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Logan Shaw wrote:

Yes. This is known as the "ideal gas law". It is only an approximation, but for air at atmospheric temperatures and pressures, it is a good approximation.
So, as humidity is added to dry air, it displaces an equal amount (number of molecules) of air. The resulting mixture is lighter than the original dry air (at the same temperature and pressure).
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Kinda has to be if you want clouds, isn't it? If you watch how clouds boil up, it is obvious.
Moist air, however, feels "thicker," leading to terms like "pea-soup fog." As a totally wild guess, I wonder if the effect is a subjective one, partly from the humdity displacing part of the air and thus reducing the oxygen content, and partly from the water being more reactive than nitrogen, and therefore wanting to bond together in larger structures, which in turn create more drag on objects moving through the vapor?
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snipped-for-privacy@hotmail.com says...

lower when it storms and higher in fair weather), however the body is heavier since lighter air is less buoyant. ...kinda like liquid water is denser than air but people float. ;-)
--
Keith

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south(middle Georgia).
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It turns out there are now two Smart Vents, a floating flood gate and a ventilation fan controlled by a vapor pressure difference...

NREL says w<0.0112 from October through May in Macon, on average...
Nick
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better .
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