Frugal dehumidification

[Robert Gammon]

in the upper

Ok, you do not believe. That's up to you. With inside temps in the mid 80s in January here and indoor RH above 75%, YOU COME SWEAT IT OUT HERE.

[digitalmaster]

I can see you have not learned yet.The people who live in a certain are KNOW if the air outside the home is drier than inside.Your number crunching is foolish.

[digitalmaster]

speaking of faith...you have much too much faith in your interpritation of 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.

>

[digitalmaster]

you are a vague article of faith...put your money where your mouth is and show us...come down here and use your idea.

[digitalmaster]

You explained that very well..I wish I could have made the point as simply and straightforward as you.

[Rod Speed]

Robert Gammon wrote

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.

[nicksanspam]

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

[digitalmaster]

now you are making sense

look for rockbed

[Robert Gammon]

In the hours when Austin, Houston, Macon, New Orleans hit their low 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.

It will be a 3 or 4 sigma case for Austin, Houston, south Georgia, etc. 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.

Save energy and money at the expense of EXTREME discomfort. Frugality can be carried to EXTREMES. It is possible to pinch a penny TOO hard.

OK, save us ALOT of money, reverting back to the houses of our 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!!

[nicksanspam]

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