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.
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.
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
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
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
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.
And from that weatherbase web site, I look at Alvin TX as it is 42 feet
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.
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.
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.
The numbers below belie the statement above.
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?"
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
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!!
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.
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).
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?
Moist air is "lighter" than dry air (air pressure tends to be
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. ;-)
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