Another thing about hot, humid air is the difference in human breathing. High humidity can actually reduce the partial pressure of oxygen in the air, so one has to breath more for the same O2. The water vapor also changes the viscosity slightly, making it 'feel' different to people that are breathing heavily (such as while exerting themselves). Not to mention the physiological response of the mucus membranes to the higher humidity.
For example, the vapor pressure at 70F and 50% is about 0.181 psia and pp of O2 is 3.048 psia. But at 110F and 90% humidity, the vapor pressure rises to
1.14 psia and the pp of O2 is reduced to about 2.85 psia (~93%). Compare this to the drop in partial pressure with altitude, and this is about the equivalent change in pp of O2 as going from sea-level to 1600 feet up. But the overall density of the mixture moving in/out the lungs is more than what one would experience from just rising in elevation.
A layman in casual conversation might use words like 'heavy' or 'weighty' to describe breathing in these conditions.
EXACTLY how I feel here on the numerous summer day where the air temp is in the mid-upper 90s and afternoon RH is well above 75% Its harder to do EVERYTHING. and the car I drive now has its AC broken.
Nick needs to spend a summer in either Macon Georgia, Mobile Alabama, New Orleans Louisiana, Houston Texas or Atlanta Georgia staying in a home that does not have Air Conditioning BEFORE says a another word about how wrong we all are, those of us who actually live in these cities, and KNOW how hot and MISERABLE the summers are. He simply has NO idea what we endure.
Did he actually ever build his homebrew sewage waste heat recovery unit? He talked about it, but never showed us the end result or showed the actual heat recovered.
I see no sign of arrogance.I don't try to tell Inuit people how to keep their home warm.In order to do so with any semblance of intelligent ideas I would need to spend some time in that locale.Some situations cannot be fully appreciated until experienced.
Then again, there is no claim, because he's already said that he was wrong about the 6000 hours/year thing.
By the way, the "smart vent" idea is a workable idea in some climates, but not in the part of Texas where I live. The low last night was
73F, and it was only that low for about an hour. Considering that I leave my thermostat set at 74F, the air inside the house has to be at a lower humidity than the air outside since the A/C has been running all night leading up to that point in the very early morning when it's almost a comfortable temperature outside for an hour or two, and during that whole time, it's been working to decrease the humidity of the indoor air.
And, it's only June. The nights will continue to get hotter for a few months. So, the smart vent might help at some time of the year in the part of Texas that I live in, but it wouldn't be for the part of the year when A/C is the most expensive, so when you consider the smart vent's benefit in terms of saving energy, you've got to consider that: if this device cuts (say) 20% off my electric bill during the few months of the year when the bill is smallest, what is the total savings over the course of a year? Not that large.
Then again, that was just one of many false claims :-) I was referring to his more recent claim than humid air is denser than dry air. Is anything denser than Mr. Gammon? :-)
We might say it's "workable" if it can significantly reduce an AC bill. It can't replace AC or dehum in places like Galveston or Key West. It IS a good way to dry out flood-damaged houses in New Orleans.
I'm unable to follow this logic, but we need to look at more than one night. The idea is to dehumidify a house by ventilating it with outdoor air if and when the absolute moisture content of the outdoor air happens to be less than the absolute moisture content of the indoor air. Or if the outdoor dew point is less than the indoor dew point, which amounts to the same thing. That may not happen every night, but most house materials can store moisture, or in this case, dryness.
For instance, Kurt Kielsgard Hanson's sorption isotherm catalog (as LBM tech report 162/86 under
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concrete stores about 1% moisture by weight as the RH of the air around it increases from 40 to 60%, and it weighs about 150 lb/ft^3, so a 4"x1000ft^2 50K pound floorslab could store 500 pints of water as a basement RH increases from 40 to 60%. Mold forms in about 2 weeks, above 60% RH. We might fire up the dehum or AC after 2 weeks...
And given the random nature of weather, we'd like to ventilate with cooler air in summertime and warmer air in wintertime, other things being equal, harvesting coolth and warmth when possible, avoiding condensation inside the house. We might say "Never ventilate if the indoor dry bulb is less than the outdoor dew point," or "Only when Tdbi-Tdpo > 5 F," or "Only when Tdbi > Tdbo."
For instance, if your house were 80 F (460+80 = 540 R) with 60% RH indoors, Tdpi = 540/(1-540ln(0.6)/9621) = 525 R (65 F). Outdoor air at 73 F and 50% RH has Tdpo = 53 F, so ventilation will dehumidify and cool the house. If Tdpo = 65, ie RH = 100e^(9621(525-533)/(525x533)) = 76%, ventilation will not dehumidify, but it will cool the house. If RH > 76%, ventilation won't help at all.
A 65 F house with 60% RH and Tdpi = 50.8 F and 80 F outdoor air might vent whenever Tdpo < Tdbi-5 = 60 F, ie RH < 100e^9621(60-80)/(520x540)) = 50% for warmth. This adds moisture to the house, but that's OK if there's no condensation and the indoor temp rises to keep the RH < 60% to avoid mold. Additional humidity makes it feel warmer and more comfortable.
Where do you live? This could work well in Abilene, with a w = 0.0130 yearly humidity ratio and 71.7 F daily min temp in August, or Amarillo (0.0119) or El Paso (0.0112) or Lubbock (0.0127) or San Angelo (0.0132.) Not every day, of course. How many hours per month are below average? We might quantify this with a TMY2 simulation.
We use last night as an EXAMPLE OF A TYPICAL NIGHT. If you are unable to see the logic of that, you need some more education. In the Humid-SemiTropical region, which includes New Orleans as well as Houston, there will be very very few nights when outdoor absolute moisture content is LESS than the indoor condition.
Again, in the Humid-SemiTropical region, such a condition will not happen on a daily or even weekly basis during the summertime. You extrapolate too much from the conditions in your Pennsylvania location.
During the summertime, in most of the region within the Humid-SemiTropical region, such a condition will exist so rarely that the ventilator might as well not be there. At this moment, 6:00am in mid-June, we have a fairly typical summertime condition for Houston TX. Temp is 72F, RH is 89%, Dew Point is 69F. My interior is currently sitting at just under 80F and RH feels like it is about 50% and that suggests Dew Point is about 45F. From this point we will climb to about 94F and RH will drop to about 50%. We will have mornings where Dew Point is within one degree of outside air temp and RH will decline only to about 80% with Dew Point tracking to within a few degrees of actual air temp.
In the Humid SemiTropical region, for most of the year, certainly May thru October, Outdoor RH will be HIGHER than indoor RH. Absolute moisture content of outside air will remain higher than indoor moisture for greater than 80% of the time. The ventilator will remain shut almost all of the time. AprilAir's 8126 ventilator is an example, it will never ventilate if indoor RH rises above 60%
Abilene is in a very different climate than Humid SemiTropical. It is on the border with Desert Southwest. It gets half the rainfall of Houston Tx. It gets much cooler there in the fall/winter than we do, average low temps are in the low 40s there.
Our point is that ONE SIZE DOES NOT FIT ALL. Building standards for Pennsylvania SHOULD be different from Humid SemiTropical areas like Houston, as you are concerning about keeping moisture inside the house for much of the year. Houston, New Orleans (never mind the flood damage).... we worry about keeping moisture OUT for much of the year.
Would you have any evidence for this rather vague article of faith? :-)
No. I use real weather data measured every hour for the last 30 years in Abilene, with w = 0.0130 and 71.7 F daily min in August, Amarillo (0.0119), El Paso (0.0112), Lubbock (0.0127), San Angelo (0.0132), and Houston...
NREL says Houston has average daily min temps and humidity ratios of 64.4 and 0.0138, 70.6 and 0.0163, 72.4 and 0.0172, 72.0 and 0.0171, and 67.9 and 0.0154 in May through September. We can cool by ventilation then, but dehum is less likely, so we probably need some AC dehumidification by day. Condensation can be avoided. It seems to me that house materials that absorb water will warm at night and lose that warmth to ventilation air and then cool by evaporation during the day as the AC removes water, for a net energy savings. You needn't reply if you have no idea what I'm talking about :-)
Would you have any evidence for this rather vague article of faith? :-)
How vague. That would depend on how you ventilate or use AC, no? And we are discussing absolute vs RH.
Agreed.
They are. This would work better in Amarillo than Phila or Houston. So?
A typical, average summer day in this region shows nighttime temps in the low to mid 70s with RH in the upper 80s to 90s. The indoor air temp will be dependent on the homeowners setting of the thermostat, but will range from the mid 70s to the low 80s. There is NO opportunity for cooling from outside air as indoor moisture levels will increase to very uncomfortable levels.
Right now, 8:15am June 10, 2006, outside air temp is 76F, winds are calm, and RH is 82%, dew point is 70F Indoor conditions are just under
80F and RH is about 50%
The figures will change from location to location in the Humid subTropical belt (or Humid SemiTropical belt as it is sometimes known), but outdoor RH will rarely be lower than indoor RH, and outdoor air will be lower in temp than indoor temps for only a very short intervals (a few hours in the depths of the night) and only when RH is 80% or higher (dew point within a few degrees of air temp)
There is NO help from the climate, in the summer months. We fight to keep moisture OUT, almost all the time. 48 inches a year of rainfall (yes Virginia, I said FOURTY EIGHT INCHES OF RAINFALL A YEAR, very flat land, high clay content in soils, so water pools, it does not drain away efficiently.
NREL data doe not correlate the time periods when low temp and humidity ratios occur. They do not occur at the same time of day. Low temps are the times when the humidity is the highest of the day, we are very close to saturation in the hours when the low temp is recorded. The low point for humidity will be late in the day when temperature is the highest.
25 years of experience living in this environment is NOT a rather vague article of faith.
Other when there is a thunderstorm in Villanova, what time of day do YOU have the lowest relative humidity????
YOU are discussing absolute, YOU are the only person using that term.
25 years of experience in THIS climate is REAL world application of information. This is not an urban myth. We invite you to to fly to Houston, or New Orleans for a few days to observe FRIST hand what we experience. As was mentioned by another poster, we do not attempt to tell the Native Canadians/Americans that live in the northern reaches of Canada and Alaska how to heat their homes in winter, not until we have been there and get some experience with local conditions, and see how people are dealing with the problems now, BEFORE we make recommendations.
SouthWest Airlines will get you here in a fully refundable RT ticket for about $600, less than half of that if you plan ahead.
We are agreed then FINALLY. This is not appropriate technology for all regions. Yes dry climates like Amarillo, or anywhere in the desert southwest could benefit from this technology, however, the INTENSE heat requires either refrigeration, or evaporative cooling to make the days bearable.
Outside of summertime, this technology will work well in any region that has a relatively mild climate, and is relatively average to dry in humidity levels.
Sounds like these opportunities will happen on non-average days, about which we know little, given this data, except that half the days will be cooler and half will be drier... 50% isn't bad odds. Nor is 25%, if cool and damp are well-correlated.
A single instant has little meaning.
Would you have any evidence for this rather vague article of faith? :-)
I disagree. You also seem to have missed this subtlety:
ACHTUNG: SUBTLETY!
Wrong again. Their monthly averages don't, but their hourly measurements do, in the form of TMY2 data files. Then again, humidity ratios don't change much over a day, since they are independent of temperature. Newer ASHRAE HOFs give mean coincident summer design temps and humidity ratios.
It is, given real numerical data ("It shore feels hot when my bunion hurts!")
Wrong again. Lots of people use that term.
Great. Send me an airplane or train ticket, s'il te plait.
This is why Nick is trying to keep to absolute humidity ratios. When you take a quantity of air with some moisture in it (say, 70F @90% 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.
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
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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
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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.
Given only that data, you can make those assumptions about percentages, and those are pretty valid assumptions[1]. But the reality, at least in Austin, TX, is different. In practice, the typical day is very hot during the daytime hours with mild temperatures (75F to 80F) and high humidity during the night.
However, if it's not a typical day, there's a 99.999% chance it's because we've had a summer afternoon thunderstorm where the heat of the day drives massive convection and we get drenched with water. On such a day, the temperatures will drop below normal during the evening and night, but the humidity is through the roof. So, in practice, the probabilities of cold and of dry are NOT independent. They are VERY strongly correlated.
- Logan
[1] Not totally valid though: if you have a group of 10 people with an average income of $145,000, it is not safe to assume that half the people make more than $145,000 and half the people make less. It's quite possible you have one person with an income of $1,000,000 and nine people with an income of $50,000. So in that case, 90% of the people have a below-average income.
Yes, Nick is arguing that he does not believe the average day is what we say it is.
Ok, these ARE averages, and we DO get a cool front to move thru and yes we have a thunderstorm. Temps drop from high 80s to mid 90s to low 80s, but then RH is close to 100% after the storm, uncomfy at best. Storms dissipate in the wake of the cool front. Daytime temps are now hitting the mid 80s vs low 90s, AH relief. But then the next High pressure system moves in and we BAKE for two to three weeks with no rain and cloudless skies. Daytime highs climb to mid to upper 90s, and with all the surface water surrounding us (Austin and Houston aren't that much different in the amount of surface water surrounding us, execpt for our proximity to Galveston Bay and Gulf of Mexico) evaporation rates climb and so does humidity. Well contractor talked to me about pumping ground water thru a geothermal heat pump and into a pond. Said pond will lose
1/2 inch to 1 inch of water level a day of more water is not supplied.
Logan is making EXCELLENT comments. However, in Nick's defense, he has backed off on saying that the system will work for US, and is now saying that it works better in dryer climates, cooler climates than what we enjoy/suffer with.
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.
The weatherbase.com web site points out some interesting facts about where I live.
COOLING DEGREE DAYS EXIST HERE EVEN IN JANUARY AND FEBRUARY!!
Yes, I have run my AC at times all thru the year, in every month. It gets cool for a short while then we have a spike that puts daytime highs in the upper 70s to low 80s, and with interior heat sources(TV, cooking, showers, people), house temps go above that so the AC runs for a few hours.
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