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re: humidifiers
9 nov 2000
ken h.  wrote:

> (william r. watt) wrote:

>> should not need a humidifier if the house is sealed okay.
>>i used to be dry and run a humidifier until i went around and
>>sealed the house. i don't need one now.

>i suppose that depends on where you live and what the rh is during
>colder months...

let's say it's january (the driest month) in phila, with an average
30.4 f outdoor temp and humidity ratio w = 0.0025 pounds of water per
pound of dry air. at saturation (100% rh) the vapor pressure of water
in 30.4 f air is approximately psat(30.4) = exp(17.863-9621/(460+30.4))
= 0.17 "hg, ie it will hold up a 0.17 inch column of mercury, and the
outdoor air has pw = 29.921/(1+0.62198/w) = 0.12", so the rh of the
outdoor air is 100pw/psat(30.4) = 69%...

>...but i disagree with the notion that rh is directly proportional
>to house sealing.

can anything be "directly proportional to house sealing," since "house
sealing" isn't a number? a house that is 100% (ie hermetically) sealed
will eventually have 100% rh, if there's a person or plant inside, ie a
source of water vapor. if the house is warmer than the outdoors, this
isn't quite true, as the water starts to condense on the cooler walls
before reaching 100% indoor rh. 

>in winter,  32 degree air outside at 60% rh will rise to 72 degrees
>inside the house with an rh of 20% or less.

yup. psat(32) = exp(17.863-9621/(460+32)) = 0.184 "hg, 60% of that
is pw = 0.111, and psat(72) = exp(17.863-9621/(460+72)) = 0.800 "hg,
so warming that air to 72 f lowers the rh to 100pw/psat(72) = 13.8%.

>unfortunately, a tightly sealed house is a polluted house...

pollution platitudes aside, ashrae recommends an average 15 cfm fresh air
per occupant, by whatever means. let's distinguish between tightly sealed
houses with and without ventilation fans or heat exchangers controlled by
humidistats, which can provide ventilation that is more reliable over the
seasons and more energy-efficient than accidental holes in the walls...

>and even  a well sealed  house will have low inside humidity during cold
>winter months.

reducing air leaks raises indoor humidity, since people add water vapor
to houses. an andersen flyer mentions an average of 14 gallons per week
for a family of 4 from breathing and showers and cooking and cleaning and
so on. nraes says a plant can add a pint of water per day to house air.
how much do you water your plants? 

condensation on windows with interior surfaces at temps less than the
room air dew point reduces humidity. better insulating windows with 
warmer interior surfaces allows higher wintertime rh. 

air at 72 f and humidity r = rh/100 has an approximate (f) dew point temp
td = (460+72)/(1-(460+72)ln(r)/9621)-460. (td = t/(1-tln(r)/9621), using
absolute temps.) condensation forms on a 52.4 f surface in 72 f air with
50% rh, on a 38.8 f surface at 30% rh, and so on. 

windows with u-value uv in a 68 f room with fractional rh r and outdoor
temp t condense when the interior pane surface temp 68-0.68(68-t)uv is
less than the dew point of the room air, for instance, if the outdoor
temp is 38 f and uv = 0.5, and the interior pane is 68-0.68(68-38)0.5 
= 57.8 f. condensation occurs whenever r > exp(-(1.596-0.02347t)uv).
here's a small table (in courier font): 

                uv     (us r2)  (us r4)
                1      0.5      0.25 btu/h-ft^2-f

     t  -10 f   16%    40%      63%
         10     26%    51%      71%
         30     41%    64%      80%

>to maintain a 3000 sq ft house at 35 to 50% rh in the coldest winter
>months in the north, it takes several  gallons of water dispersed into
>the air over a 24 hour period to maintain that level.

yup. a 3000 ft^2 house with 0.15 air changes per hour has 60 cfm airflow
with wo = 0.0025, and 72 f 50% air has wi = 0.62198/(29.921/(0.5psat(72))-1)
= 0.00844, and 60 ft^3 of 72 f air weighs about 4.5 pounds, so we need to
evaporate 24hx60m/hx4.5(wo-wi) = 38.3 pounds or 4.8 gallons of water per
day to raise the humidity to 50%. andersen says 4 people add about 2 per
day, which leaves 2.8 gallons.

that seems like a lot of water and energy to humidify a 60 cfm airstream,
about 22k btu (6.6 kwh) per day, small compared to winter house heating 
energy, but avoidable in principle. what can we do? wear fuzzy mufflers
indoors to condense the moisture out of our exhaled air and evaporate it
back into the air we breathe in, as in a camel's nose?

as you say, it's difficult and expensive to seal a house better than 0.15
ach. if we could, we might use an air-air heat exchanger to condense the
moisture out of the "exhaled house air" while heating incoming fresh air.
the exchanger drain might run onto the basement floor or water 22 plants.

>the simple fact is there's no way inside rh can be maintained at a
>comfortable level simply by sealing a house. 

the simple fact is that inside rh can be maintained at a comfortable
level simply by sealing a house. 

>...a healthy house environment must have a constant source of fresh air.

an average vs constant source of 15 cfm per occupant, when the occupants
are present...

>a dryer running for 60 minutes is exhausting house air...

and a frugaler indoor clothesline adds humidity...

>it should be noted rh is not a factor  in sealing the house.

curiouser and curiouser...

>...i recall older houses exchanged the entire volume of air inside
>the house from one to three times per hour and there was no danger
>of a polluted house environment.

perhaps you recall oil costing 10 cents per gallon. 

>with the advent of the sealed house to reduce energy costs, the current
>recommendation is about one full air exchange every three hours...

about 1/0.15 = 6.7 hours, in our 0.15 ach example.

>the only way to maintain rh in a house during winter months in very
>cold temperatures is to mechanically add several gallons of water to
>the air inside the  house over a period of 24 hours.  

that isn't the only way, and it's only needed in very cold weather,
but it might be the most practical way for a typical us or canadian
house. scandinavian houses are more airtight, with rubber gaskets to
seal air leaks. do they need winter humidification?

it takes lots of energy (about 1000 btu/lb) to evaporate all that water.
where does it go? it sometimes condenses inside the wall insulation and
rots the studs on the way out, making houses collapse in a few years... 


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