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re: pacific coastal dehumidifier
19 oct 2005
wrote:
>thanks for your comments, nick!
you are welcome.
>> >the weather on the pacific coast from northern california to british
>> >columbia tends to be warm and dryish in the summer but cool and damp
>> >in the winter...
>>
>> maybe not. nrel says seattle has these average temps and humidity ratios:
>>
>> jan feb mar apr may jun jul aug sep oct nov dec
>>
>> 40.1 43.5 45.6 49.2 55.1 60.9 65.2 65.5 60.6 52.8 45.3 50.5 f
>>
>> .0042 .0045 .0046 .0051 .0061 .0072 .0082 .0085 .0078 .0065 .0051 .0044
>>
>> the humidity ratio w is the number of pounds of water vapor per pound
>> of dry air. it does not depend on the air temperature, and it doesn't
>> change much in 24 hours. the relative humidity is the number of pounds
>> of water vapor per pound of air divided by the maximum number of pounds
>> of water vapor the air can hold (at 100% rh) at a certain temperature.
>
>i'm not sure how the absolute figures given above relate to the
>experience that everyone who lives here has. wouldn't rh be more
>pertinent?
no. that varies with temperature.
>i also doubt the accuracy of the figures. ave. temp. of 50.5 f in december?
>nonsense, or a typo.
oops. a typo. shoulda been 40.5, with 35.8 and 45.1 average daily min and max.
i rechecked the rest of the numbers, which look ok.
>...in november, on the other hand, it seems to rain constantly; your lawn
>will be a quagmire. water tables rise, and the moisture evaporates through
>your basement walls into your living space.
maybe that's where your water vapor is coming from. a 1930s house with
no vapor barrier under the basement floor?
>i'd suggest that the moisture figures above reflect a) the fact that
>warmer air is able to carry more water vapour and perhaps b) some sort
>of filtering out of the effect of rain.
dunno about rain. warmer air can carry more water vapour.
>airtight houses in canada tend to be where the climate is severe. the
>ideal place is saskatchewan, which has hot summers and severe winters
>with lots of sunny days. airtight, passive solar, summer-shading
>overhangs ... all work out well in saskatchewan. there they have a
>"continental climate" which would not likely need dehumidification in
>winter.
airtight houses need dehumidification in wintertime because they contain
humidity sources, people breathing and showering and washing floors and
cooking and so on, as well as damp basements. a perfectly airtight house
would let the indoor rh rise until condensation happens on the indoor
surface of windows.
>...our house, is a leaky house built in the 1930s. this thread is
>the result of an energuide energy audit, which resulted in the consultant
>telling us that we should make the house more airtight, but that before
>we did that, we had to deal with the humidity issue.
sounds like you have a major indoor humidity source.
>> >dehumidifiers produce heat, therefore their energy efficiency is
>> >important in continental applications. however, on the left coast,
>> >they are used mostly in the winter, so the heat they produce is
>> >mostly a slightly more expensive form of something you're going to
>> >do anyway: heat the air in the house.
>>
>> it's cheaper than electric resistance heat, with a cop of about 1.6.
>> you can measure this with a kill-a-watt meter and a measuring cup.
>
>that's reassuring. we're also looking forward to the subjective
>feeling of warmth in dry air at a temperature where we would feel cold
>in damp air.
that's backwards :-)
>> >any suggestions?
>>
>> turn on a small exhaust fan with a humidistat when the indoor rh rises to
>> 60%. in january, w = 0.0042 makes pa = 29.921/(1+0.62198/w) = 0.201 "hg.
>> indoor air at 60% rh and absolute temp t (r) has pi = 0.6e^(17.863-9621/t),
>> approximately, and pa = pi makes t = 507.5 r or 507.5-460 = 47.5 f, so
>> you can dehumidify the house with an exhaust fan as long as the indoor
>> temp is at least 47.5 f. if you want to save more energy, take advantage
>> of weather fluctuations and hygroscopic house materials and do this less
>> often, only when the outdoor air is warmer and drier than average (during
>> the day) in wintertime and cooler and drier (at night) in summertime.
>
>summertime is never an issue here. i'm afraid that the most
>hygroscopic house materials are the books.
also concrete, wood, paper, fabric, and so on.
>don't want to store moisture in our books.
the rh might range from 30 to 60% with no damage.
>so it looks like you also prefer exhaust fans to dehumidifiers. my concern
>is that the warm damp air gets replaced by cold damp air from the basement
>or outside. i'd really like to give dehumidification a chance before making
>another hole in the wall.
cold air tends to be drier than warm air. you have my numbers, which you
seem to have ignored. it isn't hard to make a hole in a window. you may
already have an exhaust fan in a kitchen or bathroom.
>by "humidity ratio", do you mean "relative humidity"?
no. i already explained the difference. you seem to have ignored that too :-)
nick
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