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Relative humidity in my house

  1. Nov 22, 2008 #1
    I am trying to get an idea of how much water vapour is in the air in my house, and how much water it would take to raise the humidity.

    Lets say that my house has a floor space of of about 170 sq meters with ceilings of 2.5 meters for a total volume of 425 cubic meters. Assume an air temperature of 20 C.

    So if my relative humidity is say 30%, how many litres of water would be in the air, and how many litres would it take to increase it to say 40%.

    The reason I am asking is I have an ongoing arguement with my wife about venting our electric dryer inside during the winter. I say it is crazy to blow all that expensive warm moist air outside, while my wife is against it My gut feeling is that the air will hold a lot of water, and the water from a load of laundry would be a "drop in the bucket".

    Now before everyone starts saying it will cause mold, let me explain how I would vent it. I would run a vent from the dryer and put it directly into the ductwork of the house. We have a 2 speed forced air natural gas furnace. The fan runs on low speed all winter, and when the thermostat calls for heat, the high speed fan kicks on. I already have a humidifier that adds water to the air, I will turn this off.


  2. jcsd
  3. Nov 23, 2008 #2


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    The main problem you'll run into with that approach is the dryer lint, but otherwise the idea is sound. Here's a psychrometric chart that tells you the properties of humid air: http://isites.harvard.edu/fs/docs/icb.topic482325.files/Psychometric Chart.pdf

    Dry bulb (regular) air temperature is in the scale at the bottom. Relative humidity is in the curves. Trace up from 20 C until you get to 30%, then go from that point straight to the right to the edge where it gives you humidity ratio.

    At 30% RH, air has an absolute humidity of 4.5 grams of water per kg of air.
    At 40% RH, air has an absolute humidity of 6 grams of water per kg air.

    Now the chart also tells you the density of air - at those conditions you have about .835 cubic meters per kg of air. So...

    Raising the humidity from 30-40% requries (6-4.5)*425/.835=763 grams or .763 liters.

    Now I don't know how much water is in a load of laundry after it's been through the spin cycle, but I suspect at least a liter or two. So doing this would have a significant impact on your house's humidity.

    But, the pressure of gases in a mixture all act independently of each other. That means that if there is more humidity (vapor pressure of water) inside your house than out, the moisture will force its way out of your house. Fast enough that you'll need to replenish it once or twice a day, depending on how "tight" your house is. So you may end up with a comfortable humidity level on laundry day, but that's about it. Still, if it takes a couple of bucks or Euros (or whatever) of money to do your laundry, you might as well dump the residual heat/humidity back into your house. It's just that the savings probably won't cover the cost of the materials.

    This is something I actually put a decent amount of effort into a few years ago... I did a project on it in college.
  4. Nov 23, 2008 #3
    Thanks for the reply.

    I am surprised it would take such a small amount of air to raise the humidity. My understanding is that the humidifiers that you mount on the side of your furnace do very little to raise humidity. I crank mine to the highest setting and it runs all winter and the house is always dry.

    As for the dryer, it probably runs 1 hour a day, at 4500 Watts x .12 cents a Kw= .54 cents per day. I could probably use the heat 180 days of the year so that is $97.20 dollars a year "out the window". I know it would actually be worse than that, because the air expelled by the dryer has to be replaced so it will come in through cracks in the house, and all that air will have to be heated. I don't know how much that would be.


  5. Nov 23, 2008 #4


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    How old is your house, how good are the windows (double pane?), and what is the outside temperature in your area in the winter? It sounds to me like the house is just not very tight.

    Regarding the humidifier on your furnace - you may want to check to make sure it is working properly. They do generally have a significant impact. Does it have its own fan or does it rely on your furnace's fan?
  6. Nov 23, 2008 #5
    Be sure to find some way to control lint... My guess is 1 hour of dryer humidity daily won't make much of a difference but might take care of things for six hours (just a guess, also depends on how big an area you are venting into).
    Because Consumer Reports doesn't even mention dryer vents as a source, my guess is BE CAREFUL as it likely is not practical....see below. Is there now a good way to do avoid lint ? Aroma is one thing... I can smell when my dryer is venting outside.

    I live in NJ and temperatures in the twenties to outside usually means modest humidity inside the house...say in the 30 to 32%. Above 32 degrees outside, especially when it rains, humidity inside is a bit higher. It seems like something in the upper thirties is about right but you have to be CAREFUL you don't get the air TOO humid because condensation can form in poorly or thinly insulated areas. That means the potential for moisture in walls, maybe mold mildew. Furnace humidifiers, as you likely know, recommend LOWER humidities with lower temperatures to avoid condensation. An outside temperature sensor is recommended by Consumer Reports.

    We gave up on furnace humidifiers because we had three different types over the years and they all caused some leakage in and around the furnace...We now use a large room humidifier now about DEC thruough Jan and although it uses some electricity and makes some noise in that area, its easy to control, see, clean,etc. I think my home of about 2500 square feet uses roughly a gallon of water per 24 hour day and keeps the humidity elevated to about 35 % a bit lower than we'd like but good enough to reduce static. I believe if you have static electricity in your house it's an indicatior you could use a bit more humidity.

    We now have a natural gas cast iron fireplace. Although it's vented to the outside, I sometimes open the glass door...heat and some humidity is added. I now wonder if an unvented type would have been preferable...higher efficiency and it would add some water vapor as a byproduct. a non vented unit could be an electric free source of heat and humidity. At the other end of the house a wood burning fireplace insert adds lot of heat but does dry things out...no humidity is available...a big pot of water does evaporate but doesn't make a dent in overall humidity even while its on.

    Consumer Reports says:
    Also some humidistats aren't accurate or reliable. And most portable humidifiers won't let you set relative humidity levels below 30 percent. When outside temperatures drop below 20° F, even an indoor humidity level of 30 percent can lead to condensation on windows, doors or other cold surfaces. Be sure to lower the humidity level as outdoor temperatures drop.
    Consumer Reports recommends you clean portable humidifiers regularly to prevent mold growth, which can become an airborne allergen. Mold can grow in as little as 48 hours on wet surfaces. If you're not willing to make the effort to refill and clean your humidifiers, consider a permanently installed system (you still need to clean and maintain such a setup) or don't humidify--mold can pose much more of a health problem than low humidity.

    Operating costs. In-duct systems and other evaporative models deliver the most energy efficiency. You can easily spend $350 per year to run four tabletop models compared with about $30 for a single in-duct model.
  7. Nov 23, 2008 #6


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    My wife and I have a much smaller house, but humidity gets quite high when we run the heat from the dryer into the house. We even get fogging on the windows. The lynt does get everywhere, and we are working on improved filtration.

    However, on days when we run the dryer, we usually don't have the furnace kick on at all. And this is Michigan winter we're talkin' about here!
  8. Nov 24, 2008 #7
    I checked with my wife and she reminded me when we run our large floor humidifier 24 hours in cold dry weather we likely use three or more gallons daily....if you get fogging on single pane windows I'm not aso sure it's a real problem except maybe on the windows; but if double pane windows are fogging likely the humidity is too high for that cold a day...

    For filtration: you need to be careful not the block the vent air as overheating of the dryer can occur. I would think a sort of hollow box frame which the dryer vent feeds into, lined with air conditioner filtration material (say, from Home Cheapo) would capture most of the lint....
    Did you find that "dryer vent damp clothes smell" objectionable??

    If you know the wattage of your clothes dryer and the amount of time it runs you can easily calculate the value of the heat saved. That plus the cost saving of NOT running a humidifier will give you a feel for overall savings.
    Say the clothes dryer uses 1200 watts for an hour (I don't know actual figures). thats 1.2 KW; if your cost for electric is 18 cents then you save 1.2 x 18 or about 22 cents...if run daily, in a month that's say 30 x 22 or about $6.60/month for the dryer savings...(I assume nearly all the dryer power is used for heat, only a small portion for the motor. Cut my figure by 10% if you like to account for the motor.)
    Last edited: Nov 24, 2008
  9. Nov 24, 2008 #8
    Could you perhaps install a flap of some sort to regulate the amount that gets vented outside relative to the amount that gets vented into your house? Then you could adjust that incrementally and monitor how your humidity is affected. It would also be nice in the summer when you might not want any of that air in your house.

    If you want to reclaim some of the heat to cut heating costs, you could use a countercurrent heat exchanger instead. This would eliminate the issue of lint
  10. Nov 24, 2008 #9
    Thats a great idea. You call them HRV's and we have one. Unfortunately, it is not close to the dryer, but it is a great idea for new houses. They could all be vented through the HRV and get free heat.

    Our house is in the Toronto area. It is fairly tight with double pane windows.

    As for dryer useage, in my second post I did the math, 4500 watts x 1 hr day x .12 cents kh/hr x 180 days = $97.20 for the winter

    Unfortunately, this is a project I will never get to do. My wife just won't hear of it. Some battles are not worth fighting
  11. Nov 25, 2008 #10
    Well whch is it; if the latter, don't "battle" just do it....ALL woman get bossy after a while and want things THEIR way...you have rights as well...exercise them!!!
  12. Nov 26, 2008 #11
    Don't do it! Instead, contact someone who has already done it who can reassure your wife.
  13. Oct 26, 2010 #12
    I suppose you've considered saving a whole lot of energy by hanging your laundry in the furnace room; you'll get a more even distribution of moisture and save loads of £££! We notice much fabric dust from our weekly ironing stint, so I'd be less than happy blowing even more round the house.
  14. Dec 3, 2011 #13
    Here is my Rub Goldberg System

    I discharge my dryer a few inches above a bucket full of water. The lint then sticks in the water and air deflects off the top. So I don't have a lint issue. I then blow the air out of the laundry room with a fan. The problem is the air in the laundry room is high in humidity, so the dryer is pulling in moist air and working twice as hard to dry the load.

    note: don't submerge the end of the dryer vent in the bucket. This would put unnecessary pressure on the motor.

    note: don't do if you have a gas dryer (obvious but still needed to say it)

    I burn wood for heat, like the idea of free dryer heat and need humidity but find some rooms of my house have too much humidity.
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