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  1. #1
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    Default Heat recovery from clothes dryer

    I'm planning a laundry room for my house and have wondered if there is any way to capture some of the heat that is put out by the clothes dryer. It seems like a lot of heat going to waste, and a pretty simple thing to save some of it. Maybe like a double-wall exhaust pipe, with a fan that blows air between the inner and outer pipe?

    What is the theory on this, and why aren't more people doing anything like this (am I just a nutcase?) Is there a device on the market that accomplishes this?

    The flip side of this is, how much heat is lost through the dryer exhaust pipe when the dryer is not in use, and is there a way to minimize this?

  2. #2
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    Default Re: Heat recovery from clothes dryer

    HD Rider,
    1. Why aren't more people doing it? Because the amount of lost heat is so low that it does not justify the investment in extra equipment (special vent pipe) or energy (additional fans) required to harvest it.

    2. If you use a fan to cool the exhaust, you will encourage condensation of moisture in the exhaust gases. You have to provide a way to drain the liquid water without freeze-up problems in cold weather.

    3. There are devices on the market to do what you suggest; for example:
    www.indoorlinttrapfilter.com
    The short version: bad idea. (a) No good for gas dryers -- dangerous exhaust fumes shouldn't be vented indoors. (b) A bad idea for electric dryers too, since adding extra moisture inside the house is risky.

  3. #3
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    Default Re: Heat recovery from clothes dryer

    Quote Originally Posted by Martin Holladay View Post
    HD Rider,
    1. Why aren't more people doing it? Because the amount of lost heat is so low that it does not justify the investment in extra equipment (special vent pipe) or energy (additional fans) required to harvest it.

    2. If you use a fan to cool the exhaust, you will encourage condensation of moisture in the exhaust gases. You have to provide a way to drain the liquid water without freeze-up problems in cold weather.

    3. There are devices on the market to do what you suggest; for example:
    www.indoorlinttrapfilter.com
    The short version: bad idea. (a) No good for gas dryers -- dangerous exhaust fumes shouldn't be vented indoors. (b) A bad idea for electric dryers too, since adding extra moisture inside the house is risky.
    I'm NOT suggesting venting the dryer to indoors, rather capturing just the heat. Now, if the Heat Recovery Ventilator concept is to try to save the heat from the indoor air that is vented outside (air that is roughly 70 degrees), why would it not pay to save the heat from the clothes dryer air, which is a much higher temperature? And as far as the condensation goes, as I understand it, the same issue exists in high-efficiency sealed-combustion furnaces, and that is easily dealt with by adding a drain.

    Finally, when the winter wind is blowing against the dryer vent, you certainly must have a significant amount of air infiltration back through that exhaust pipe. Or not?

  4. #4
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    Default Re: Heat recovery from clothes dryer

    HD Rider,
    I never said you couldn't do it. I just said it probably wouldn't be worth the investment in extra equipment (special vent pipe) or electricity to run an additional fan.
    Go ahead. You can build one. The simplest way would just be to extend a long zig-zag run of 6-inch galvanized pipe all around your house, within the conditioned envelope, to cool the exhaust gases. Oh, yes -- you need to catch the water and run it to the drain. But you know that. I'm sure you can rig it up. Be sure it's easy to disassemble to clean the lint twice a year. Remember, those joints have to be both watertight and easy to take apart and reassemble.
    Or, you can rig up a double-wall pipe as you originally suggested. Again, make it both watertight and easy to dissassemble. Slope it to a drain; hook it up to a plumbing trap. Put in a fan, and calculate how much electricity it will take to run the fan; then calculate whether the cost of the electricity is more than or less than the value of the thermal heat recovered. Put in a control to interlock the operation of the fan with the operation of the dryer. Include several good backdraft dampers. Let's see-- are you done yet? Send me a picture when you've got it built.

  5. #5
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    Default Re: Heat recovery from clothes dryer

    Before I get into a little snit over the need to interlock the operation of the dryer with the circulating fan, I'd like to do some math.

    Does anyone know a quick way to find out how much gas is consumed to dry a load of laundry, and perhaps the amount of heated air that is removed from the house in the process? That would help me decide whether to pursue the idea or drop it.

    And what about cold air blowing in through the dryer? Certainly I can't be the only one who's noticed it.
    Last edited by hdrider_chgo; 11-27-2007 at 10:34 AM.

  6. #6
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    Default Re: Heat recovery from clothes dryer

    HD Rider,
    One clothes dryer manufacturer (Siemens) lists the BTU input for one model of gas dryer at 18,500 BTUs:
    http://www.siemens-home.com/Applianc...-Pedestal#NULL
    Remember:
    1. The gas burner is not on for the entire time that the clothes drying cycle is operational.
    2. No heat-recovery device will recover 100% of the heat.

    Finally, before you go into the business of building heat-recovery devices for clothes dryers, have your lawyer talk to the lawyer of this patent holder:
    http://www.patentstorm.us/patents/72...scription.html

  7. #7
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    Default Re: Heat recovery from clothes dryer

    Martin,

    I'm not sure how you convert the BTU output that you gave to the amount of gas actually used to dry a load of clothes, or how much heated indoor air is exhausted to the outdoors in the process.

    I have not been able to get your patent link to work. Although I am curious about what you are pointing to, I am not interested in going into business on this. I doubt whether something as simple as what I would envision would be patentable anyway.

    I have no idea whether it's feasible to capture the dryer heat. I simply thought it was a logical question, to which I am still looking for answers.

  8. #8
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    Default Re: Heat recovery from clothes dryer

    HD Rider,
    The Forum software seems to clip long Web addresses. If you are trying to copy the Web address for the patent, some characters may be missing. On my screen, my post has been altered -- the Web address is shown with "..." in the middle. The missing characters between "72" and "scription.html" are "13349-de". Or you can just try clicking on the address to see if that works.
    Hope that helps.

  9. #9
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    Default Re: Heat recovery from clothes dryer

    Quote Originally Posted by hdrider_chgo View Post
    And what about cold air blowing in through the dryer? Certainly I can't be the only one who's noticed it.
    A customer of mine completed a major remodel 2 summers ago, the washer & dryer ended up in an interior closet. Dryer duct ran down into the basement and had a longish run of about 25ft in an enclosed joist bay (finished basement) and out. The duct shared the bay with plumbing. They had 3 seperate burst pipe incidents until I re-routed the dryer vent to a seperate joist bay and insulated it. So yeah, the duct is nice conduit for not just cold but frigid air.

    The flappers on the exterior vent are worthless. I would isolate the ducting, insulate it and use foam to seal around any openings in the walls or flooring the duct passes through. In my own house, as long as the dryer door is kept closed there's almost zero draft. But if clothes are left in the dryer overnight in the winter, watch out, nothing wakes you up quicker than putting on chilled underwear.

    -Norm

  10. #10
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    Default Re: Heat recovery from clothes dryer

    This is slightly off topic, but relevant; dryer vents have come out of the IMC, so we won't be seeing them anymore. I posted a question about this on the ICC Forum and never got an answer to my basic question, which had to do with whether it was coming out of the UMC since California isn't going tot he IMC but remaining on the UMC. BTW, there is a mistake in my question, we ICC members receive the free Building Safety Bulletin, which is not the Building Safety Journal (I thought they were the same), the November issue of Building Safety Bulletin states that come January, the publications are being combined so we will be receiving the Building Safety Journal which is online. I can't link you to the Building Safety Bulletin article, but here is my question. If anyone is really interested I'll scan the article and upload it (if I haven't thrown it away).
    “It is not an endlessly expanding list of rights —the “right” to an education; the “right” to health care; the “right” to food and housing. That is not freedom. That is dependency. Those are not rights. Those are the rations of slavery – hay and a barn for human cattle.” - Alexis de Tocqueville

  11. #11
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    Default Re: Heat recovery from clothes dryer

    Dick:
    See Section 504 Clothes Dryer Exhaust. 504.6 Domestic clothes dryer ducts. Section 504.6.2 is quite specific about providing an exhaust duct if a dryer space is provided. Section 504.1 includes an exception: This section shall not apply to listed and labeled condensing (ductless) clothes dryers.

  12. #12
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    Default Re: Heat recovery from clothes dryer

    HD rider,
    not only are you sending a lot of heat out the vent pipe but you are drawing in a lot of unheated air in the winter. A dryer blower that runs at 125 CFM will take 7500 cubic feet of heated air out of your house for every hour it runs. To heat the make up air 60 degrees you need about 8100 BTU's or .06 gallons of oil on an 85% efficient unit. If you can use an HRV or something to recover 45 % of the 18500 BTU's you will recover 8325 BTU's Hope this helps

    Lou

  13. #13
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    Default Re: Heat recovery from clothes dryer

    Most of the folks I know who have electric dryers run them into the inside of the house in the winter. You add some humidity but for most folks with a gas furnace you need some humidity added to the house. Solves you having a humidifier added to your furnace.

    Not sure why it seems to be so difficult to consider. Martin, you seem to be in kind of a snit about the whole idea. I have seen everything from an old panty hose over the end of the outlet hose which helps catch lint and can be changed easily to systems that you buy and install with a filter already in them. About the only problem anyone ever mentioned to me was if they did all their laundry on the same day. If they did 10 loads then it was an overload of humid air in the house. But if they switched to doing one or two loads a day it was not a problem. Plus lots of the new washers really wring out the clothes so mine does not make the dryer run as long so not as much humid air.

    My contraption cost something like 10 bucks, took 15 minutes to install, and I am getting free heat and humidity out of my dryer. If I need to do lots of laundry and do not want all the heat and humidity in the house I flip a little door and it shoots outside again.

  14. #14
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    Feb 2007
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    Default Re: Heat recovery from clothes dryer

    Quote Originally Posted by m beezo View Post
    My contraption cost something like 10 bucks, took 15 minutes to install, and I am getting free heat and humidity out of my dryer.
    I have the little plastic box with a flap for summer and winter, probably same one you have. Outlet hole to basement has a screen on it for lint, it still exhausts some air out the regular outdoor vent, just reduces hole for flow from 4" to maybe 1-2". My dehumidifier is right next to the box and it really doesnt run much, winter is so dry anyways.

    I had the same thought as OP, why heat the outside? I dont know if it really does anything but for $10 and a "maybe", what the hell.

    Travis
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt. ~Bertrand Russell

    wausaubuilder.com

  15. #15
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    Default Re: Heat recovery from clothes dryer

    Roger:

    You wouldn't still have the October issue of Building Safety Bulletin would you? I've apparently thrown mine away, it made the case that builders could use the exception to leave the ducts out, I would surmise that the appliance industry pushed for that to force people to buy new dryers, but that's just speculation. Notice the paucity of response I got on the ICC Bulletin Board?
    “It is not an endlessly expanding list of rights —the “right” to an education; the “right” to health care; the “right” to food and housing. That is not freedom. That is dependency. Those are not rights. Those are the rations of slavery – hay and a barn for human cattle.” - Alexis de Tocqueville

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