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Thread: Clothes Dryer Makeup Air - A Better Way?

  1. #1
    Plumber in Previous Life sixlashes's Avatar
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    Lightbulb Clothes Dryer Makeup Air - A Better Way?

    I am building an extremely tight ICF house in NW Florida. I have not had the blower door test performed yet, but I know I will well be under .1 ACH infiltration. I used ICF wall construction, icynene foam insulation, and high attention to detail. I also am installing a 180 cfm energy recovery ventilator to precondition incoming fresh air. I say this to illustrate how important energy efficiency is to me.

    Here is my conundrum. Running a clothes dryer blows approx 100 cfm of air out of the house if installed within the building envelope. My concern is paying for the makeup air coming in at 90+ degrees and 55%+ humidity. But my wife absolutely does not want the washer and/or dryer in the garage.

    The first question which I cannot find the answer to on any of the energy conservation websites is: I understand the clothes dryer is more efficient utilizing 75 degree 50% humidity incoming air than 90+ degree, 55%+ humid incoming air. Is the increased efficiency worth keeping the clothes dryer inside the house envelope and pulling in all of that sensible and latent heat?

    My hair brained idea is to create a "closet" with a 32" door beside the washer and put the dryer in it. The door to this closet opens away from the washer to stand conveniently against the exterior wall when open. When the dryer is running, the door is closed and a electrically operated damper opens on an 8" very short duct to the soffit outside. That way, the dryer circulates air from the outside while being conveniently located inside the house.

    Am I trying to push water uphill with a broom, or will this be an energy saver?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    One thing you didn't say is whether it is a gas or electric dryer. The ERV will recover some, but not all of the energy used to condition the internal air. It might be more efficient to pull in external air directly into the dryer. Note, you'd need a screen on it to prevent critters from roaming in which will decrease the effective size of the duct, so size appropriately.

    From a comfort standpoint, that room won't feel all that nice from all of the air moving through it - washing clothes and running the dryer will increase the humidity, and mold and mildew will be a potential problem, but if the door is open the majority of the time, it may not end up being a factor.

    WIthout running numbers, it might be a tossup whether getting the air from erv or a dedicated duct might be more efficient when you consider trying to cool that room off after you're done. See what others have to say...
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Plumber in Previous Life sixlashes's Avatar
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    I don't see that the ERV would have any impact in this situation. It works on a balanced air transfer system. When it runs, 190 cfm blows in - 190 cfm blows out. Since the dryer is mechanically pushing the air out, not the ERV, even if the air came in the ERV inlet, there would not be any outflow to absorb any heat or humidity from the incoming air.

    My plan is to locate only the electric dryer within the sealed closet with the 8" round air intake. My goal is to not bring so much makeup air into the house. My heat pump will deal with the heat and humidity given off from the washer. I also realize there will be a blast of hot, humid outside air when you open the door to the dryer closet. That will only be 50 cubic feet or so, since the damper would immediately close off the outside air when the door opens. I am trying to mimic my direct-vent gas water heater by creating a somewhat closed loop that does not utilize any insde air.

    I guess my fundamental question is: Will running the clothes dryer with the reduced efficiency from the hot, humid air cost less than air conditioning and dehumidifying all of that incoming air if fed with inside air? Maybe my geothermal heat pump (21.0 EER) would use less energy to condition 100 cfm of infiltration than the increased energy the dryer would use to dry a load of clothes with hot, humid air.

    I know this is an unusual question, but I believe folks have dealt the concept before. I am sure the issue of whether the dryer should be in the garage or the house from the energy usage standpoint has been dealt with. My twist is to create a micro-garage environment with the closet in the house. I have four teenagers / near teens and anticipate a lot of dryer usage. The sun bleaches out anything you hang out here in Fla.

    I appreciate everyone's input.
    Last edited by sixlashes; 08-07-2008 at 02:02 PM. Reason: Incomplete

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I was under the impression that some were able to help compensate for pressure imbalences...wouldn't be the first time I was wrong.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Like an engineer alternety's Avatar
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    There is one ERV I am aware of that does pressure balancing. I don't remember manufacturer. Search for ERV not HRV. It is not the best way. Unbalance ruins the transfer efficiency.

    What I have done is to install a cold air duct to supply makeup air directly over the dryer. The washer/dryer are in a small room with a door (I call it the laundry roomhttp://www.terrylove.com/forums/images/smilies/biggrin.gif). It should probably be larger than the dryer exhaust pipe. Buy an electrically operated vent damper designed to go into a duct. They come as a short piece of duct, motor, and damper. Get a good one that has good seals. Some are very crappy. Get a device designed to turn things on when current is used and plug the dryer into that. Or if you have the skills, whatever electronic approach you desire. This approach is OK for gas or electric as long as it operates properly. A gas dryer must always have a combustion air source.

    If you have any combustion in the house that is not sealed and directly ducted to an external air source, you must not allow a negative pressure in the house. It can reverse the flow of the combustion products with a possible subsequent issuing of death certificates.

    Consumer CO detectors are not sufficient protection.

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    Master Plumber nhmaster's Avatar
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    Try to weigh the aggravation and expense against the dollars saved and see where you stand. Often you can spend a whole lot of money and time to solve a problem that really isn't one in the first place. Though sometimes the research can be fun.

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    Like an engineer alternety's Avatar
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    Tight house and dryer IS a problem. Opening a window is the only zero cost alternative as a means of supplying air and it can make the house uncomfortable. Air leakage in a modern well designed structure will simply not supply dryer air, or range hood air, combustion air, or healthy breathable air. If the dryer is to work and not overheat, there has to be an air supply. Second cheapest, duct with manual damper. But there has to be something.

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    Plumber in Previous Life sixlashes's Avatar
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    alternety,

    Wow, someone else who has gone down this strange and twisted road!

    I do still have a question though. It seems you have installed this setup with the goal of ensuring adequate makeup air. While undeniably a great plan, you could have fulfilled this requirement by ducting in makeup air anywhere in the house as long as there is an open path for the air to get to the dryer.

    I am having a hard time getting my question across. My laundry room in 10' X 18'. What is more economical:

    1. Running a clothes dryer for a longer period utilizing 95 degree 80% Relative Humidity air?

    or

    2. Running the same dryer for a shorter period utilizing 75 degree 50% RH air while also using the air conditioning to cool and dehumidify the air?

    There must be an objective way to figure this out. I am just not smart enough to tackle it. I always have more questions than answers...

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    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, the 95-degree air has a lot more actual moisture in it than the 75-degree air, a lot more than the 30% since the hotter air can hold a bunch more moisture than the cooler air. That's why they call it relative humidity - it is based on how much the air can hold, not an absolute amount.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Master Plumber nhmaster's Avatar
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    If the house is that tight then you're going to need an air x air exchanger anyway.

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    Master Plumber nhmaster's Avatar
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    Here's an alternative though, cut a 4" round hole in the side of the dryer near the bottom and put it's own air intake from the outside in. Then it won't effect your house at all.

  12. #12

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    Mechanical autodamper under, next to, close to the dryer???

    Keep it simple.

  13. #13
    Forum Admin, Expert Plumber Terry's Avatar
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    the dryer circulates air from the outside while being conveniently located inside the house.
    Be very careful that the return air is far away from the exhaust.
    If not, the return air will pull the moist warm air from the exhaust and you well be cycling wet air through your dryer.

    I found this out the hard way when kids or some female type would open the laundry room window. The clothes would never dry until I went downstairs and closed the darn window.

  14. #14
    Like an engineer alternety's Avatar
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    If doing a duct through the attic, prepare to explain to the inspector why there is an (maybe) untaped, uninsulated duct up there. Aparantly they see less of this than they should.

    And of course the outside opening needs to have appropriate covering for critters and bugs. Remember that bug screening significantly restricts air flow. That is part of the reason why the mack-up pipe should be bigger than the exhaust.

    Through all of this you need to know the total air resistance your dryer can overcome. I believe that ought to be treated as the sum of make-up and discharge. When you start making elbows you allowable pipe length goes down quickly.

    Having gone through this recently, at tip - keep the laundry room near an outside wall. It can save so much grief (and metal work). Mine is in almost the center of the house.

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    Plumber in Previous Life sixlashes's Avatar
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    Thanks everyone for your posts.

    I have settled on doing this with an 8" makeup air duct coming in through the soffit just above the dryer. Since the dryer is on the outside wall, the duct length is minimal. As per Terry's suggestion, I am offsetting the exhaust 5' laterally from the intake.

    I will put the dryer in a 3' square closet to minimize hot air coming into the rest of the laundry room. I hope to utilize a mechanical damper that opens when the closet door is closed. If not, I'll use an electrically operated one. Either way, I want one with an end-of-travel switch hooked up in series with the dryer door-open switch. That way, the dryer will not operate unless the fresh air damper is open.

    My goal of not heating & humidifying the house when the clothes dryer is running will be accomplished.

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