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Is there a “direct vent” clothes dryer or kitchen hood?

Andrew Homoly | Posted in Green Building Techniques on

It seems to me we focus on creating super tight envelopes and then cut holes in it to purposely blow out an enormous amount of air with clothers dryers and kitchen hoods. We either create negative pressure in the home or bring in unconditioned air with make up dampers. HRV’s and ERV’s now solve that issue with bath fans, but isn’t there anything out there for dryers and hoods? My thought is there should be some kind of venting system like the direct vent on the back of a fireplace where the incoming air gets conditioned by the outgoing air. Does anyone know of such devices or has anyone jury rigged up a solution?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Andrew,
    For the clothes dryer, you can crack a window open in the laundry room -- or choose a condensing dryer if that bugs you.

    There are solutions for range hoods, but they aren't simple or cheap.

    Here is more information for you:

    Alternatives to Clothes Dryers

    Makeup Air for Range Hoods

  2. David Meiland | | #2

    Seems to me that cracking a window is a good idea, but you're really not saving much because the dryer has to heat cold outside air as opposed to using warmer interior air. Either the dryer does the air heating or the furnace does it, take your pick. The idea of using heat from the attic is a very good one and it'd be nice if systems for this were available (i.e. a dryer with an intake connection and maybe some logic to tell it where to get its air).

  3. Gale Mack | | #3

    Home Builder/Designer since 2006 - A clothes dryer draws close to 200 cf/min of conditioned air from your structure. In a 1500 sq ft structure and the dryer running for just one hour, you have drawn all the conditioned air from you structure. Martin is right. Crack a window AND shut the door to laundry room. I have the parts to make a direct vent dryer system to try it out. Parts cost $200., but I figure I can make that up quick in the heating/cooling savings. Also, it depends on where you live. I read in Arizona, people are screwing a sheet metal intake plenum over the intake air vents on the back of dryer and running a 5" duct up through the ceiling. It terminates 1.5' from the top of roof peak. Run dryer only during the day, drawing the very hot air out of the attic. Heating element never turns on AND the system does not mess with their A/C in the structure! Ingenious huh?

    G. Mack - 5 star+ energy home builder Michigan ([email protected])
    My motto is: FINDING LOW TECH SOLUTIONS FOR LIVING IN A HIGH TECH WORLD!

  4. John Klingel | | #4

    The volumetric heat capacity of air is 0.0182 btu/cf·F. Calculate your heat loss based on your hours/yr of dryer use, delta T, etc, and see if any solution is economical. I roughed a calculation for a 35 degree delta T and one hour/day of dryer time and got 16 gallons of heating fuel used per year. At $3/gal, that is not real cheap, but your data may give different results. Question: If you have an HRV, wouldn't the dryer be causing air to flow through the HRV, reducing the heat loss by the efficiency of the HRV? If so, figure that in, too.

  5. Jeff Nelson | | #5

    John Klingel said, "If you have an HRV, wouldn't the dryer be causing air to flow through the HRV, reducing the heat loss by the efficiency of the HRV?" Would that also keep the HRV running for a longer cycle, thereby negating the effects of negative pressure in the house? I'm still learning about HRVs, so if this question is completely ignorant, please correct and pardon me.

  6. David Meiland | | #6

    I don't think a HRV factors into clothes drying. Heated air is being exhausted directly outdoors by the dryer. Make-up air is probably being pulled in through the HRV ductwork, but it's not a balanced flow, it's just incoming air, and no heat from the dryer is being recovered.

    Came across this a while back, a few flaws but worth watching http://www.kickstarter.com/projects/1729220401/intelligent-dryer-energy-recovery-unit

  7. Jeff Nelson | | #7

    David --

    I was thinking more for the makeup air. But yes, you mentioned the "balanced" flow. I'm thinking just in terms of makeup, not really thinking about the H part - the heat - in HRV. I have this exact situation coming up and I was actually thinking of separating the laundry room (in the corner of the basement) from the rest of the house within the thermal envelope and/or air barrier. Instead of insisting on keeping a window cracked, I could have a vent from outside to the laundry room and gasket the laundry room door to keep the cold air in there. Of course, I'm just kicking around ideas at this point - I just realized that this is going to be a problem for me.

  8. David Meiland | | #8

    Jeff, here's the question I have (and don't have an answer to). Scenario 1: the dryer is inside the heated envelope, supplies itself with indoor air, then heats it further to dry the clothes. Scenario 2: the dryer is in unheated space, uses outdoor air, and heats it as needed to dry the clothes. Which scenario uses more energy? Either way you need air at a certain temperature in the dryer. Getting it from outside presumably means the dryer runs longer.

    I suspect the indoor air supply is more efficient IF the house is heated with a heat pump system that's performing well. I suspect the indoor air supply is cheaper IF the house is heated with natural gas and the dryer is electric.

    Suspicions is all I got. Anyone else?

  9. John Klingel | | #9

    Interesting problem that many of us face. Regarding " Make-up air is probably being pulled in through the HRV ductwork, but it's not a balanced flow, it's just incoming air, and no heat from the dryer is being recovered." The incoming air is at least being warmed a little through the HRV, though, is it not? (Likely not worth getting excited about.) I've thought of having a somewhat sealed laundry room and having the dryer exhaust through a pipe w/in a pipe, allowing incoming air to enter in the outer pipe, in a crude HRV manner. Any solution needs to be simple and cheap unless you run your dryer hours/day.

  10. Keith Gustafson | | #10

    I doubt the outside air would make much of a difference in energy usage. I would think the outside air in the winter would be dryer in addition to colder. I am thinking that for the majority of the drying cycle the clothes are not 'hot' but the air blowing by them can hold more moisture because it is warmer. That last 10 percent of drying they start to get hot.

    I think in a former posting there were several ideas about how to accomplish intake air cheaply

  11. User avatar
    Michael Chandler | | #11

    We use a 6" Cape back draft damper located through the wall to out doors directly behind the dryer. the incoming air blows on the warm metal of the dryer so it doesn't seem to be so much of a cold draft.
    It does make the laundry room warm and humid if the clients run the dryer a lot in the summer. in some of our homes this serves as make-up air for the kitchen hood as well. Hopefully they will use the clothes line when the weather is good.

    Cape Back draft damper $21.50 http://www.rewci.com/noname6.html

  12. David Meiland | | #12

    Keith, I don't have equipment to data-log 240V loads, so I don't actually know how my dryer runs (or any, for that matter), i.e. are the heating elements on for most of the cycle, do they turn on and off during the cycle, or what? It would certainly make a difference. All I know is the thing has a 30A breaker and from that I assume it has a ~4000 watt heating element. That's a lot of electric heat. I think Martin has stated before that recovering dryer heat doesn't have much upside, which may be why there don't seem to be commercial setups for doing so.

  13. John Klingel | | #13

    Michael: Those look like they may do well. Do you know if they ice up in colder zones, 7 & 8?

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