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Community and Q&A

Venting dryer inside house

Matt Culik | Posted in Mechanicals on

I’m going to get absolutely roasted for asking this, but…

We’re moving into a new house and will be replacing the old electric hot water heater with a new heat pump unit (probably a Stiebel Eltron) in the basement.

Separate but related, it appears the laundry room does not have a vent connection for the dryer (there is a second laundry area upstairs that the current owner appears to use as primary).

Rather than drill a hole in the side of the house for the vent, would it make sense to vent the hot, humid dryer exhaust into the basement (obviously would need to address the lint issue, but stay with me…)? My thought is that this would provide “better” air for the heat pump, and the heat pump would cool and dehumidify the basement.

I know going in that this is probably a TERRIBLE idea, but figured I’d ask just in case… I’m guessing the air is way too wet to be properly dehumidified by the heat pump, and I’m asking for mold and general dampness problems.

Thoughts? Thanks.

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  1. Stephen Sheehy | | #1

    Get a heat pump dryer. No vent is needed. Moisture condenses and goes into the washer drain.
    There's no reason to assume that the moisture from the dryer will be offset by the hpwh.

  2. Steve Knapp CZ 3A Georgia | | #2

    I second Stephen's suggestion. I have a heat pump dryer (unvented), and it works quite well. Venting a conventional dryer to the inside is asking for trouble.

  3. GBA Editor
    Martin Holladay | | #3

    The lint problem is insoluable. And damp lint sticks to everything (with the potential of gumming up the HPWH compressor).

    There is also the problem that operation of these appliances is non-simultaneous. There is no guarantee that the heat pump compressor operation will coincide with clothes dryer operation.

    You should consider installing a heat-pump clothes dryer or a condensing clothes dryer (dryers that don't need to be vented to the exterior of your home). For more information on these options, see Alternatives to Clothes Dryers.

    For more information on heat-pump clothes dryers, see these two articles:
    Heat-Pump Clothes Dryers
    Energy-Saving Clothes Dryers Hit U.S. Markets.

  4. Charlie Sullivan | | #4

    Matt, I agree that a ventless heat pump dryer is the best solution. The LG model is designed to be vented; the Whirlpool one is not. The Whirlpool will put some heat into the room it's in though not nearly as much as a conventional dryer would if it were vented directly into the room. So you can feel good about that heat being later used by the water heater. The whirlpool doesn't put any humidity into the room. Overall that's good. The water heater might be slightly more efficient in high humidity but the other risks introduced by the high humidity aren't worth it.

    If you have a conventional dryer and for some reason you want to wait to get a heat pump dryer, you might be OK temporarily venting it inside, as long as you monitor the humidity and make sure you aren't putting it in any faster than the water heater is taking it out. That would be most likely to work OK if you don't use the dryer much and line dry most of your clothes outside. As Martin notes, the dryer operation is unlikely to coincide with the water heater operation, so there is no reason to try to duct the dryer output through the heat pump, especially considering the lint problems. But if you simple direct the heat into the space, the space can store some heat and humidity until the water heater later pulls that back out of the space. There are many products designed to catch the lint for dryers vented indoors.

    I do want to reiterate that venting a dryer indoors is generally a very bad idea, and I would only consider that if you do monitor the humidity.

  5. Matt Culik | | #5

    Thanks for the responses. Answers are what I expected. The biggest thing I learned is that heat pump dryers are available in the US now! Didn't realize that, and will be seriously considering that route.

  6. Expert Member
    Malcolm Taylor | | #6

    Why not just vent a conventional dryer to the outside?

  7. Brad H | | #7

    Martin, why do you say " The lint problem is insoluable" ?

  8. GBA Editor
    Martin Holladay | | #8

    Because a lint filter still allows some of the small particles of lint to get past the filter (which is why dryer exhaust ducts eventually get clogged or catch fire).

    These small particles don't matter too much if the exhaust is directed outdoors. If the exhaust is direct indoors, the small lint particles can cause problems.

  9. Brad H | | #9

    Lint doesn't seem like really small particles. It seems like putting a better filter in would solve that problem. In a dry climate the added humidity would be a benefit.

  10. Steve Knapp CZ 3A Georgia | | #10

    @Brad. I was curious if your statement on lint size was accurately. I couldn't find a document that directly addressed this issue, but a couple of sources suggested that cotton dust particles can be from 3 to 100 microns in size (that is, very small). One article noted that dryer filters are designed to catch large particles since most machines vent to the outside.

  11. Brad H | | #11

    Can you share those source links? I agree you'd probably need more than the typical dryer lint filter.

  12. Charlie Sullivan | | #12

    The cotton dust, which is mostly cellulose, is insoluble in all but a few exotic solvents, but if you react it with concentrated acetic acid, you make cellulose acetate, which is soluble in water. In the first half of the 20th century, cellulose acetate was used to make film for motion pictures. Perhaps you could use it to make a smart vapor retarder film, or if that fails, a smart film about vapor retarders.

  13. Andrew C | | #13

    Ok, I'll admit it, I snorted. Thanks.

    Back in the 70's, my dad put a deflector into the dryer exhaust vent pipe, so that exhaust could be directed outside during summer, and inside during the winter. Bad idea, shortly abandoned. Humidity in basement, plus that damp lint everywhere.

  14. Steve Knapp CZ 3A Georgia | | #14

    Brad. Here are the links:

    This document describes many types of particles. The closest one I could find that seemed relevant to the dryer discussion involved byproducts of cotton ginning:

    This Wikipedia article describes potential problems associated with lint particles:

    As I said, these sources don't address the original topic directly, but they do seem to suggest that lint can include very small particles. A Filtrete website lists a number of filters for capturing lint particles. ( The least efficient filter the company recommends is rated at MERV 8, for what that is worth.

    I guess my concern is that indoor air quality is already an issue in most homes, and introducing more pollutants is probably going to further degrade conditions.

  15. Brad H | | #15

    A $15 merv-8 filter sounds reasonable to me. Exhausting warm moist air in a cold dry climate just seems wrong. We dry most of our clothes on a drying rack inside, so an expensive heat pump dryer probably doesn't make sense.

  16. Steve Knapp CZ 3A Georgia | | #16

    Note that the Merv 8 filter only captures up to 85% of the large particles. To capture 95% of the smallest particles, you'll need Merv 16. But I understand what you are saying.

  17. GBA Editor
    Martin Holladay | | #17

    One more factor to consider: the better the filter, the higher the static pressure. If you install a high-performance filter, the clothes dryer fan may be so restricted by the filter that the air flow in cfm drops considerably, lengthening drying times and potentially violating the terms of the dryer warranty.

  18. Reid Baldwin | | #18

    How do condensing dryers and heat pump dryers that vent inside handle the lint issue?

  19. Stephen Sheehy | | #19

    Our Whirlpool heat pump dryer has two lint filters. It just circulates the warm air instead of venting it to the outside. It works very well.

  20. Steve Knapp CZ 3A Georgia | | #20

    The filter on our LG heat pump dryer generally needs to be cleaned after every use. The lint is usually slightly moist, which I'm sure helps with particle clumping.

  21. Brad H | | #21

    the hp dryers get the large particles in the lint filter, the small ones get stuck to the clothes. you take the clothes out and give them a shake and scatter those particles in the air.

  22. Brad H | | #22

    re pressure drop, agreed, the filter would need to be sized appropriately

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