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The short answer
Only on an electric dryer, and only as a last resort. Never vent a gas dryer indoors: its exhaust can carry carbon monoxide. On an electric dryer, an indoor lint trap catches lint, but the water from each load, about a gallon for a typical one, stays in the room. The model code requires dryer exhaust to go outdoors unless the dryer is a listed ventless model, so use a trap only when no outdoor route exists and your dryer's installation instructions, local code and lease or HOA allow it. Vent outside if you possibly can.
Two designs, one shared limitation.
The common style
Upkeep: Dundas Jafine's use and care guide (PDF) says to empty the reservoir and refill it to the fill line after each use.
The other style
Upkeep: ADR's installation brochure (PDF) says to clean the screen every 3 to 5 loads. Its FAQ says to vacuum or replace the filter as lint loads it; a flap you calibrate at install opens if you wait too long.
The BetterVent is the filter-style kit in this comparison. Neither design removes water from the exhaust.
Both designs
No lint trap removes that humidity. The same guide advises against devices that remove lint and return dryer exhaust to the home, because they add moisture.
This is a carbon monoxide rule, not a preference.
Both kit makers on this page draw the same line. Dundas Jafine says its indoor kit should not be used with gas dryers, and ADR says never to vent a gas dryer anywhere but outside. The model code requires gas dryer exhaust to carry moisture and combustion products outside the building (IRC G2439.1). If your dryer has a gas line behind it and no working exterior vent, the machine should sit idle until the duct is fixed.
Code says one thing, your drywall says another.
Code
Check first: read your dryer's installation instructions. Speed Queen's (PDF, p. 10), for example, say the dryer must be exhausted to the outdoors. Then call your building department rather than guessing. In a condo the HOA gets a vote too.
Legal is the low bar
Repeated condensation is the moisture mold needs. Over a winter it can also show up as peeling paint and swollen trim.
Renters and interior closets, with eyes open.
Here is the honest use case.
In that case a kit is better than a disconnected foil hose blowing lint into the room. It does nothing about the water.
In a dry, heated home the added humidity may stay in range. Let the meter decide.
If the meter holds above 50 percent between loads, the top of EPA's ideal range, or a window starts sweating, stop using the kit.
Most "no vent possible" rooms have a vent option.
A room that seems unventable may only be awkward.
The room will tell you before the drywall does.
Any one of these means the room is absorbing what the duct should be carrying outside.
Then: stop using the kit and go back to the exterior and ventless options above.
For the rest of the cluster, head back to the dryer vents hub.
The safety and code questions people search most.
Never on a gas dryer. Its exhaust carries combustion products, which can include carbon monoxide, and both indoor-kit makers named here rule gas dryers out. On an electric dryer, the kits are sold for rooms where outdoor venting is not possible. Many dryer manuals still require outdoor exhaust, and PNNL's Department of Energy guidance advises against devices that return dryer exhaust to the home because they add moisture. Check your dryer's installation instructions first, and watch room humidity if you use one.
It depends on your jurisdiction, but the model code does not carve out indoor kits. The International Residential Code requires dryer exhaust to carry moisture outdoors and exempts only listed and labeled condensing (ductless) dryers. It also points to the dryer manufacturer's instructions, and many of those require outdoor exhaust. Local codes can amend the model text, so call your building department before installing one, and check your lease or condo rules.
They are built to catch lint, and that is all they do. The water, about a gallon for a typical load by PNNL's estimate, goes into the room as vapor. They work as a lint filter. They do not work as a substitute for moving humid air out of the house.
The water-trap style routes the exhaust across a reservoir so lint sticks to the water surface instead of drifting into the room. The water is the filter. Let it run low and the kit stops doing its job, which is why Dundas Jafine's instructions say to empty the reservoir and refill it to the fill line after each use. Filter-box designs such as the BetterVent use a polyester filter instead of water.
No. Gas dryer exhaust carries combustion products, which can include carbon monoxide, and an indoor kit would release them into your living space. The model code requires gas dryer exhaust to go outside the building, and both indoor-kit makers named here rule out gas dryers. If your gas dryer has no exterior duct, stop using it until one is installed.
They can, and this is the dealbreaker for many homes. Every load adds water to the room air. In a small laundry closet that shows up as fogged windows, damp drywall and a musty smell. PNNL advises against devices that return dryer exhaust to the home for this reason. A hygrometer on the shelf tells you whether the room stays within EPA's ideal range of 30 to 50 percent relative humidity. If it does not, the kit has to go.
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