What is the key characteristic of Type 1 clothes dryers?

Type 1 clothes dryers are specifically designed to vent to the outside. This characteristic is crucial because venting helps remove moisture and heat from the drying process, allowing for more efficient drying of clothes. By expelling the hot, damp air outside, these dryers prevent the accumulation of humidity within the home, which could lead to issues like mold growth and increased energy consumption due to additional air conditioning requirements.

In contrast, the other options do not accurately describe Type 1 dryers. While some dryers may be water-efficient or use alternative energy sources, these features are not defining characteristics of Type 1 clothes dryers. Additionally, the notion of portability is not typical for this type, as they are generally designed for permanent installation purposes with proper venting systems in place. Thus, the ability to vent to the outside is the standout feature that distinguishes Type 1 dryers from other types.