Which type of heating system is referred to as 'wet' systems?

The term "wet" systems in heating typically refers to those systems that utilize water as the primary medium for heat transfer. In this context, hot water heating systems fit the definition perfectly, as they involve the circulation of heated water through pipes or radiators to distribute warmth throughout a space.

This process relies on the principles of hydronics—the use of water for heating. By heating water in a boiler and circulating it through a closed-loop system, these systems efficiently deliver heat to various areas within a building. Unlike air heating systems, which use air to carry heat, or electric heating systems, which convert electrical energy directly into heat, hot water heating systems capitalize on the thermal properties of water, which can efficiently transfer and retain heat.

Radiant heating systems can also be considered "wet" when they rely on water running through pipes embedded in floors or walls to radiate heat. However, they are a subset of hot water heating systems. So, the most straightforward classification of a wet heating system is hot water heating, directly affirming the rationale for choosing that answer.