The wallheater is based on infrared heat. This is the heat you feel in the sun and UV is the burn you get in the sun. Infrared heat is invisible light with lots of energy. When the “light” hits an object, heat is created.
When we are near a warm source like a fire, we feel a flood of infrared light as warmth. When we walk into a hot room such as a greenhouse, heat in the warm air is transferred to our bodies, again perceived as warmth. But we don’t directly perceive ultraviolet radiation. It slams into our bodies and sometimes breaks molecules, but very little of it is converted into heat. The heat we feel from sunlight is infrared. So infrared feels hot because we sense it as heat, and because when we feel its heat, there is a lot of radiation, which makes up for its lower energy content. We don’t feel ultraviolet radiation, which is usually less abundant, at least until we get sunburn.