To answer this commonly asked question we need to make it clear there are two main types of heated towel rails; dry electric and liquid filled, and they heat differently.
Dry Electric Element Heated Towel Rail
A Thermorail dry element electric heated towel rail will generally heat up to around 35 degrees and heats up a lot quicker than a hydronic rail. When a towel is placed over the bars the towel insulates the rail and absorbs the heat, meaning that beneath the towel the temperature will rise to between 50 – 55 degrees. This will vary depending on the ambient temperature of the room and how folded your towels are. It is important to note a folded towel will absorb more warmth than a towel just draped over a rail. The reason for the difference in temperature between the covered and uncovered bars of the rail are that and electric dry element rail is heating the air in the tubes and when the towel is placed over the rail it insulates the tube causing it to heat up. This means the exposed parts of the rail are not as hot to the touch and makes it safer in the bathroom.
Liquid Filled Heated Towel Rail
A liquid filled rail heats the entire rail to 55 degrees and takes longer to reach temperature but will help with adding warmth to the bathroom. As the element is heating the water and not the air, covering the rail with a towel will not cause it to get any hotter. A rail connected to a hydronic heating system will heat up to a similar temperature to a liquid filled rails depending on how the hydronic system is set and used. Both Liquid Filled and Hydronic Rails are designed to not only heat your towels but also add some warmth to the room.
For more information visit https://www.thermogroup.com.au/heated-towel-rails/more-about-heated-towel-rails/