Yes, heated towel rails can be used as the primary heating source in a bathroom if they have enough heat output. To ensure adequate warmth and comfort, calculate the BTU requirements for your bathroom, considering factors like room size, insulation, and desired temperature. Choose a heated towel rail with a heat output that meets or exceeds these requirements. However, for additional heating flexibility and efficiency, you may still consider using them in conjunction with other heating sources, such as underfloor heating or a separate radiator.