Vaillant's aroTHERM is an air source heat pump that uses 75% of renewable energy to run, making it sustainable which saves you money on your energy bills. The Vaillant Glycol Discharge Vessel is a necessary component which collects glycol from this heating system in the possible event of the opening of the PRV. This accessory ensures that glycol, most commonly known as antifreeze, isn't discharged into the aroTHERMs drainage system.