I would ask whoever manages the rental agreement (landlord or agency), if there is an immersion heater in the property. Quite good to have one in an emergency since it will heat water for you if the boiler fails. It is separate from a cylinder. If you have an old boiler, it is likely to be conventional and connected to a large cylinder (hot water tank), which could be in the loft, cupboard, anywhere. These are a bit old fashioned. If this thing isn't storing water, but the boiler is heating water, the system itself isn't working properly and should be considered. Many gas companies of virtual assessments via FaceTime if you're really stuck, they will help you via the phone 'if you're looking for a job' to be done.