Small 47 sq meter 2nd floor flat in London - 50 flat block Built 1998. All electric , no gas possible. 2 x 20 yr old original Creda storage heaters - lounge and bedroom - both have windows. The flat possibly wont be occupied 2-4 weeks in some months during the year. Because of this will probably be on standard tariff electric rather than economy 7. My worry is preventing water pipes freezing ( Kitchen, lounge and bedroom have an outside wall ) ; ventilation and protection from condensation / mould during these periods. The rest of the block, other flats / communal areas are very well heated 24/7 so we do benefit from this heat. We can leave windows slightly open ? Any advice please.