You would expect storage heaters to be wired on a separate circuit, same as a cooker or immersion heater will almost always be on there own circuit. As such if a house already has storage heaters it won't need to be rewired to use Economy 7. If a separate meter with a separate timed supply is installed the relevant circuits can just be recounted at the consumer unit (fuse-box of old). I'll remove the comment to that effect as it's misleading.