Hello, I've got plumbers in to quote for installation of an upgraded electric hot water system. I have very low hot water pressure and the system is probably 20+ years old. It's an economy 7 immersion heater system. I've been told by one plumber I can get an unvented cylinder installed, with the tundish running into a plastic soil stack. I've been told by another plumber that an unvented cannot discharge into a plastic soil stack and therefore I need a thermal store to upgrade my system and increase the hot water pressure. I cannot discharge outside as this is a flat roof and I cannot get gas installed. Any help? I want to get this sorted but don't know where to turn and I don't have an quotes back for a thermal store so no idea of installation cost.