We have a Mira mixer shower which worked with a condensing boiler and also had a pump fitted due to low water pressure. We had new Worcester Bosch 30i combi boiler fitted last week. Since then the shower comes on at normal pressure for 90 seconds then pressure drastically reduces - if we turn up heat pressure returns but after 90 seconds drops to practically nothing again. Installer saying must be shower fault but it was working fine before new boiler installed. Any ideas please?