Hi,Been to a customer's house who requires their old open vented pumped ch with indirect dhw cylinder converting to a sealed combi system,The old system can independantly heat dhw but ch can only be heated whilst dhw is being heated,looked in the airing cupboard to see if there was any port valves,there was none to be seen,am i right in assuming that there must be a valve under the floorboards somewhere that shuts of ch when only dhw heating is required?