I think I know the basic principles, but wondered if there's more to it - pros and connies. The main 'idea' of them is that is acts as a large heat store which can then be used of DHW and CH. It's big pro is that is can be heated from numerous sources - solid fuel, PVs, solar panels etc. as well as a boiler. DHW is obtained from a heating coil inside the tank - cold mains water is passed through this coil and is heated 'instantly' as required? CH supply is taken directly from the tank - the tank essentially stores a large quantity of 'system' water? The tank is kept up to required temp - summer and winter - by the main boiler? In Summer only DHW is required, so there's still the need to keep a whole tank heated? Would you only ever consider one if you wanted more than one source of heating - eg PV panels powering an immersion, or a solar panel directly heating the stored water? And just how effective are they if supplied by PVs for example? I mean, if the PVs managed to get the store fully up to max temp during the day, just how much CH would that provide for the house in the evening? Cheers.