A calorifier doesn’t heat water directly — it stores it, and lets a separate heat source (a boiler, heat pump or both) do the actual heating through an internal coil. That indirect setup is what gives you a buffer of hot water on demand rather than waiting for it to heat up each time.
Dual coil versions are worth considering for anyone part-way through moving off fossil fuel heating: you can run a heat pump and a boiler into the same tank, so the newer, lower-carbon source does most of the work with the boiler as backup rather than needing two separate hot water systems.
Sizing comes down to peak demand and how quickly the tank needs to recover between uses, which we work out from your actual usage rather than a standard formula. We install calorifiers as part of a full heating system or as a standalone hot water upgrade.
