Hot water storage versus thermal storage ultimately comes down to one question: how long does the heat need to be preserved? Water is the standard choice for bridging a day. Want to bridge an entire season? Then you run into a physical limit that no amount of pipe pressure or insulation solves.
Why water is the standard choice
Water has one of the highest specific heat capacities of all commonly used materials: it takes relatively a lot of energy to heat it, so it can also release relatively a lot of heat back. It’s also cheap, safe, widely available, and fully established in installation practice. For a boiler or a small day buffer, water is therefore almost always the right choice: cheap, simple, and more than sufficient for bridging a few hours to a day without complicated installation.
Where water falls short
The problem lies in temperature and volume. Water can’t be boiled above 100°C at atmospheric pressure, and in practice a residential water buffer stays well below that to avoid pressure buildup and evaporation. That limited temperature range, combined with a relatively low energy density per cubic metre at those temperatures, means that storing a whole season’s worth of heat requires a buffer that simply becomes too large to place practically in or next to a home. Our thermal storage product page explains why we chose a different storage medium for that reason.
What does a salt buffer do differently?
Caldum works with two storage media, each with a temperature range well above water’s roughly 90°C: a salt mixture up to 225°C (Caldum 1, for homes) and an ore up to 450°C (Caldum 2, for larger buildings and district heating). At those higher temperatures, the material holds much more heat per kilogram, making the buffer four to six times more compact than a water buffer with the same storage capacity.
Why that difference matters so much in practice
Between a few hours and an entire season lies a completely different order of magnitude, not just a matter of somewhat more storage. A day buffer for hot water fits in a cupboard. That same amount of heat, but stored for the whole heating season, would require a volume that simply doesn’t fit in most homes — a basement or garden isn’t enough. By working at higher temperatures with a denser material, a seasonal buffer stays manageable in size. That’s also why seasonal storage long remained mostly a theoretical concept, until salt mixtures and ores actually solved that volume problem.
So when is hot water storage still the right choice?
Honestly: often. For daily buffering, low temperatures, or small installations, water is still the simplest and cheapest option, and Caldum isn’t trying to replace that either. In fact, a small water buffer for hot tap water usually remains present in most installations, even alongside a large seasonal buffer. The choice only becomes relevant once the question shifts from “how do I bridge a few hours” to “how do I bridge an entire season” — and how much space is practically available for that. How much buffer you exactly need depends on your heat demand; we calculate that in our article on sizing a thermal buffer correctly.
Who does this matter to?
This weighs heaviest for anyone at the start of a choice between storage techniques: installers specifying a system, homeowners orienting themselves, and building managers looking to replace or supplement an existing water buffer. The longer the desired storage period and the more limited the available space, the sooner the physical limits of water tip the decision toward a different storage medium. For a day buffer, water remains the logical starting point; only with seasonal storage does that balance shift.