Stratification is the natural layering of water in a storage tank by temperature: hot water, being less dense, sits at the top while cooler water stays at the bottom. A well-stratified tank delivers usable hot water from the top while feeding its coolest bottom water to the collector or heat pump, raising whole-system efficiency by 10–20% compared to a fully mixed tank.
Why stratification matters
A storage tank is not just a bucket of warm water — it is a temperature ladder. Every consumer and generator connected to it prefers a different rung:
- Taps and heating circuits draw from the hot top.
- Solar collectors work most efficiently when fed the coldest possible water, because collector efficiency falls as inlet temperature rises.
- Heat pumps achieve their best COP charging the cooler middle zone rather than fighting an already-hot tank.
Destroy the layering — through high-velocity inlets or badly placed connections — and everything degrades at once: the collector runs hotter and wastes yield, the heat pump condenses against a higher temperature, and the tap runs lukewarm even though the tank holds plenty of energy.
How tank design protects stratification
- Tall, slim geometry. Height-to-diameter ratios of 2.5:1 or more give layers room to form.
- Diffusers / baffle plates at inlets slow incoming water so it slides into the layer matching its temperature instead of churning the tank.
- Correct port heights. Solar coils sit low, boiler or heat pump coils mid-height, DHW draw-off at the very top.
- Internal coil heat exchangers transfer heat without physically mixing volumes.
- Thermosiphon-style charging lances in advanced stores guide charged water upward to its density-matched level.
Measuring it: why two tanks with equal volume perform differently
Standards for solar stores (EN 12977-3) test how well a tank builds and holds layers. In practice, a stratified 300 L tank can deliver more usable 45 °C water than a mixed 400 L tank — the reason serious solar and heat pump systems specify stratification-optimised stores rather than the cheapest vessel of the right volume.
Frequently asked questions
Does stratification survive when the circulation pump runs?
Yes, if flow velocities at the ports are kept low by design (diffusers, generous port diameters). Layering is destroyed by jet-like inlets, not by circulation as such.
Is a stratified tank relevant for heat pumps, or only solar?
Both. Feeding the heat pump condenser from a cooler layer lowers condensing temperature and lifts COP; solar circuits gain even more because collector efficiency is highly sensitive to inlet temperature.