Instant vs. Storage DHW Heating

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When choosing a condensing boiler you will run into one fundamental question: should the preparation of domestic hot water (DHW) be instant, or storage-based? Both principles have their merits - the decision depends on the size of the household, the number of points of use used simultaneously, and the level of comfort you expect from hot water.

How instant (combi) DHW heating works

With instant heating, the boiler does not heat water in advance into a reserve, but only at the moment you open the tap. Cold water passes through a plate heat exchanger directly in the boiler, the boiler automatically switches priority to DHW heating, increases burner output, and within a few seconds begins delivering heated water. The advantage is a theoretically unlimited amount of hot water - only the boiler's instantaneous output is a limit, not the volume of a reserve.

How storage heating works

Storage heating works on the principle of accumulation - the boiler continuously heats water in a separate tank (typically to 55-60 °C), and when drawn off, hot water simply flows out while the boiler reheats the reserve in the meantime. This means a large volume of hot water is instantly available regardless of the burner's current output. If you need to connect a boiler to a storage tank, see boilers with the option to connect a storage tank or ready-made boiler and storage tank sets.

Maximum flow rate - what it means in liters per minute

The key parameter of instant heating is its maximum DHW flow rate at a given temperature rise (Δt), commonly stated at Δt 30 K (heating from 10 °C to 40 °C). A typical shower uses approximately 6-10 l/min, a washbasin or sink somewhat less (4-6 l/min).

The "cold water sandwich" phenomenon

If you open and close the hot water tap twice in quick succession, a section of cooled water remains between the boiler and the outlet - when you reopen it, you will feel a brief cold burst before hot water flows again. This phenomenon does not occur with storage heating, since the tank always has a homogeneous reserve of heated water ready.

Guideline flow rate by number of bathrooms

1 bathroom (shower or bath) - a flow rate of approximately 10-12 l/min at a typical Δt is sufficient.
1 bathroom + kitchen (simultaneous draw-off) - a boiler with output closer to the upper end of the range (e.g. a 24-25 kW variant) is suitable, so that the temperature drop during simultaneous draw-off is not noticeable.
2 bathrooms used simultaneously - here instant heating generally is not enough without compromises; it is more sensible to consider a storage system or a boiler and storage tank set.

Space requirements and costs

An instant-heating boiler is a more space-efficient solution - the entire unit fits on a wall even in a small utility room or a flat. Storage heating requires space for the tank itself (from around 50 to 200+ liters). In terms of purchase price, instant heating is generally cheaper, while storage heating represents a higher initial investment that pays off in the form of greater comfort at higher consumption.

Summary: who each type suits

Instant heating is suitable if you live in a flat or a smaller house with one bathroom, points of use are practically never used simultaneously on a large scale in your home, and you prefer a compact solution with a lower purchase price.

Storage heating is more suitable if you have a house with several bathrooms, several people live in the household and hot water draw-off commonly overlaps, or you dislike the cold water sandwich effect and prefer an even water temperature every time you open the tap.

You can find the full range in the condensing boilers category, which includes models with both instant and storage DHW heating.

Frequently asked questions

Is it worth investing in a condensing boiler? Yes - by utilizing the heat from flue gases it typically saves 15-20% of fuel compared to a conventional boiler.

Which condensing boiler should I choose? The decision depends on output, the method of DHW preparation and the installation location - a detailed procedure can be found in the article How to choose a condensing boiler.