The Legionella and district heating articles mention tank and flow-through — here we size the hot water supply itself. We look at the balance between tank size and capacity, simultaneity and peak load for domestic hot water, charge circuit and flow-through water heater, and how Legionella and district heating considerations play a role.
Introduction
Hot water can be produced in two basic ways: you can store it heated in a tank, or you can heat it on-demand as it is drawn. The choice is a balance between output and storage volume — and it is a separate design topic at the same level as floor heating and expansion.
Peak load and simultaneity
Hot water consumption comes in short, powerful peaks: morning shower, washing up, multiple taps at the same time. You do not design according to the sum of all taps, because they are rarely used at once — you design according to the simultaneous peak load. DS 439:2024 gives the method for determining the design hot water flow from the assumed water flows of the fittings and a simultaneity consideration, in the same way as for cold water (see »DS 439:2024 — the new standard for water installations«).
Flow-through — high efficiency, no storage
A flow-through water heater (electric, gas or a district heating exchanger) heats the water the moment it flows through. There is no storage — so the entire output must be delivered in the same second the tap is on. It requires large instantaneous power.
The advantage of flow-through systems is that there is no standing hot water waiting — it is hygienically favourable against legionella, because the water volume is small.
Tank — low power, large storage
A hot water tank reverses the calculation: it stores a quantity of heated water, so a high draw-off peak can be covered by the tank while the actual heating (charging power) can be much lower. A tank with low input power can thus cover a high peak — as long as the tank is large enough and the reheating time matches how often the peaks come.
- 01Large tank + low output: suitable where output is limited (e.g. a heat pump or modest district heating connection), but requires space and holds more standing hot water.
- 02Small tank/flow-through + large power: compact and hygienic, but requires sufficient power to be available.
- 03The vessel's performance is often described with a peak capacity or NL figure (performance/nominal value) from the manufacturer — it tells how large a draw-off point the vessel can handle, and is used directly in sizing.
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