DHW storage volume: sizing it precisely

Clarifying DHW usage on site to start on the right footing

Identifying the occupancy profile: occupants, water outlets, simultaneous draw-offs

Before sizing a tank or a combined system, establish the occupancy profile. How many occupants day to day. How many water outlets. And above all, which draw-offs can happen at the same time (two showers, shower plus sink). This point drives both the storage volume and the recovery speed needed for consistent hot water.

Accounting for habits: showers, baths, life cycles (weekday/weekend, rental)

Habits make the difference. Quick shower or bath, fixed or staggered schedules, remote work, children. Identify draw-off peaks rather than the average. Also think about life cycles: weekday versus weekend, second home, rental with turnover. This avoids equipment that's too big and heats for nothing, or too tight and runs cold at the wrong moment.

Checking site constraints: available space, access, drainage, electrical supply

On site, check maintenance access and logistics. Footprint, doorway clearance, positioning near water points, possible drainage. For a heat pump water heater, anticipate available air, condensate handling, and noise. On the electrical side, check the subscribed power, protections, and the value of off-peak hour control.

Calculating DHW volume without getting it wrong: a simple method and field benchmarks

Estimating daily hot water needs: litres per person and per use

For DHW used at 40°C, keep a simple benchmark. Count 40 to 60 L per person per day, then adjust based on habits and peaks (showers back to back, baths, kitchen use).

• Shower. 35 to 60 L
• Bath. 120 to 150 L
• Washbasin. 2 to 5 L
• Hand-washing dishes. 10 to 20 L

Translating the need into tank volume: usable storage vs advertised volume (don't confuse the two)

A "200 L" tank doesn't guarantee 200 L of water at 40°C. What matters is the usable volume, the amount actually available at the tap. If production is slow, for example with a heat pump, generally allow 20 to 30% margin to get through peaks without lukewarm water.

Factoring in storage temperature and the mixing valve: direct impact on volume actually available

Storage temperature changes everything. With storage at 55 or 60°C and a mixing valve at 40°C, the tank "produces more litres" thanks to the blend. In practice, V40 = Vtank x (Thot - Tcold) / (40 - Tcold). With cold water at 10°C, 200 L at 60°C gives roughly 330 L at 40°C. For more on the health aspects linked to storage temperature, see anti-Legionella devices.

Matching DHW sizing to the generator: heat pump, boiler, solar and backup

Heat pump and DHW: recovery time, available power and the risk of an undersized tank

With a heat pump, the DHW tank is sized mainly on recovery time. Usable power drops in cold weather and if the set point is high. A tank that's too small gives lukewarm showers or forces frequent recovery cycles. Aim for a volume consistent with actual usage, and check the anti-Legionella mode.

Boiler and tank: comfort, cycles, DHW priority and settings to plan for

With a boiler, comfort depends on volume, but also on cycles. An oversized tank limits restarts, provided you set the DHW priority and the charging flow rate. Plan for a suitable set point to avoid scalding and limescale, and check the timing delays.

Solar thermal: volume, stratification and backup to secure hot water

In solar thermal systems, the tank is the link between production and demand. Stratification is key, hence the value of a well-connected vertical tank. Without sun, the backup (electric, boiler or heat pump) secures hot water. Size it to cover peaks without undermining the solar gains.

Avoiding oversizing: losses, costs and non-compliance to anticipate in 2026

Understanding losses: heat loss, recirculation loop, unheated space and tank insulation

An oversized tank means more hot surface and therefore more heat loss. Add a recirculation loop running continuously and pipework in an unheated space, and you're heating the garage as much as the DHW. Priority: insulate the tank and pipework, reduce lengths, and control the recirculation loop (timer, interlock, on-demand operation).

Limiting hidden costs: footprint, maintenance, replacement, wasted consumption

Oversizing costs you in floor space, accessories (valves, circulators, insulation) and hidden maintenance costs. A large volume can also speed up scaling and increase standby consumption. The result: a higher bill, and a more expensive replacement when the tank fails.

Anticipating 2026 requirements: consistency with the energy audit, supporting documents and RGE best practices

In 2026, incentive schemes and inspections require simple proof. Sizing consistent with the energy audit, occupancy assumptions, technical data sheet, photos of the installation and insulation, and a commissioning report. On the RGE side, keep clean traceability. A tank judged oversized can end up flagged, or lead to a rejected application: hence the value of knowing how to avoid MaPrimeRénov' application rejections.

Validating the DHW volume with the client: checklist and concrete cases

Validation checklist: comfort, recovery time, settings, anti-scalding safety

Before finalising the tank, validate actual usage with the client. Number of showers or baths, peak times, simultaneous draw-offs, and expected comfort level. Get them to specify the acceptable recovery time, especially with a heat pump or heat pump water heater. On settings, clarify the set point, absence mode, and the presence of a thermostatic mixing valve to secure the DHW.

• Consumption profile. Morning, evening, weekend.
• Recovery time. Comfort or economy.
• Anti-scalding. Limiting temperature at the draw-off point.

Concrete sizing examples: 1-2 bed / 2-3 bed flat, family, house with two bathrooms

Rough orders of magnitude. A 1-2 or 2-3 bed flat with 1 to 2 people, 120 to 150 L is often enough if showers are spread out. A family of 4, aim for 200 L instead, especially if showers run back to back in the evening. A house with two bathrooms, anticipate parallel draw-offs and frequently move up to 270 or 300 L, or a solution with better recovery power.

Points to check at handover: temperature, mixing valve, anti-Legionella setting, draw-off tests

At handover, measure the temperature at the outlet and at the draw-off point. Check the mixing valve, its installation direction and its setting. Confirm activation of the anti-Legionella cycle per the manual, and run draw-off tests. A long shower, two points open, checking comfort and recovery time.