Calculating DHW needs for a multi-unit building: a simple, reliable method for 2026

Defining your DHW calculation scope in a multi-unit building (before reaching for the calculator)

Clarifying uses: showers, baths, kitchens and ancillary needs

Before pricing, set the framework. Count the hot water for showers and baths, kitchens, and ancillary needs (cleaning cupboard, laundry room, common areas). Also note excluded items, for example a laundry room connected to a separate meter. A well-defined scope avoids oversizing the production system.

Assessing occupancy: number of dwellings, unit types and occupancy rate

In a multi-unit building, the number of dwellings isn't enough on its own. Record the unit types (studios, two-bedroom, families), vacancy periods, and the presence of non-residential premises. A realistic occupancy rate gives a reliable basis for your DHW volumes.

Setting the assumptions: cold water temperature, DHW setpoint and draw-off profiles

Lock in assumptions common to the whole building. Cold water temperature according to season and region. DHW setpoint suited to hygiene and network losses. Finally, choose draw-off profiles (morning and evening peaks) to size the flow rate and the usable storage volume.

Choosing a calculation method suited to the multi-unit building

Consumption-per-dwelling approach: quick for feasibility studies

For a first DHW estimate, start from an average consumption per dwelling, adjusted for actual occupancy and uses (showers, baths, small shops). This method gives a reliable order of magnitude for comparing scenarios (plant room renovation, heat pump, solar) and checking that the available energy holds up, without spending days on calculations.

Draw-off profile approach: finer-grained for sizing production and storage

When it's time to size the system, daily volume alone isn't enough. Draw-off profiles describe peaks (morning, evening) and troughs. By working on an hourly profile, you size the production capacity, the tank volume, recirculation and control, with less oversizing and less discomfort.

Relying on existing data: history, metering, plant room readings

The best basis remains your real data: bills, BMS history, hot water meters, energy meters, run hours, flow/return temperatures, recirculation loop readings. These elements also help spot drift (leaks, an overheated recirculation loop) before finalising the calculation. To go further on network-related losses, see overheated recirculation loop.

Carrying out the DHW needs calculation: steps, formulas and points of attention

Calculating the energy to heat the water: volume, ΔT and conversion to kWh

Start by estimating the daily DHW volume (L/day). Fix the incoming cold water temperature and the setpoint. The core of the calculation is simple. Useful energy = V (L) x 1.163 x ΔT. You get Wh. Divide by 1,000 to convert to kWh. Remember to distinguish weekdays, weekends and specific uses.

Factoring in losses: recirculation, storage, distribution and pipe insulation

Add the losses that don't serve the shower but heat the building: active recirculation loop, network length, unheated rooms, a poorly insulated tank. Pipe insulation on pipework and fittings significantly reduces these losses. Record diameters, lengths and circulation hours. For storage, account for the tank's static losses as stated by the manufacturer.

Moving from need to capacity: daily peak, simultaneity and margin

Convert energy into capacity by looking at the peak period. Capacity (kW) = energy over the peak (kWh) / duration (h). Apply a simultaneity coefficient based on the number of draw-off points, then keep a margin for inlet temperature variations and usage drift. Without this step, the generator is either overstretched or oversized.

Translating the DHW calculation into equipment sizing for the multi-unit building

Determining the storage volume: tank, stratification and recharge time

From the peak DHW needs, convert the useful litres into tank volume. Storage must cover the peak without a drop in comfort, while remaining "rechargeable" during operation. A well-stratified tank delivers more genuinely usable litres. Take care with the connections, limit velocities, and check the target recharge time (often 1 to 2 hours) to avoid oversizing.

Adjusting production: boiler, heat pump, solar thermal and hybrid solutions

Translate the energy to be supplied into capacity, factoring in local cold water, the setpoint and losses. A boiler can handle short peaks. A heat pump requires looking at capacity at low outdoor temperature and the temperature rise. For solar, size the coverage primarily on the consumption "baseline". Hybrid systems secure the peaks with a controllable backup.

Checking the DHW recirculation loop: flow rate, balancing, temperature and legionella risk

The recirculation loop guarantees the temperature on each floor. Calculate the recirculation flow rate to offset losses, then balance each riser. Check the flow temperature, the return, and the wait time at draw-off. Keep temperatures consistent with legionella prevention, avoid dead legs, and plan measurable monitoring (probes, metering, adjustments) — see also the DHW sanitary safety devices.

Proving and securing your DHW calculation in 2026: documents, checks and subsidies

Supporting documents to prepare: energy audit, calculation notes, plans and diagrams

For a solid file, gather from the outset the energy audit (if required), your DHW calculation note (needs, occupancy profiles, setpoint temperatures), and the assumptions used. Add equipment data sheets, plus the hydraulic plans and diagrams of the network (recirculation loop, pipe insulation, draw-off points).

• Floor plans and location of risers and plant rooms.
• A dated, versioned schematic diagram, with sections and lengths.
• Pro forma invoices or detailed quotes consistent with the quantities.

Field checks: temperature measurements, flow rates, settings and metering

On site, secure your figures with simple, traceable measurements. Record temperatures (generator outlet, recirculation return, representative points), check recirculation flow rates and balancing, then log the settings (schedules, anti-legionella, setpoints). Reliable metering (hot water, energy) helps objectify discrepancies.

Link with MaPrimeRénov' Copropriété and CEE: consistency of assumptions and traceability

In 2026, the key issue is consistency between the DHW calculation, the sizing, and the subsidy paperwork. For MaPrimeRénov' Copropriété, rely on the audit and keep clear traceability. For CEE, align your assumptions with the targeted standardised operation and keep certificates, detailed invoices and proof of commissioning, with a complete file archived. To better frame the requirements and points of attention, also see our guide to understanding energy savings certificates.