The French EPC calculation method does not derive hot water demand from the number of occupants. It works through a number of equivalent adults derived from the habitable floor area, then applies 56 litres at 40 °C per equivalent adult per day for conventional behaviour and 79 litres for high-consumption behaviour. Above 70 m², the occupancy coefficient is 0.025 times the floor area, which makes the demand saturate: a 140 m² house is credited with 2.275 equivalent adults, that is 127 litres a day at 40 °C. That figure exists to calculate a consumption, not to size a cylinder, and confusing the two is the most common error on this item.
What the assessment method takes as demand
Conventional demand is derived from floor area
The method first defines a maximum occupancy coefficient, equal to 1 below 30 m², to 1.75 − 0.01875 × (70 − area) between 30 and 70 m², and to 0.025 × area above that. The number of equivalent adults follows: it equals the occupancy coefficient while that stays below 1.75, and then becomes 1.75 + 0.3 × (coefficient − 1.75). The progression is therefore deliberately damped on large floor areas.
The resulting daily demand
Values calculated for a detached house, in litres of water at 40 °C per day.
| Habitable floor area | Occupancy coefficient | Equivalent adults | Conventional behaviour | High consumption |
|---|---|---|---|---|
| 40 m² | 1.188 | 1.188 | 66 L | 94 L |
| 60 m² | 1.563 | 1.563 | 88 L | 123 L |
| 80 m² | 2.000 | 1.825 | 102 L | 144 L |
| 100 m² | 2.500 | 1.975 | 111 L | 156 L |
| 120 m² | 3.000 | 2.125 | 119 L | 168 L |
| 140 m² | 3.500 | 2.275 | 127 L | 180 L |
The gap between the last two columns is 41 %, which gives the order of magnitude of the uncertainty the method itself accepts on this item.
What that demand is not
It is a daily average, spread over a year in which seven days are conventionally unoccupied. It is not a peak, nor a flow rate, nor a simultaneity. No cylinder is sized on that figure alone, and an assessor using it to validate a volume is working outside the method's intended use.
From demand to cylinder volume
Mixed water volume at 40 °C is not nominal volume
A cylinder advertised at 200 litres does not deliver 200 litres at 40 °C. Since the store runs hotter than the temperature in use, the mixing valve reconstitutes a volume larger than the nominal one by blending with cold water. The heat balance gives mixed volume equal to nominal volume times the ratio of the two temperature differences, with the cold water temperature being a monthly climate figure in the assessment method. That is a calculation identity, not a regulatory value: the figure that counts is the one the manufacturer declares.
The floors ecodesign sets
Point 1.3 of Annex II to Regulation (EU) No 814/2013 has imposed, since 26 September 2015, a minimum volume of mixed water at 40 °C according to the declared load profile.
| Declared profile | M | L | XL | XXL | 3XL | 4XL |
|---|---|---|---|---|---|---|
| Minimum mixed water at 40 °C | 65 L | 130 L | 210 L | 300 L | 520 L | 1,040 L |
These are a regulatory floor, not a sizing target. Their main use is comparing two units of the same profile, by reading the declared figure rather than the nominal volume.
The upper bounds on small profiles
Point 1.2 of the same annex caps the storage volume of the small profiles the other way: 7 litres at most in profile 3XS, 15 litres in XXS and XS, 36 litres in S. Those bounds concern isolated draw-off points, a sink or a hand basin, not a dwelling's main cylinder. The corresponding test cycles are covered in the article on DHW load profiles.
Sizing on the peak, not on the average
The peak is recorded, not deduced
The sizing parameter is the volume drawn during the peak, not the daily consumption. What gets noted during the survey comes down to four lines, recorded on the phone as the technician works through the visit.
- Number of draws following one another with no recovery interval, morning and evening.
- Simultaneous draws possible given the number of wet rooms.
- Presence of a bath and how often it is actually used.
- Acceptable delay for recovery between two peaks.
Recovery power is part of the equation
Two cylinders of the same volume are not equivalent if one recovers in two hours and the other in eight. The pair to record is volume and recovery time, taken from the unit's documentation across the room's temperature range. A generous volume compensates for slow recovery, and the reverse holds too; it is that trade-off which gets documented on the estimate.
The heat pump case
On a heat pump water heater, the available power varies with the temperature of the air drawn in. In an unheated room and in cold weather, recovery lengthens and the electric backup takes over, which cancels part of the benefit. The volume is then set to cover two closely spaced peaks without a restart, and the location is validated before the capacity is chosen.
What an oversized cylinder costs
Losses grow with volume
Annual storage losses from the assessment method, for a class C vertical unit, rounded to the nearest kWh.
| Volume | 100 L | 150 L | 200 L | 250 L | 300 L |
|---|---|---|---|---|---|
| Annual losses | 403 | 483 | 644 | 725 | 870 |
Going from 200 to 300 litres "to be on the safe side" costs 226 kWh a year, every year, and it shows up in the assessment. That is the price of a margin that was never measured.
Format weighs as much as capacity
At equal volume, a horizontal cylinder is calculated with a markedly higher loss coefficient than a vertical one, which can cancel out the benefit of a well-chosen capacity. The quantified comparison appears in the article on the assessment storage-loss table, and the installation trade-off in the one on choosing between a vertical and a horizontal cylinder.
Footprint and maintenance
A larger volume increases the footprint, the weight when full and therefore the fixing requirements, and it sometimes pushes the cylinder further from the draw-off points. Each of those effects is paid for once at installation and again at every visit. Oversizing is not free insurance.
Validating the volume and documenting it
Handover tests
- Extended draw-off test, volume delivered before the outlet falls below the target useful temperature.
- Temperature measured at the cylinder outlet and at the furthest draw-off point.
- Recovery time after a peak, timed.
The temperature limits that apply
National safety rules cap hot water at the outlets, in France at 50 °C in rooms intended for washing and 60 °C in other rooms, which makes a thermostatic mixing valve necessary as soon as the store runs hotter. The requirement to hold 55 °C permanently at the outlet only applies where total storage reaches 400 litres, which is rare in a single dwelling.
What goes on the estimate
Nominal volume, declared mixed water volume at 40 °C, load profile, cylinder format and stated recovery time. An estimate carrying only the capacity leaves the argument open at handover, whereas those five items close it. On the expected wording, see the mandatory information on a renovation works quote.
