Moving a water cylinder onto the 11:00 to 17:00 slot
The hot water cylinder is the only large load in a house that stores what it makes. That is what makes it the first appliance to shift, and the easiest.
Updated on 14 September 2026
Why the cylinder comes before everything else
A washing machine runs when you load it. An oven heats when you cook. The cylinder makes hot water hours before anyone uses it, and holds on to it. Shifting when it produces changes nothing about your day.
It is also the heaviest item on the board after heating. On a 2 400 W element it draws more on its own than every light in the house put together.
The arithmetic, on a 200 litre cylinder
| Quantity | Value | Where it comes from |
|---|---|---|
| Volume | 200 litres | The common size for four people |
| Temperature rise | 45 °C | From 15 °C at the tap to 60 °C in the cylinder |
| Energy required | 10.5 kWh | 200 kg × 4 186 J × 45 °C, converted to kWh |
| Element rating | 2 400 W | The standard figure at this volume |
| Heating time | 4 h 22 | 10.5 kWh divided by 2.4 kW |
| Length of the daytime off-peak window | 6 h 00 | From 11:00 to 17:00 |
A full reheat therefore fits inside the window with an hour and a half to spare. A half empty cylinder, which is the normal case, refills in two hours.
A contactor does not read a clock, it follows a signal
The classic day/night contactor has no timer: it closes the circuit when the network operator sends the reduced-rate signal. As long as that signal carried a night window only, one module and two wires were the whole job.
The 2026 grid has two off-peak windows, one of them in the middle of the day. Depending on the age of the meter and the existing wiring, the contactor follows both or only one, and the installation looks perfectly healthy either way. That is the first thing to check, and on site it takes ten minutes.
Where the signal is not enough, the answer is a programmable time switch in the board or a controlled module. One more module, not a new board.
What we check before fitting anything
- The age of the meter, and whether it is a smart one: that dictates the wiring.
- Whether the cylinder already has its own circuit, or shares one with something else.
- The cross-section of the cable feeding it, because an underfed cylinder is rewired before it is ever scheduled.
- The night-only supply, which has its own meter and its own logic.
- The boost override, which has to stay reachable and legible: an empty cylinder on a Sunday evening will not wait until 11:00 on Monday.
Tell us what is in your consumer unit
Describe what sits on your board and we will say what can be shifted and what cannot. By phone on +32 495 88 75 92 or through the questionnaire.
Sources for this page
- Public Service of Wallonia, the two-rate schedule changes on 1 January 2026Checked on 2026-09-14