Three-phase instantaneous water heater supply
Updated: 17.10.2026
You have found a 21 kW heater, but the label says 400 V and your home may have only a normal single-phase service. That is the decision to settle before comparing controls or prices. A three-phase instantaneous water heater in this class is not an 11 kW marketplace unit with a higher number in the title. It is a different electrical load. See the captured products on the [21 kW instantaneous water heater comparison](/hub/hot-water/21kw-instantaneous-water-heater), then confirm the incoming service before you order a three-phase-only appliance.
You have found a 21 kW heater, but the label says 400 V and your home may have only a normal single-phase service. That is the decision to settle before comparing controls or prices.
You have found a 21 kW heater, but the label says 400 V and your home may have only a normal single-phase service. That is the decision to settle before comparing controls or prices. A three-phase instantaneous water heater in this class is not an 11 kW marketplace unit with a higher number in the title. It is a different electrical load. See the captured products on the 21 kW instantaneous water heater comparison, then confirm the incoming service before you order a three-phase-only appliance.
Our top picks
| Badge | Product | Power | Supply | Stated flow | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Stiebel Eltron Stiebel Eltron DCE-X 10/12 | 10/12 kW | 220/240 V | 5 L/min | 8.2 | about £412.86 | View offer |
| Alternative |  WINTEMP WINTEMP 11 kW tankless | 11 kW | 240 V | n/a | 5.8 | about £176.47 | View offer |
| Preis-Tipp |  Triton Showers Triton Instaflow 10.1 kW | 10.1 kW | n/a | 2.7 L/min | 7.0 | about £154.99 | View offer |
On this page
What 21 kW asks of the supply
Flexiheat specifies its 21 kW electric tankless heater at 400 V three-phase and 3 × 30.3 A. The three conductors share the load. This is not one 30.3 A circuit on a 230 V supply. The same table gives about 10.1 litres per minute at a 30°C rise.
Stiebel Eltron's DHE 18/21/24 is also 400 V and states 29, 31 and 35 A at 18, 21 and 24 kW. Use 31 A as that model's 21 kW current, not as a correction to Flexiheat's 3 × 30.3 A. The circuit follows the rating plate and the manual.
An 18/21 kW label does not mean one phase is enough. Veito and Quality Heating both describe the Blue S as three-phase only. A selector changes heat within the designed supply. It does not redesign the incoming service.
What an 11 kW heater does instead
The captured amazon.co.uk listings top out at about 10–12 kW. The Stiebel Eltron DCE-X 10/12 is 220/240 V with selectable 10, 11 or 12 kW. The WINTEMP is listed at 11 kW and 240 V. Both need dedicated circuits. Neither is a turned-down 400 V 21 kW machine.
The Stiebel listing states 5 litres per minute. A 21 kW class figure is about 10.1 litres per minute at a 30°C rise, so an 11 kW listing can cut the available flow.
Use 5.5–12 kW only when the demand is smaller: one basin, one sink or sequential use, depending on winter inlet temperature. A bathroom and kitchen drawing together are more likely to show the limit. How to resize that demand is in the buying guide.
How to tell, and who confirms it
One service fuse often means single-phase and three fuses can mean three-phase. That is a clue, not a reason to remove covers. A block can have three-phase in the riser while a flat receives only one phase, so a large communal intake does not prove the dwelling can run 21 kW.
Ask an electrician to inspect the accessible intake and the proposed load. The distribution network operator confirms whether three-phase is available and what the upgrade involves. Your supplier bills you. It is not necessarily the organisation that alters the connection. Who does the internal work is in the installation guide.
The decision if you have only single-phase
If only single-phase is present, do not order a three-phase-only 21 kW heater on the hope that an electrician can adapt it. Ask whether an upgrade is available, and include intake, meter, consumer unit, circuit and building-management work in the total.
Where an upgrade is unavailable or disproportionate, resize the job: 5.5–8 kW for one basin, 10–12 kW for one harder outlet, or a dedicated electric shower for one shower. Keep 21 kW only when three-phase is present or the network operator will provide it.
Common questions
Will 21 kW run on a normal UK single-phase supply?
The UK 21 kW electric models cited here are designed for 400 V three-phase, not a common 230 V single-phase service. Flexiheat explicitly says its three-phase range cannot run on single-phase. Do not attempt to adapt the connection. Ask the network operator about the incoming service and use an electrician for the load assessment.
What does 3 × 30.3 A mean?
It is Flexiheat's published current figure for its 21 kW, 400 V three-phase model: approximately 30.3 A on each of three phases under that design. It is not a single 30.3 A supply and not a universal figure for every 21 kW heater. Follow the exact model's rating plate and manual.
Why does Stiebel list 31 A at 21 kW?
Stiebel Eltron's DHE 18/21/24 is a different appliance and publishes 29/31/35 A for its three selectable settings. Flexiheat publishes 3 × 30.3 A for its 21 kW model. Small differences between model-specific data should not be averaged. The electrician designs to the chosen appliance's declared ratings.
How can I see whether my home has three-phase?
One service fuse often indicates single-phase and three fuses can indicate three-phase, but this is only a clue. Do not open sealed equipment. Flats can also sit in a three-phase building while receiving only one phase individually. Ask an electrician to inspect and the distribution network operator to confirm the service.
Can an electrician turn a 21 kW heater down to one phase?
Not unless the manufacturer explicitly designs and documents that exact model for the alternative supply. The cited 18/21 kW Blue S remains three-phase only at its selectable settings. An electrician cannot make a three-phase-only appliance into a compliant single-phase product by selecting less output or moving conductors.
Who arranges a three-phase upgrade?
The local distribution network operator controls the incoming network connection and determines whether an upgrade is available. An electrician can assess demand and prepare the internal design, but cannot authorise or create the external service. Obtain both the network proposal and the installation quote before buying the heater.
Is a 13 A plug enough for any 21 kW model?
No. A normal 13 A UK socket provides roughly 3 kW, far below 21 kW. Even the 5.5–12 kW products in the captured comparison are not ordinary plug-in loads. Treat any conflicting marketplace wording as a reason to stop and verify the rating plate with a competent electrician.
Related pages
Sources
Public UK sources from the research digest, plus the six amazon.co.uk listings in this comparison. The Veito amazon.co.uk offer was checked on 16 October 2026.
- Approved Document P (retrieved 16 October 2026)
- Building Regulations 2010 (retrieved 16 October 2026)
- GOV.UK repairs and competent persons (retrieved 16 October 2026)
- Quality Heating instant-heater guide (retrieved 16 October 2026)
- Flexiheat UK tankless range (retrieved 16 October 2026)
- Veito Blue S at Quality Heating (retrieved 16 October 2026)
- Veito UK Blue S (retrieved 16 October 2026)
- B&Q Veito Blue S (retrieved 16 October 2026)