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How to use an instantaneous water heater

Updated: 21 October 2026

The heater is already on the wall, a qualified person has connected it, and you want to know what you set and what a second tap will do. How to use an instantaneous water heater at this 27 kW class is about outlet temperature and flow, not wiring. You open a tap and the unit heats while water is moving. You do not add a plug or take the cover off. Controls follow the Stiebel Eltron DHE 27 Set, the 400 V listing in this comparison. The listings are on the [27kW instantaneous water heater comparison](/hub/hot-water/27kw-instantaneous-water-heater). Peter F. did not operate a unit for this page. A qualifying purchase can earn a commission at no extra cost to you.

The heater is already on the wall, a qualified person has connected it, and you want to know what you set and what a second tap will do.

Our top picks

WINTEMP WINTEMP YQ-WN27K

Best overall

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer
GRS GRS 27 kW

Rank 2

GRS GRS 27 kW
1.6/10about £388.71
View offer
Stiebel Eltron Stiebel Eltron DHE 27 Set

Rank 3

Stiebel Eltron Stiebel Eltron DHE 27 Set
6.9/10about £821.57
View offer
On this page
  1. 1.What happens when you open the tap
  2. 2.Temperature, memory and ECO
  3. 3.A second tap
  4. 4.When you stop

What happens when you open the tap

An instantaneous heater does not keep a cylinder of hot water. It heats mains water only while a tap is open and flow is high enough to switch it on. On the DHE 27 Set that point is above about 2.5 litres per minute. A dribble can run cold because the elements have not started. Open the outlet properly, then adjust. For normal use the isolator stays on. If the unit was never connected to 400 V, no temperature setting will fix it. That question is on the three-phase page.

Temperature, memory and ECO

On the DHE 27 the outlet range published by Stiebel is Off and about 20 to 60 °C. The installer can cap it at 55, 50 or 43 °C, so the dial may stop short of 60 °C. Do not defeat that cap by opening the case. The 4i control is meant to hold the set temperature until output runs out. Memory buttons store settings. Use the manual in the carton for which button does what. ECO limits flow so the heater is not asked for more hot water than that mode should give. It does not add kilowatts. A 240 V listing that mentions a plug is not a set of UK instructions.
WINTEMP WINTEMP YQ-WN27K

Testsieger

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer

A second tap

The heater has one output, about 27 kW when the supply is 400 V, not a separate 27 kW for every tap. Stiebel's FAQ says a sink opened during a shower may affect the shower. That change is not automatically a breakdown. If you need the shower stable, keep the other hot taps shut. A winter main near 10 °C asks for a rise of about 30 °C, which sits behind the roughly 13 litres per minute figure on the buying guide. If the shower was stable before, at the same taps and season, a sudden change is a reason to stop.

When you stop

Stop, and turn the isolator off if you can do that safely, if you see a leak, smell burning, or the circuit trips again after a reset. Do not remove the cover. Call an electrician or a plumber for no heat when the unit used to heat, or for water where water should not be. A fine spray that never gets hot can be the 2.5 litres per minute switch-on. Open the outlet further before you assume the elements have failed. Landlords still have a legionella duty where people shower or the property is let. The Health and Safety Executive treats instantaneous hot water as a lower risk than a stored cylinder in a small domestic system, not as zero risk. Cleaning the showerhead is ordinary control, not a wiring job. Only the DHE row matches the controls described here.

Common questions

Do I leave an instantaneous water heater switched on?
Yes for normal use. The isolator stays on so the heater can start when flow exceeds the switch-on point, above about 2.5 litres per minute on the DHE 27 Set. You do not strike it like a boiler each morning. Turn it off at the isolator if you can do so safely and there is a leak, a burning smell, or repeated tripping. Leaving it on is not the same as leaving a hot tap running.
What temperature should I set?
On the DHE 27 the published range is Off and about 20 to 60 °C, unless the installer has capped it at 55, 50 or 43 °C. Set the outlet temperature you actually want, rather than maximum, and let the control hold it until output runs out. A shower that needs a large winter rise will still be limited by about 13 litres per minute at a 30 °C rise. The manual in the box is the button sequence. This page does not replace it.
What does ECO change?
ECO is the economy mode on the DHE display. Stiebel describes it as a flow limit, so the heater is asked for less hot water, not as an extra heat source. The shower can feel weaker while ECO is on. It does not change a 240 V unit into a 400 V one, and it does not let two showers run as if each had 27 kW. Use the carton manual to enter and leave the mode.
Why is there a short hot or cold burst at the start?
The heater starts when flow is high enough, and the first seconds can be unsettled while it catches up. A very small flow can stay cold because it never reaches about 2.5 litres per minute. That is expected behaviour on this class of control, not a measurement from a UK safety test. If the burst becomes a long run of cold water at a normal shower flow, stop and call a qualified person rather than opening the case.
When do I call an electrician or a plumber?
Call one for a leak, a burning smell, repeated tripping, or no heat when the heater used to work at a normal flow. Turn the isolator off if you can do that safely, and leave the cover on. Also call before any new circuit or any question about whether the flat is single-phase. Daily temperature setting is yours. Connection to the supply and to the water main is not.

Related pages

Best 27kW instantaneous water heater

27kW instantaneous water heater

27kW instantaneous water heater UK: the buying checks

A 27kW water heater three phase supply for UK homes

240V tankless water heater UK, not the 400 V heater

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these three listings were read on 19 October 2026. Page updated 21 October 2026.

  1. Approved Document G, with 2024 amendments (retrieved 19 October 2026)
  2. Approved Document P, 2013 (retrieved 19 October 2026)
  3. Water Supply (Water Fittings) Regulations 1999 (retrieved 19 October 2026)
  4. HSE, landlords and legionella (retrieved 19 October 2026)
  5. Stiebel Eltron UK, DHE 27 Set (retrieved 19 October 2026)
  6. Stiebel Eltron UK comparison sheet (retrieved 19 October 2026)
  7. Stiebel Eltron UK, three-phase water heating (retrieved 19 October 2026)
  8. Flexiheat UK, electric instantaneous water heaters (retrieved 19 October 2026)
  9. amazon.co.uk, Stiebel Eltron DHE 27 Set B08L5B34YH (retrieved 19 October 2026)