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18kW instantaneous water heater shower

Updated: 21 October 2026

You are fitting out a bathroom and you want one heater to cover the shower. An 18kW instantaneous water heater shower is a fair match for one ordinary shower, and a weak match if the bath has to run as well. The heater warms mains water as it flows, so the hot flow depends on the kilowatts and on how cold the inlet is. Published figures put a standard shower at about 7–8 litres a minute and an 18 kW unit near 9 litres a minute at shower temperature. The [18kW instantaneous water heater comparison](/hub/hot-water/18kw-instantaneous-water-heater) sets the kept listings side by side. This page stays with the outlets and assumes the supply is already suitable.

You are fitting out a bathroom and you want one heater to cover the shower. An 18kW instantaneous water heater shower is a fair match for one ordinary shower, and a weak match if the bath has to run as well.

Our top picks

WINTEMP WINTEMP YQ-WN18K 18 kW

Best overall

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
View offer
Rheem Rheem RTEX-18

Rank 2

Rheem Rheem RTEX-18
8.4/10about £364.17
View offer
Bosch Bosch Tronic 4000 18 EB

Rank 3

Bosch Bosch Tronic 4000 18 EB
7.0/10about £428.72
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On this page
  1. 1.What one shower draws
  2. 2.Two showers or a bath
  3. 3.A second tap
  4. 4.When 24 kW is the next step

What one shower draws

A standard shower in UK merchant notes runs at about 7 to 8 litres a minute. Stiebel Eltron’s rule of thumb divides kilowatts by two at about 38°C, which puts 18 kW at about 9 litres a minute. Flexiheat prints 8.7 litres a minute at a 30°C rise, and CLAGE lists 9.2 litres a minute for the DEX 18 Next at a 28°C rise. You have a little spare on one ordinary spray, and very little if you ask for hotter water. At about 50°C the same rule is nearer 6 litres a minute. A colder mains inlet needs a bigger rise, so the hot flow falls.

Two showers or a bath

Heater Shop treats two showers, or a bath plus basins, as alternative jobs for the switchable Stiebel DHE 18/21/24. A fixed 18 kW plate cannot be turned up. Two showers at 7 to 8 litres a minute each want about 14 to 16 litres, while the heater stays near 9. Flexiheat notes a bath mixer at about 12 litres a minute. You can fill a bath if you accept that slower flow and leave the shower off. A bath symbol on the CLAGE DEX 18 Next is not a promise that both outlets run together. If children or older people use the bath, the installation guide covers the 48°C limit and when it applies.
WINTEMP WINTEMP YQ-WN18K 18 kW

Testsieger

WINTEMP WINTEMP YQ-WN18K 18 kW
8.8/10about £276.46
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A second tap

A basin or a kitchen tap does not receive a private supply. Open it during a shower and both outlets share whatever 18 kW can raise, still near 9 litres a minute. The spray then weakens or cools. A Bosch line about showers and bathrooms is a listing claim, not a measured flow when the second tap opens. On a weekday morning the shared flow is the regular complaint. Compare the control notes on the best 18 kW heaters shortlist before you rely on a second outlet.

When 24 kW is the next step

Step up if you want a bath mixer or two showers without taking turns. Stiebel’s rule of thumb puts 24 kW at about 12 litres a minute around 38°C, and only where the supply can carry it. The buying guide checks that supply. One standard shower still fits 18 kW more closely. Peter F. drew these figures on 20 October 2026 from Stiebel Eltron’s UK sizing guide, Flexiheat, CLAGE’s DEX 18 Next data and a Heater Shop line. This is not a lab test. A qualifying purchase can earn a commission at no extra cost to you.

Common questions

Will 18 kW run a normal shower?
Yes, for one standard shower at about 7 to 8 litres a minute. Stiebel’s rule of thumb puts 18 kW at about 9 litres a minute around 38°C, and Flexiheat prints 8.7 litres a minute for a 30°C rise. You have a small margin on one ordinary head, and a colder winter inlet uses it up. These are published figures, not a test of your bathroom.
Will it run two showers at once?
Not at a normal flow on a fixed 18 kW rating. Two showers at 7 to 8 litres a minute each need about 14 to 16 litres, while the heater stays near 9. Heater Shop’s note on two showers refers to the switchable Stiebel DHE 18/21/24, which can also be set to 21 or 24 kW, not to every fixed 18 kW plate.
Can it fill a bath?
It can fill a bath on its own, nearer the heater’s flow than a bath-mixer rate of about 12 litres a minute. An 18 kW unit sits around 9 litres on the published figures, including 9.2 litres a minute for the CLAGE DEX 18 Next at a 28°C rise. Leave the shower off while the bath fills.
What happens if someone opens the kitchen tap?
The tap shares the heated flow. Opening it does not create a second supply of about 9 litres. A shower at 7 litres plus a few litres at the sink already meets what 18 kW can raise, so the spray weakens or cools, depending on the model. You may not notice when those outlets run at different times.
Why is a drench head a problem?
Merchant notes often put a drench head at 15 to 30 litres a minute, well above the roughly 9 litres an 18 kW heater can heat. The rose never gets a full hot flow, so the shower feels weak or lukewarm while the heater is already at its limit. A standard head at 7 to 8 litres a minute is the match for this rating.
Does a thermostatic mixer change the heater size?
No. A thermostatic mixer blends hot and cold at the outlet. It does not add kilowatts or raise the litres 18 kW can heat through a given rise. If the hot feed is already at the limit, the valve can only trade flow against temperature. It cannot turn about 9 litres of heat into two full showers.
When is 24 kW the next step?
Step up for a bath mixer near 12 litres a minute, two showers at once, or a shower plus a tap you will not stagger. Stiebel’s rule of thumb puts 24 kW at about 12 litres a minute around 38°C, and only if the supply can carry that rating. One standard shower still fits 18 kW more closely.

Related pages

18kW instantaneous water heater on three-phase

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What size 18 kW instantaneous heater do I need

How to install an 18kW instantaneous heater in the UK

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. Part G FAQ, bath temperature limit (retrieved 19 October 2026)
  3. Approved Document P, 2013 (retrieved 19 October 2026)
  4. UK Power Networks, single-phase and three-phase (retrieved 19 October 2026)
  5. Stiebel Eltron UK, what size instantaneous water heater (retrieved 19 October 2026)
  6. Stiebel Eltron UK, Tempra products in the UK (retrieved 19 October 2026)
  7. Zip DBX18 installation sheet (retrieved 19 October 2026)
  8. Rheem RTEX product sheet (retrieved 19 October 2026)
  9. Flexiheat UK, electric multipoint water heaters (retrieved 19 October 2026)
  10. amazon.co.uk, Bosch Tronic 4000 18 EB (retrieved 19 October 2026)