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24kW water heater shower flow rate in a British winter

Updated: 21 October 2026

You want a shower, and the box says 12 litres a minute, so the 24kW water heater shower flow rate sounds settled before you have asked how cold the mains is. It is not settled. A 24 kW instantaneous heater puts a fixed amount of heat into the water that is passing. The [24kW instantaneous water heater comparison](/hub/hot-water/24kw-instantaneous-water-heater) lists the heaters that print a flow. Peter F. wrote this on 20 October 2026 from supplier tables and the amazon.co.uk listings captured on 19 October 2026. No shower was measured in a lab. A purchase through our links can earn a commission at no extra cost to you.

You want a shower, and the box says 12 litres a minute, so the 24kW water heater shower flow rate sounds settled before you have asked how cold the mains is. It is not settled.

Our top picks

GASLAND GASLAND GS318NG

Best overall

GASLAND GASLAND GS318NG
8.4/10about £151.99
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CAMPLUX CAMPLUX GX318LP

Best value

CAMPLUX CAMPLUX GX318LP
8.4/10about £239.99
View offer
GEESEN GEESEN RV05

Rank 3

GEESEN GEESEN RV05
7.6/10about £219.00
View offer
On this page
  1. 1.What people mean by a decent shower
  2. 2.The winter sum at 24 kW
  3. 3.Why the box can still say 12 litres a minute
  4. 4.Two outlets on one heater

What people mean by a decent shower

A comfortable mixer shower in UK plumbing talk is often 8–12 litres a minute. A United Utilities project with LJMU put mixer showers roughly in a 5–15 litre band and electric showers roughly in a 3–8 litre band. An electric shower limits flow because the element is in the shower. That is a different machine from a gas instantaneous heater feeding a mixer. Part G is about litres per person per day in a new dwelling. It is not a minimum shower flow and not a cap of 8 litres on your bathroom. A drench head happy at 15 litres will not hold 40°C from a January mains on 24 kW.

The winter sum at 24 kW

Heat into water is roughly flow times temperature rise times the specific heat of water. Using 4.186 kJ per kilogram per degree, 24 kW divided by 4.186 and then by a 30 degree rise is about 11.5 litres a minute. That is a rule of thumb, not a test of any ASIN. AIZO Quality Heating’s table prints the same about 11.5 litres a minute at 30°C and about 9.1 litres a minute at 38°C, and says real flow is often 5–10% lower. Use the table, not the same article’s 14–16 litres for a 24–30 kW band. Flexiheat puts UK winter mains near 10°C, and about 5°C in parts of Scotland. From 5°C to 40°C the rise is 35°C and the theoretical flow falls towards the 9 litre region. A Worcester Bosch text for a 24 kW combi, which also heats the rooms, quotes 11.8 litres a minute at a 30°C rise and 9.0 litres a minute at a 40°C rise. AIZO puts two bathrooms at once on a 27–30 kW row.
GASLAND GASLAND GS318NG

Testsieger

GASLAND GASLAND GS318NG
8.4/10about £151.99
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Why the box can still say 12 litres a minute

The GASLAND GS318NG and the CAMPLUX GX318LP both state 12 L/min, and neither states the temperature rise at which that flow is measured. A 12 litre claim and an 11.5 litre winter sum can both be true if the 12 litres was taken at a smaller rise. Do not read 12 L/min as a strong 40°C shower in January. The GEESEN RV05 states 10 L/min and also prints 24 kW, 55,000 BTU, 55,000 watts and 25,000 BTU. Fifty-five thousand BTU per hour is about 16 kW, not 24 kW. Do not run the winter sum on that title. Ten per cent losses on 11.5 litres is about 10 litres at the tap.

Two outlets on one heater

A second tap does not get a second 24 kW. It shares the same heat. The GX318LP bullets say two water points at once, a shower and a kitchen, and they do not say both at the winter rise. The pattern that fits the sum is the shower alone, then the kitchen. Neither indoor listing is described for two bathrooms. If the flame dies when you turn the shower down, you have likely fallen through the minimum flow. That behaviour is on the how to use page. Buy a higher kilowatt class, on the correct gas, if two showers at once are the brief.

Common questions

Is 12 litres a minute a strong shower?
It can be, if the heater is only adding a modest rise. As a winter shower from about 10°C to about 40°C, a bare 12 L/min on the GS318NG or GX318LP is not a measured strong shower. Comfort talk of 8–12 litres a minute is a guide, not a promise from those listings.
What do I get from 10°C mains if I want about 40°C?
The rise is 30°C. At 24 kW the rule of thumb is about 11.5 litres a minute before a 5–10% reduction for real losses, so around 10–11 litres at one shower. In a Scottish mains nearer 5°C the same 40°C outlet is a bigger rise and a lower flow, closer to 9 litres. Neither listing states that it was tested at those inlets.
Can two showers run together?
Not as two unrestricted winter showers. About 11 litres a minute is one mixer. You can use a second outlet in turn. The GX318LP line about two points does not include a temperature for both of them open in January.
Will a second tap steal the shower?
Yes, in the sense that both outlets share one heat input. Opening the kitchen tap can cool the shower or drop the burner out. Finish the shower, then run the tap. A burner that only stays lit at a high flow is minimum-flow behaviour, not a second hidden kilowatt.
Is an electric shower’s 6–8 litres a minute the same machine?
No. An electric shower heats in the handset or the box on the wall and limits flow so an 8–10 kW element can reach temperature. A 24 kW gas instantaneous heater feeds a mixer from outside the cubicle. You cannot read 7 litres on an electric shower as the flow this gas heater will give.
What if the listing’s 24 kW and its BTU figure disagree?
Do not calculate a shower from that title. Until one duty is clear, the winter sum does not apply. The GS318NG and GX318LP at least state 24 kW without that BTU clash, and they still omit the rise at 12 L/min.

Related pages

Buying guide

24kW instantaneous water heater

Best 24kW instantaneous water heater for the gas you have

How to use a 24kW instantaneous water heater daily

Natural gas vs LPG instantaneous water heater choice

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. Gas Safety (Installation and Use) Regulations 1998, regulation 30 (retrieved 19 October 2026)
  3. HSE: gas safety in rented homes (retrieved 19 October 2026)
  4. Approved Document J (2022) (retrieved 19 October 2026)
  5. BS EN 26:2023, gas-fired instantaneous water heaters (retrieved 19 October 2026)
  6. GOV.UK: energy labelling of products (retrieved 19 October 2026)
  7. Water Supply (Water Fittings) Regulations 1999, regulation 5 (retrieved 19 October 2026)
  8. Instant water heaters: electric and gas flow notes (retrieved 19 October 2026)
  9. HSE: who may carry out a landlord gas check (retrieved 19 October 2026)
  10. BS EN 26:2023 CEN summary (retrieved 19 October 2026)
  11. Flexiheat instant gas water heaters (retrieved 19 October 2026)