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How to use a 24kW instantaneous water heater

Updated: 21 October 2026

The heater is already on the wall, a Gas Safe engineer has commissioned it, and you want to know how to use a 24kW instantaneous water heater. These appliances heat only while water is flowing. Open a tap and the burner should fire. Close the tap and the burner should go out. This is not the error-code list for a GASLAND GS318NG or a CAMPLUX GX318LP. The [24kW instantaneous water heater comparison](/hub/hot-water/24kw-instantaneous-water-heater) is the model set. Peter F. wrote this on 20 October 2026 from listing text, stored reviews and public UK guidance. A purchase through our links can earn a commission at no extra cost to you.

The heater is already on the wall, a Gas Safe engineer has commissioned it, and you want to know how to use a 24kW instantaneous water heater. These appliances heat only while water is flowing.

Our top picks

GASLAND GASLAND GS318NG

Best overall

GASLAND GASLAND GS318NG
8.4/10about £151.99
View offer
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.It heats only while water is moving
  2. 2.Flow, a second outlet, and a cold patch
  3. 3.When it will not stay lit
  4. 4.Cold weather and power left on

It heats only while water is moving

You leave the gas and the electrical supply as the installer left them, and you start a draw. Opening the outlet starts ignition, and closing it puts the flame out. On the Rinnai pattern published by Calor there is no standing pilot. A trickle often does nothing, because the burner will not light below a minimum flow. Reviews of the GX318LP include a unit that would not light until the flow was raised. Do not treat a reviewer’s US gallon figure as a GS318NG specification. The GS318NG names low-water-flow protection and an error-code indicator, and it does not publish the code table here, so read the manual that came with your unit. Approved Document G caps a bath at 48°C only for a new dwelling or a relevant change of use, through a blending valve, not for every existing bath.

Flow, a second outlet, and a cold patch

A 24 kW heater has one stream of heat to share. If you are in the shower and someone opens the kitchen tap, the shower can swing hotter or colder, or the burner can drop out. The GX318LP bullets say two outlets at once, naming a shower and a kitchen, and they do not state the temperature rise at 12 L/min. The GS318NG states 12 L/min with no rise either. If the shower goes cold when you turn the tap down, you may have dropped below the flow that keeps the burner alight. Open the outlet a little rather than cycling the power. The litres behind that limit are on the shower flow page.
GASLAND GASLAND GS318NG

Testsieger

GASLAND GASLAND GS318NG
8.4/10about £151.99
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When it will not stay lit

Air in a new pipe can stop the first start. A 15-minute purge in an RV review is not a procedure to copy onto a house heater. A UK review of the GS318NG describes a spark that returned only after a battery disconnect. That is a reported fault, not a user step. Another UK review describes a leaking gas valve. If you smell gas, do not reset anything. Shut off the supply if you can do that safely at the valve the installer showed you, ventilate, leave, and call the gas emergency number and then a Gas Safe engineer. Do not keep relighting a unit that dies quickly.

Cold weather and power left on

Frost protection on these listings is not a heater that looks after itself with the isolator off. The GS318NG says it heats if the water falls below 5°C while the unit is switched on. The GX318LP names four internal anti-freeze modules and does not say they work with the supply removed. The RV05 says anti-freeze stops when power and gas are off, and that you should empty the tank. Do not copy Navien’s drain sequence onto a GS318NG. No drain steps for that model were in the sources used here. If the unit has already frozen, or a pipe has split, stop and call a Gas Safe engineer.

Common questions

Do I leave a 24 kW instantaneous heater on?
Leave it as commissioned if the frost protection you are relying on needs power. Turning the isolator off in a freeze removes that protection. You do not leave a pilot to babysit on the Rinnai-style pattern, because opening a tap is what starts the burner. Follow the isolator instruction in your own manual if it differs.
Why does nothing happen until the tap is well open?
The burner waits for a minimum flow. A drip or a basin tap barely cracked often sits below that threshold, so you get cold water and no ignition. Open the outlet further. If a normal shower flow still produces no flame, stop trying and use the manual’s code list, then a Gas Safe engineer if it will not stay lit.
It sparks and dies. What may I try?
You may check that the outlet is open enough, that other taps are not stealing the flow, and that the isolator is on. Read the error indication against the manual supplied with the heater. Do not invent a battery-reset routine from a review, and do not open the burner. If it will not stay lit, or you smell gas, that is Gas Safe, not another attempt.
Should I unplug it in a cold snap?
Unplugging it removes the frost protection the listings describe. The GS318NG line applies while the unit is switched on. If you must isolate a house heater for a winter absence, ask the commissioning engineer how that model is drained. Do not assume a Navien drain order applies.
Why run water before the RV heater fires the first time?
A UK review of the RV05 needed about 15 minutes of flow to clear air before it lit. Purge until water, not sputtering air, is what that reviewer was doing. If the burner then runs and the tap stays cold, as one Canadian review reported, stop. That is a fault, not a longer purge.
Is a smell of gas a reset?
No. Do not relight, do not cycle batteries, and do not look for an error code with the supply still open. If you can safely turn the valve the installer pointed out, do that, ventilate, leave, and call the emergency gas service. A Gas Safe engineer investigates afterwards.

Related pages

Buying guide

24kW instantaneous water heater

Best 24kW instantaneous water heater for the gas you have

Natural gas vs LPG instantaneous water heater choice

24kW water heater shower flow rate in a British winter

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)