Gas Water Heater: How It Works and the Safety Rules

A gas water heater heats water by burning gas in a combustion chamber and passing the water through a heat exchanger. Where piped town gas is available, it offers a continuous supply of hot water at good flow, and it remains a familiar choice in many Singapore homes that already cook with gas.

It is also the type of water heater with the most serious safety implications, because it involves both a fuel supply and combustion products. This article explains how gas heaters work, the ventilation and installation principles that keep them safe, and the licensing rules that apply to any work on them in Singapore.

How a gas water heater works

Most household gas heaters in Singapore are instantaneous. When a hot tap opens, water flow is detected and the burner ignites. Hot combustion gases pass through a heat exchanger, transferring heat to the water flowing through it. When the tap closes, the burner shuts off. Modern units have controls that modulate the flame to hold a steady outlet temperature as flow changes.

Because the burner can deliver a lot of heat quickly, a gas instantaneous heater can often supply higher flows than an electric instant water heater, and some models serve more than one outlet. Gas storage heaters, which heat a tank with a gas burner, also exist but are less common in local homes.

Combustion, flueing and ventilation

Burning gas consumes oxygen and produces flue gases, including carbon dioxide, water vapour and, if combustion is incomplete, carbon monoxide. Carbon monoxide is colourless and odourless, and it can be fatal in an enclosed space. For this reason the location, air supply and flueing of a gas water heater are not matters of preference.

Key principles:

Follow the appliance type. Gas heaters are designed for particular flue arrangements and locations. Install only as the manufacturer and the applicable installation code allow.
Do not enclose it. Boxing a heater into a cabinet or behind a false ceiling without the required ventilation or flue route can starve it of air and trap flue gases.
Keep flues clear and intact. A disconnected, blocked or improvised flue is dangerous.
Do not block ventilation openings. Openings provided for combustion air or ventilation must stay clear.

In Singapore, gas installation work by licensed workers must comply with the relevant Singapore Standard code of practice. EMA has issued a circular requiring licensed gas service workers and professional engineers to comply with SS 608:2024, the updated Code of Practice for Gas Installation, from 1 September 2025.

The licensing rule: only licensed gas service workers

This is the most important point in the article. Under Singapore’s Gas Act and gas supply regulations, the installation, testing, maintenance and repair of gas appliances connected to gas pipeline systems, including gas water heaters, must be carried out by a Licensed Gas Service Worker (LGSW) licensed by the Energy Market Authority. EMA’s consumer guidance states that piped gas appliances must be installed by a licensed worker, who will issue a written statement certifying that the appliance has been tested and is safe to use.

The scope of a gas service worker licence is defined by EMA, including limits on the operating pressure of installations they may work on; work beyond that scope requires a Professional Engineer. EMA also notes that engaging an unlicensed person for gas service work is an offence.

Before engaging anyone, check the worker’s licence on EMA’s website. Do not allow a general renovation contractor, plumber or handyman to connect, move or modify gas pipes or appliances unless they hold the appropriate EMA licence. In HDB flats, HDB’s renovation guidelines also state that gas service works must be done by an EMA-licensed gas service worker.

If you are using bottled liquefied petroleum gas rather than piped gas, check the supplier’s requirements and the applicable rules for cylinder installations before buying or installing an appliance.

Water pressure and electrical supply

A gas instantaneous heater needs adequate water flow to activate the burner and to deliver a steady temperature. On low-pressure supplies, flow at the shower and taps may be disappointing. Check the appliance’s minimum operating pressure against your supply before choosing a model, particularly on upper floors or where several outlets draw at once.

Many modern gas heaters also need electricity for ignition, controls or a fan-assisted flue. Any new electrical connection point should be installed by an EMA Licensed Electrical Worker.

Placement in a renovation

Gas heaters are commonly located in kitchens, service yards or ventilated areas near the bathroom, with hot water piped to the outlets. In a renovation, plan the heater position, the gas pipe route, the flue path and the hot water pipes together, before carpentry and false ceilings are built. Changing any of these later means opening finished work.

Avoid locating a gas heater where its flue discharges into an enclosed space, near openable windows of bedrooms, or where it could be obstructed by future cabinetry. Our overview of water heater types compares gas with the alternatives, and our guide to hot water installation covers pipe routing and placement more broadly.

Warning signs and maintenance

Have a gas heater serviced periodically by a licensed gas service worker. Stop using the appliance and seek help if you notice:

A smell of gas.
Yellow or sooty flames, or soot marks around the heater or flue.
Headaches, dizziness or nausea when the heater is running, which can be signs of carbon monoxide exposure.
Water leaks, unusual noises or repeated ignition failure.

If you smell gas, follow standard gas safety practice: avoid anything that could create a spark, including electrical switches, open windows and doors, turn off the gas supply if it is safe to do so, leave the area and call your gas supplier’s emergency line or the emergency services. Gas supplied to consumers in Singapore is odourised so that leaks can be detected.

For bathrooms more generally, good ventilation matters for moisture as well as safety. Our article on natural ventilation explains how air movement works through homes.

Frequently asked questions

Who can install a gas water heater in Singapore?

Only a Licensed Gas Service Worker licensed by the Energy Market Authority may install, test, maintain or repair gas appliances connected to a gas pipeline system. The worker should certify in writing that the appliance has been tested and is safe. Check the licence on EMA’s website.

Can a gas water heater be installed inside a bathroom?

Only if the appliance type, flue arrangement and ventilation permit it under the manufacturer’s instructions and the applicable installation code. Incorrectly located gas heaters can cause carbon monoxide build-up. A licensed gas service worker should confirm the location.

Why is the water flow from my gas heater weak?

Gas instantaneous heaters need a minimum water pressure and flow to operate properly. Weak flow can result from low supply pressure, partly closed valves, blocked filters or scale in the heat exchanger. A licensed worker can check the appliance; supply issues may need a plumber.

Can I move my gas water heater myself during renovation?

No. Disconnecting, relocating or reconnecting a gas appliance or gas pipe is gas service work and must be done by an EMA Licensed Gas Service Worker. Unlicensed gas work is an offence and creates a serious safety risk.

Planning a renovation that moves service routes through walls and ceilings? Talk to our consultancy or get in touch for independent technical advice.