Gas guide
How does a gas geyser work?
A gas geyser works by heating water only while it flows. Opening a hot tap pushes water through the unit, the water flow opens the gas valve, and an igniter lights the burner. The flame heats a copper heat exchanger, and the water leaves hot. Close the tap and the burner goes out, so no stored water is kept hot.
Step by step: from tap to hot water
- Water flow is detected — above a minimum flow, a diaphragm valve or flow sensor in the geyser reacts to water moving through it.
- The gas valve opens — gas from the regulator (2.8 kPa on a domestic LPG installation) reaches the burner.
- The burner lights — usually from a battery-powered spark, on some models a piezo or mains igniter.
- The heat exchanger heats the water — a finned copper coil above the burner transfers heat to the water flowing through it.
- The flue takes the fumes out — combustion products, including carbon monoxide, are vented to open air.
What the knobs do
Most gas geysers have two controls: a gas (flame) control and a water (temperature) control. More gas means a hotter flame; less water flow means each litre spends longer in the heat exchanger and comes out hotter. Set them once for your house and leave them.
Litres per minute, not tank size
A gas geyser is rated in litres per minute at a given temperature rise, because it has no tank. A small unit feeds one outlet at a time; a household running a shower and a kitchen tap together needs a bigger one. See gas geyser installation for how we size a unit for a house.
Safety devices inside the unit
- Flame failure protection — shuts the gas off if the flame goes out.
- Overheat cut-out — stops the burner if the water gets too hot.
- Minimum water flow — the burner won't light on a trickle, which protects the heat exchanger.
These protect the appliance. The installation around it — cylinders, regulator, pipework and flue — is what the gas certificate of compliance covers.
Why the flue and position matter
A gas geyser burns gas, so it produces combustion gases that must go outside. That's why most are mounted on an outside wall. SANS 10087-1 sets out how flues must terminate; an indoor unit needs a flue designed for it and enough ventilation. A geyser in a closed bathroom without a proper flue is dangerous.
Load-shedding
A gas geyser with a battery igniter keeps heating water when the power is off. Fan-assisted models need mains power, so check the specification before you buy.
Get it installed or certified
- Gas geyser installationSized, mounted, piped, certified
- Gas stove installationStove, hob or oven, tested
- Gas certificate of complianceInspection, test, COC
- Get a priceCall 083 498 1693 or use the form
More gas guides
- Geyser types compared
- Gas vs electric stove
- Gas installation regulations
- SAQCC Gas explained
- Gas cylinder sizes
- Checking for a gas leak
Part of Gas Wizards — gas installers in Johannesburg and the West Rand.
Quick answers
Does a gas geyser need electricity?
No, not for most models. A battery-spark gas geyser runs without mains power; fan-assisted models do need it.
Does a gas geyser keep water hot all day?
No. An instantaneous gas geyser only heats water while a hot tap is open, so there is no tank to keep hot.
Does a gas geyser need a minimum water pressure?
Yes — the burner only lights above a minimum flow. Each model states its minimum working pressure; low-pressure homes need a model chosen for it.