Gas geyser model & size guide
What size gas geyser do I need?
A family of four usually lands on a 16L gas geyser — but the real answer depends on how many taps and showers you run at the same time. Three questions will confirm your size.
3 Questions will find your gas geyser model and size
Question 1
Will the gas geyser be installed indoors or outdoors?
Your installation location will determine which model of gas geyser is suitable.
Question 2
Will you use one or many outlets at the same time?
Variable demand
Multiple use
A multiple-use application is ideal for when the flow will vary, because multiple taps are running at the same time and need to supply a shower and basin, two showers, or a bath and shower simultaneously.
vs
Budget Friendly
Single use
A single-use application are cheaper models but work best where the water flow stays steady — typically supplies one outlet at a time (shower, basin or kitchen sink).
Question 3
What is your hot water demand?
A gas geyser is sized in litres per minute – how much hot water it can deliver while a tap is running. Unlike an electric geyser, which stores a tank of heated water, a gas geyser heats the water as it flows through the appliance. So choosing the right size gas geyser depends on your flow rate.
Your total water demand is the combined flow rate – in litres per minute – of all the outlets you plan to run off a single geyser at the same time. Measure each one with the bucket test and add them together to find the right size gas geyser.
→ Measure your flow rate
→ Run the bucket test
For multiple-use models, repeat for each tap with all of them running together, then add the totals.
Can’t do the bucket test?
An example how to put it all together
Once you know your three answers, they combine into the exact product category to shop for.
NOTE: It is recommended to size up to give your geyser enough capacity to maintain consistent hot water when demand, water temperature or pressure changes. If you choose a geyser that is too small, the available flow won’t be enough.
Typical gas geyser size chart
These are the standard gas geyser sizes available in South Africa. The figures are a guide only — large luxury shower heads and high-flow taps can change the size you actually need, so always check the actual flow rate of your taps and showerheads with the bucket test.
5/6L models
Basin/sinkor
Eco shower head8 – 12L models
Standard shower head + basin/sink16L models
Two standard shower headsor
Standard bath + shower head20 & 21L models
Two large shower headsor
Standard bath + shower head26 – 30L models
Large bath + two luxury shower headsNOTE: when several outlets run at once, the available flow is shared between them — the more you load at the same time, the less each one gets. Most households don’t use every outlet simultaneously, so size for your normal peak usage, not every tap in the house.
Important if you’re replacing an electric geyser
A 16L gas geyser gives you more hot water than a 150L electric geyser.
If you’re used to electric geysers, our numbers measure two completely different things.
Gas geyser size capacity
A gas geyser’s litres are how much hot water it delivers every minute. A 16L gas geyser heats 16 litres a minute, for as long as you need it. That’s a 150l electric geyser’s whole tank in ten minutes – and the gas heated water never runs out – as long as you have gas.
Electric geyser size capacity
An electric geyser’s litres are the size of its tank. A 150L electric geyser holds 150 litres. Use it up and you wait an hour or more for it to reheat.
Choosing the right gas geyser FAQs
Yes, but you need an indoor model that is designed for indoor safety. Indoor models are fitted with a flue that carries the exhaust safely outside. Outdoor models have no flue, so they can only be installed outdoors.
You’ll see indoor models listed as Type B and outdoor as Type D — that’s the industry naming, and it’s what your installer will use.
It depends on whether both bathrooms get used at the same time, not on how many bathrooms you have. If only one shower runs at a time, a 12L model is usually enough. If two showers can run together, you’ll want a 16L, if the collective flow rate is less than 16L. Two large or high-flow shower heads — or a bath filling while someone showers — points to a 20L.
Not sure? Run the bucket test on each outlet and add the totals, or see the typical gas geyser sizing guide.
Not if you use the same amount of hot water. A gas geyser only burns gas while hot water is running, and it uses what it needs to heat the water passing through it. A 20L model heating 10 litres a minute uses much the same gas as a 12L model heating 10 litres a minute — the larger unit simply can deliver more when you ask for it.
So sizing up costs more upfront, not more every month. Your gas bill is driven by how much hot water you use, not by the model you install.
Yes. Choosing a multiple use geyser over a single use geyser has many advantages. However, never choose a single use geyser when you require a multiple use geyser. Multiple use geysers can do everything that a single use geyser can do; it does not work vice versa.
Yes. A multiple-use geyser is a better choice when your water flow or heating requirements vary significantly.
You regularly use outlets with different flow rates
If your household regularly uses both a bath and a shower, the difference in flow between the two can cause the water temperature to fluctuate with a single-use geyser. A multiple-use geyser can adjust to the changing flow. Choose a model sized for your highest-flow outlet.
Your seasonal temperatures vary significantly
If summer temperatures regularly exceed 30°C and winter temperatures drop below 5°C, a multiple-use geyser is better able to adjust its gas flow for the changing conditions. A single-use geyser may run too hot in summer and struggle to reach the desired temperature in winter.
Your water pressure or flow is inconsistent
If your water flow varies — for example, with a gravity-fed tank or inconsistent municipal supply — the temperature from a single-use geyser can vary too. A multiple-use geyser adjusts its gas flow as the water flow changes, helping maintain a more consistent temperature.
Indoor gas geysers require both flueing and ventilation for safe operation.
Flueing
The flue carries the burnt gases produced by the geyser safely outside. The flue requires an opening through an external wall, typically around 130 mm in diameter.
Ventilation
The geyser also needs a continuous supply of fresh air for combustion. Ventilation openings are therefore required to allow fresh air into the room.
The ventilation requirements depend on the size of the gas geyser, so check what will be required before committing to an indoor installation.
Use our typical sizing guide to estimate the gas geyser size you’ll need. It’s based on the average flow rates we commonly see for different household outlets and is a reliable starting point.
Keep in mind that every home is different, so your actual flow rate may vary.
The accessories you need will depend on your geyser and where it is installed.
Outdoor installations
A weather box is the main accessory required to protect the geyser from the elements.
Battery-operated geysers
You’ll need a set of Duracell batteries.
Indoor installations
You’ll need a flexible flue and an exhaust box cover to safely direct burnt gases outside.
Yes. If you live in a coastal region or a particularly windy area, ask us about our coastal protection kit and cyclone flue. These help protect your gas geyser from strong winds and minimise exposure to salt-laden air.
It depends on how far the furthest hot-water outlet is from the geyser and how often you use it.
As a general rule of thumb, allow roughly 1 second of waiting time for every metre of pipe. So, if a tap is 20 metres away by pipe, you could wait around 20 seconds for hot water to reach it.
Remember, pipework rarely runs in a straight line. An outlet 10–15 metres from the geyser may have around 20 metres of piping.
If an outlet is more than 10–15 metres away, it may be worth considering a second gas geyser — particularly if you use hot water there frequently.
If your electric geyser is still working, you can keep it as a backup. Ask your plumber about the correct valve configuration to allow for this.
If your electric geyser has burst or is no longer needed, it can either be removed or drained and left in the roof.
The good news is that your gas geyser can use your existing hot-water plumbing, so you don’t need an entirely new plumbing system.
Gas geysers are sized according to water flow, not the number of people in your household.
People can use hot water one after another without running out — as long as you have gas.
The difference comes when hot-water outlets are used at the same time. Single-use geysers are designed to supply one hot-water outlet at a time. Multiple-use geysers can supply more than one outlet at the same time, provided the geyser is sized for their combined flow rate.
Use our sizing guide and bucket test to work out which type and size of geyser will suit your household.
The logic tree is a tool that guides you through a series of questions to help find the gas water heater solution the would be most suitable for your individual home set up.