Continuous Flow or Storage? Sizing Hot Water by Litres Per Minute and Household Size

Ask two suppliers to quote a hot water replacement and you can get one number in litres and another in litres per minute, with nothing obvious to compare them on. That is not sales fog. The two technologies are sized on different units because they solve the problem in different ways.

A storage tank is sized in litres held. It is a buffer: it delivers a lot at once, then it is empty until it recovers. A continuous flow unit is sized in litres per minute at a given temperature rise. It is a pipeline: it never runs out, but it can only heat so much at once, forever.

Continuous flow: the question is how many outlets at once

Because a continuous flow unit cannot run out, the failure people actually experience is different. You do not get a cold shower at the end. You get a shower that goes weak or cool because someone else turned on a tap.

So the sizing question is simultaneity. How many hot outlets can plausibly run at once on a bad morning? A modern water-efficient showerhead uses a fairly modest flow. Two showers together needs roughly double. Add a kitchen tap and you need a meaningfully larger unit again. A one-bathroom household with staggered showers has completely different requirements to a two-bathroom house where four people leave at 7:30am.

The trap in the numbers is temperature rise. A flow rating means nothing without the rise it was measured at, because heating water takes energy in proportion to how far you lift it. The same unit that looks generous on a 25 degree rise looks much smaller on a 40 degree rise. That matters here because incoming mains is far colder in July than January, and colder again in Canberra or Tasmania than Darwin. A unit sized on summer performance disappoints in winter, exactly when the household notices. Insist on figures quoted at the same rise, and size for your winter mains.

Storage: size for peak demand, not the daily total

Tanks get sized by household size in most quoting conversations, a reasonable starting point, but the number that decides whether a tank works is peak demand rather than daily consumption. A household that spreads usage across the day lives comfortably on a tank that would fail a household using the same total volume in a 45 minute burst before school.

The second variable is recovery rate, how fast the tank reheats after you empty it. A gas storage tank with a burner underneath recovers considerably faster than an electric tank with a resistive element, which is why a gas tank can be physically smaller than an electric one serving the same family.

Then there is the tariff, the biggest sizing lever on electric storage. A tank on a controlled load or off-peak tariff only heats overnight and does not recover during the day at all, so it has to hold a full day of hot water in one go. That is why off-peak tanks are noticeably larger than continuous tariff tanks serving the same household. If you inherit an off-peak system and it runs out at 7pm, the honest diagnosis is often that the tank is undersized for the tariff rather than broken. Rule out a failed element or thermostat first, because a twin-element tank running on one dead element behaves exactly like a tank that is too small. Replacements turn up under electric storage and gas storage listings.

The trade-offs nobody mentions in the showroom

Each technology has an annoyance that only shows up once you live with it.

• Standing loss on storage. A tank holds hot water around the clock and slowly loses heat to the room, so you pay to reheat water you never used.
• The cold water sandwich on continuous flow. Stop a tap and start it again shortly after, and the slug sitting in the pipe and heat exchanger arrives at a different temperature to what follows. A brief cold patch mid-shower, inherent to the design.
• Minimum activation flow. The burner needs a certain flow before it fires. Crack a tap open gently, as people do when shaving, and you get cold water because the unit never woke up.
• Dead leg and lag. Every system waits while the pipe run clears, but continuous flow adds ignition time on top, and a long run to a distant bathroom makes it worse whatever you buy.

Gas continuous flow has real install constraints

A large continuous flow unit is a substantial gas appliance drawing a serious rate when it fires, so gas supply and line sizing genuinely matter. An older property with an undersized run to the appliance location can be perfectly capable of running a cooktop and completely incapable of running a big instantaneous heater. Flueing, clearances from windows, vents and boundaries, and condensate handling on condensing models all have to be right.

LPG and natural gas units are not interchangeable. They are built for different supply pressures and injectors, and running the wrong one is dangerous rather than merely inefficient. Moving from bottled gas to reticulated means a different appliance, not a setting.

All of this is licensed work. Hot water installation is plumbing, and anything touching gas requires a licensed gasfitter in Australia. Gas sizing is exactly the question to put to a professional before you buy rather than after the unit arrives, so find a hot water plumber and get the supply assessed first. Units show up regularly under continuous flow gas listings, alongside the full hot water listings.

Choosing between them

One detail applies whichever you pick. Bathroom outlets in Australian homes are limited to a maximum of 50 degrees, so the water arriving at the shower is not the water leaving the heater. If temperature rather than volume is your real complaint, the problem may not be sizing at all, and the tempering valve and the 50 degree rule is worth reading before you spend money on a bigger unit.

Otherwise, continuous flow suits households that want endless hot water, have the gas supply to feed it, run a predictable number of outlets at once, and do not want a tank taking up space. Storage suits heavy simultaneous demand, properties without adequate gas, anyone on a controlled load tariff, and any system where the heat source is intermittent, which is why solar and heat pump systems are tank-based.

If you are still choosing the technology rather than the size, start with the comparison of electric, gas, heat pump and solar. If you are buying secondhand, run through the used hot water system checklist first, because a bargain unit with the wrong flow rating is not a bargain.