Instantaneous (Continuous Flow) Hot Water
Endless hot water from a unit the size of a briefcase: we install, service and repair gas and electric instantaneous systems across the Northern Beaches and North Shore.
What is an instantaneous hot water system?
An instantaneous (continuous flow) system heats water on demand as it flows through the unit, instead of storing pre-heated water in a tank. Because there's no tank to empty, you never run out of hot water, and because it only heats what you use, there are no standing heat losses. Ready Plumbing installs and repairs both gas and electric instantaneous systems.
Continuous Flow
Endless hot water, no tank
Ready Plumbing installs, services and repairs instantaneous hot water systems across the Northern Beaches and North Shore. Also called continuous flow, these compact wall-mounted units (from brands we trust like Rinnai, Rheem and Thermann) heat water the moment you open the tap and keep heating for as long as it runs.
Best for:
- Families tired of the last shower running cold.
- Homes short on space: the unit mounts on a wall and the old tank footprint is freed up.
- Households with variable or low usage, where a storage tank wastes energy keeping water hot all day.
Common Fault
The stop-start problem (and why it happens)
The most common complaint we hear about instantaneous units: the hot water cuts in and out mid-shower. These heaters only ignite when enough water flows through them to trip the internal flow switch. When flow drops below that threshold, even briefly, the burner shuts off.
In Our Experience
Most likely causes, in order
Blocked inlet filters
They clog up regularly and choke the flow. Cleaning them is quick and often fixes the problem outright.
Worn or restrictive tapware
Dropping flow below the switch threshold stops the burner igniting.
A tired flow switch
Or another minor internal part. Most are an easy swap rather than a unit replacement.
Check The Whole System
Sometimes it's not the heater, it's the valves
Sometimes the fault isn’t in the heater at all but in the associated valves. Tempering valves and pressure limiting valves can mimic heater faults when they fail. Our diagnostics check the whole system before we replace anything. If a unit is genuinely at end-of-life, replacements are often a straightforward swap-over. See our repairs service: we’re available 24/7.
Sizing
Sizing it right
Continuous flow capacity is rated in litres per minute, and the right size depends on how many outlets run at once, not how many people live in the house. As a guide: one bathroom, 16–20 L/min; two bathrooms, 26 L/min; three or more, 32 L/min or twinned units. Gas supply also matters: bigger units need adequately sized gas lines, which we check before quoting. More detail in what size hot water system do I need.
Why Choose Us
Continuous flow, sized and installed right
Sized to your household
Not just the cheapest unit; we size to outlets used at once so you don't get the stop-start problem.
All major brands
Rinnai, Rheem, Thermann and other trusted continuous flow units, installed and serviced.
Diagnostics before replacement
We check filters, tapware, flow switches and valves before recommending a new unit.
24/7 repairs
Genuine around-the-clock service if your continuous flow unit stops working.
Compare Your Options
Comparing against a tank or a heat pump?
Start at the hot water overview, or compare storage systems and heat pumps.
Last reviewed: 8 July 2026 by the Ready Plumbing team.
FAQ
Instantaneous Hot Water: your questions answered
Plumbing problems can be extremely frustrating. Our licensed Sydney plumbers are here to provide fast, reliable answers and solutions.
Why does my instantaneous hot water keep stopping and starting?
These units are notorious for it, and the cause is usually water flow: the internal flow switch needs a minimum flow rate to keep the burner ignited. Partially blocked inlet filters are the most common culprit, and they clog up regularly but are quick to clean. Low-flow taps and worn parts can also contribute. It's usually a simple, inexpensive fix.
Is instantaneous hot water cheaper to run than a storage tank?
Generally yes for gas units, since you're not paying to keep a tank of water hot around the clock. The saving is most noticeable in smaller households where a storage tank spends most of its day just maintaining temperature.
Can an instantaneous unit run more than one shower at once?
Yes, if it's sized correctly. Units are rated in litres per minute, and a 26 L/min unit comfortably runs two outlets at once. Undersizing is the main cause of disappointing continuous flow systems, which is why we size to your household rather than defaulting to the cheapest unit.
Where does the unit get installed?
Usually on an external wall near the highest-use outlets (kitchen or bathrooms). Internal installations are possible with proper flueing. The compact size frees up the space your old tank occupied.
Do instantaneous systems need servicing?
Light servicing keeps them reliable, mainly cleaning the inlet filters and checking the burner and valves. Well-maintained units commonly last 15–20 years, roughly double a steel storage tank.
Guides & Real Jobs
Helpful reading from our plumbers
Guide
Gas vs Heat Pump Hot Water: What's Cheaper to Run?
Replacing a hot water system and stuck between gas and a heat pump? Here's how the running costs, upfront prices and rebates actually compare for Sydney homes.
Guide
Signs Your Hot Water System Is Failing
We keep hearing the same question from clients across the Northern Beaches and North Shore: what are the signs of a failing hot water system? Here's what 20 years of call-outs has taught us to look for.
Guide
What Size Hot Water System Do I Need?
Undersize a hot water system and the last shower runs cold; oversize it and you pay to heat water nobody uses. Here's how to size storage tanks and continuous flow units properly.
Case Study · Dee Why
No Hot Water in Dee Why: Fixed Within the Hour
A Dee Why home lost hot water when their gas system wouldn't re-ignite. Our emergency call-out found a failed thermocouple and had hot water back within the hour.
Get In Touch
Contact Your Local Plumber in Sydney
Ready Plumbing is a Sydney-based and fully-licensed plumbing business with more than 20 years of experience, offering a range of residential and commercial plumbing solutions and emergency plumbing 24/7. Get in touch and we'll respond promptly.
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