How long does a gas instantaneous hot water system last?

Many installers put a well-maintained gas continuous flow unit at around 15 to 20 years, depending on water quality, how hard it works and whether it is serviced. Units near the coast can wear faster because salt air attacks the casing and fittings. Your owner's manual lists the warranty and service intervals for your model.

Can a new instantaneous unit go into my existing recess box?

Often, yes, if the new model is listed as compatible with that box. Rinnai publishes which heaters fit each of its recess boxes. If the box is damaged or not compatible, it can be replaced, but reusing a sound box keeps the job simpler and avoids changes to the wall.

Should I repair or replace an old gas hot water unit?

It depends on the fault and the age. A failed part on a younger unit is usually worth repairing. On an older unit, especially one with corroded fittings, a repair can be followed by another fault soon after. In our experience, it is worth weighing the repair cost against the unit's remaining life before deciding.

Do I need a licensed gas fitter to replace a gas hot water system?

Yes. In NSW, only licensed gasfitters can replace or relocate gas appliances, including a gas water heater. When the work is finished, the gasfitter must give you a certificate of compliance for network gas, or a certificate of inspection for bottled gas.

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Case study · 5 min read

Gas instantaneous hot water, Clovelly

A Clovelly house lost its hot water when its old Rinnai Infinity 26 failed. Adam replaced it with a new Rinnai Infinity B26 in the same wall recess box.

Adam Norton · 26 October 2026

Adam from Norton Plumbing giving a thumbs up beside a new Rinnai Infinity B26 gas instantaneous hot water system in a wall recess box at a Clovelly house

The call from Clovelly was simple: no hot water. The house had a gas instantaneous hot water system, a Rinnai Infinity 26, set into a recess box in the side wall. The unit had failed and was no longer heating at all, and the brass valves and fittings underneath it were green with corrosion. Rather than chase a repair on a failed unit with tired fittings, Adam recommended replacing it.

Adam from Norton Plumbing working on the old Rinnai Infinity 26 gas instantaneous hot water system with its cover off in a wall recess box at a Clovelly house — The old Rinnai Infinity 26 with its cover off, before Adam disconnected it. The corroded valves and fittings are underneath the unit.

What a gas instantaneous hot water system is

A gas instantaneous system, also called continuous flow, heats water only when a hot tap is turned on. There is no storage tank. Cold water runs through a heat exchanger, a gas burner heats it on the way through, and it comes out the other side hot. Because there is no tank to empty, it does not run out of hot water partway through a shower the way a storage system can.

Units like the Infinity are often installed in a recess box, a steel frame set into the outside wall so the heater sits flush with the house rather than hanging off it. It is tidy, and it keeps the unit out of the way along a narrow side path.

Replacing the gas instantaneous hot water system

Adam turned off the gas and water, took the cover off the old unit and disconnected it. He lifted it out of the recess box and carried it out to the truck.

The replacement was a new Rinnai Infinity B26, Rinnai's current model in the same size range, which Rinnai lists as compatible with its recess boxes. Adam hung it in the existing box, so there was no change to the wall.

The old connections underneath had corroded, so Adam re-piped them in new copper, with new isolation valves for the gas and water. He joined the copper with press fittings, made with a hydraulic press tool rather than a flame, and wiped down every joint.

Adam from Norton Plumbing using a press tool on new copper connections under a Rinnai Infinity B26 gas instantaneous hot water system in Clovelly — Adam using the press tool on the new copper connections under the Rinnai Infinity B26.

He then turned the water back on at the meter, checked the joints for leaks, fired up the new unit and tested it for hot water at the taps before fitting the cover. The house had hot water back the same day.

Gas hot water in Clovelly homes

Clovelly's semis and freestanding houses sit close together, often with only a narrow side passage between them. That is why so many gas instantaneous units here are recessed into a side wall: there is rarely room to mount anything that sticks out. The salt air close to the water is also hard on exposed brass and copper, and the green corrosion on this unit's old valves is typical of fittings that have been outside near the coast for years.

If your recess box has been there a while, ask your plumber to check the fittings underneath at the same time as the heater. Replacing tired valves and pipework along with the unit saves a second visit later.

We have written about another Clovelly gas hot water job, where bogong moths jammed the fan of a Rinnai in an apartment roof cavity. See Bogong moths in hot water unit, Clovelly.

Signs your gas instantaneous hot water system is on its way out

An error code on the controller that keeps coming back after you reset it.
Hot water that fluctuates between hot and lukewarm while the tap is running.
The burner taking longer to fire when you turn on a hot tap, or a new rumbling or roaring noise.
Green or white corrosion on the valves and fittings under the unit, or water dripping inside the recess box.

If you notice any of these, turn the hot water off at the controller and have a licensed gas fitter look at it. Do not take the cover off a gas appliance yourself. The NSW Government's guide to using a gasfitter explains that only licensed gasfitters can replace gas appliances in NSW.

Contact Norton Plumbing

Norton Plumbing has repaired, serviced and replaced gas and electric hot water systems across the Eastern Suburbs since 2019, from Clovelly and Coogee through to Bondi, Randwick and Maroubra. Adam Norton is the primary plumber on the tools and holds NSW plumbing licence 397768C. Phone 0477 858 951. Based at 10/11a-15 Berwick Street, Coogee NSW 2034. See our hot water service page or our Clovelly plumber page.

Frequently asked

Common questions

How long does a gas instantaneous hot water system last?: Many installers put a well-maintained gas continuous flow unit at around 15 to 20 years, depending on water quality, how hard it works and whether it is serviced. Units near the coast can wear faster because salt air attacks the casing and fittings. Your owner's manual lists the warranty and service intervals for your model.
Can a new instantaneous unit go into my existing recess box?: Often, yes, if the new model is listed as compatible with that box. Rinnai publishes which heaters fit each of its recess boxes. If the box is damaged or not compatible, it can be replaced, but reusing a sound box keeps the job simpler and avoids changes to the wall.
Should I repair or replace an old gas hot water unit?: It depends on the fault and the age. A failed part on a younger unit is usually worth repairing. On an older unit, especially one with corroded fittings, a repair can be followed by another fault soon after. In our experience, it is worth weighing the repair cost against the unit's remaining life before deciding.
Do I need a licensed gas fitter to replace a gas hot water system?: Yes. In NSW, only licensed gasfitters can replace or relocate gas appliances, including a gas water heater. When the work is finished, the gasfitter must give you a certificate of compliance for network gas, or a certificate of inspection for bottled gas.

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