Bosch Hot Water System Repairs Adelaide: What Actually Goes Wrong

Nearly every Bosch hot water system in Adelaide is a continuous flow gas unit rather than a tank, so it fails in continuous flow ways: ignition and flame faults, a blocked inlet water filter, a scaled heat exchanger, or a flow rate too low to fire the burner at all. Most of those are a repair, not a replacement. A Bosch that still lights and holds temperature is worth fixing well past 10 years. The failure that usually ends one is a scaled or split heat exchanger, because that single part costs close to a whole new unit.

Which Bosch you actually have

Bosch built its Australian name on continuous flow gas, and that is what hangs on the wall of most Adelaide homes carrying the badge. There is no tank and no stored water. Cold water passes through a heat exchanger, the burner fires while the water is moving, and everything shuts down when you close the tap. No thermostat, no anode, no tank to rust through. The model family decides the rest.

Hydropower models generate ignition power from the water flowing through them. No power point, no lead, and they keep producing hot water through a blackout, which is why they turn up so often in the Adelaide Hills.
Electronic ignition models need a 240V outlet. After a storm, check the outlet and the circuit breaker before blaming the appliance.
External units vent straight to atmosphere. Internal units depend on a clear flue, and condensing models add a condensate drain that locks the unit out when it blocks.
Natural gas and LPG units are not interchangeable. Moving one to a bottled gas property in the Hills means a conversion by a licensed gas fitter, not a different regulator.

The faults that stop a continuous flow unit

The list is short and predictable. The good news is that the heat exchanger is the part you least want to replace, and the heat exchanger is rarely what has failed.

Ignition or flame failure. Fouled electrodes or a dirty flame sensor: the unit clicks, tries, and locks out. On LPG properties an empty cylinder gives an identical symptom.
A blocked inlet water filter. The strainer on the cold inlet collects Adelaide scale and sediment, choking flow below the rate needed to fire the burner.
A sticking water flow sensor. The turbine that tells the board water is moving gums up, so the burner never gets the signal.
Scale in the heat exchanger. Output drops, the unit runs hotter to compensate, then trips the overheat lockout. Progressive, so it is worth catching early.
Fan or air pressure faults. A failing fan, a blocked flue terminal or a bird nest will stop the unit lighting for safety reasons, which is the system working correctly.
Hydro-generator failure on Hydropower models. A unit that never needed power suddenly produces no hot water and no sound at all.

Gas is a licensed trade in South Australia. Marc is a licensed plumber and gas fitter with 20 years behind him, so the burner, gas valve, flue and combustion side get assessed on the same visit as the water side rather than in 2 visits and 2 opinions.

What Adelaide does to a continuous flow unit

Adelaide mains water is hard by Australian standards, and hardest when the metropolitan system draws more heavily on the River Murray. SA Water publishes a figure for every supply zone and it is worth looking yours up. Hardness matters far more here than to a tank: a tank drops its sediment to the bottom where it sits harmlessly, while a continuous flow heat exchanger pushes every litre through narrow waterways at high temperature, which is exactly the condition that deposits calcium on the wall.

The Hills add their own. Much of Blackwood, Belair and Stirling has no reticulated gas, so the LPG cylinder becomes part of the diagnosis, and plenty of properties run rainwater to the house. A continuous flow unit needs a minimum flow to fire, so a cycling pressure pump, a blocked tank outlet strainer or a filter overdue for a change reads as a hot water fault and is actually a supply problem. In a frost pocket, an external unit with no power connection has limited freeze protection, and the answer is insulation or draining down over an empty winter.

The Bosch problems that are not faults

The tap is barely open. Continuous flow units need a minimum flow, commonly around 2.5 litres a minute, before the burner lights. A trickling basin tap or a silted aerator never triggers it.
The cold slug between draws. Turn a hot tap off and back on and you get a burst of cold that was sitting in the pipe. That is the cold water sandwich, it is inherent to the technology, and there is nothing to repair.
The mixer is doing the work. Set the unit to the temperature you want and run the tap on full hot rather than blending cold in at the tap.
Every gas appliance is out, not just the hot water. That is a supply, meter or cylinder problem, and the water heater is only the messenger.

The same honesty runs the other way. Descaling a heat exchanger that has never shown a temperature drop or an overheat lockout is work you do not need, and we will tell you that rather than book it in.

Repair or replace

A well-installed continuous flow unit commonly reaches 15 to 20 years, where a gas storage tank is finished between 8 and 12. Replacing a 10 year old Bosch because it will not light throws away half its service life. Repair the ignition, flame sensor, flow sensor, inlet filter, fan, thermistor or hydro-generator, because all of those return the unit to full output. Replace it when the heat exchanger has scaled beyond recovery or split, when control board parts for a discontinued model have dried up, or when the installation no longer meets clearance requirements where it stands.

The useful comparison is the repair against the replacement over the remaining service life, not against doing nothing. An ignition repair on a 9 year old Bosch buys years on a unit already installed, already compliant and already sized for the house. The same repair on a 19 year old unit with a scaling heat exchanger is a job you will be doing again shortly. Whichever way it goes, the price is agreed with you up front, before any work starts.

Other hot water pages

Heat Pump Hot Water: Heat pump hot water supply, siting and repair across Adelaide. What the running cost really is, where the unit can sensibly go, and when a heat pump is not worth the change.
Solar Hot Water: Repairs and diagnosis for solar hot water across Adelaide and the Hills: boosters, tempering valves, pumps, controllers, frost protection and evacuated tubes, plus a straight read on when the system is finished.
Rheem Hot Water Repairs: Repairs to Rheem electric storage, gas storage and continuous flow units across Adelaide, with a straight answer on when the cylinder is beyond saving.
Rinnai Hot Water Repairs: Diagnosis and repair of Rinnai Infinity continuous flow units across Adelaide. Fault codes read properly, gas and water sorted on the same visit by a licensed gas fitter.
Dux Hot Water Repairs: Repairs to Dux electric, gas, solar and heat pump hot water units across Adelaide. We read the compliance plate first, because on a Dux it settles the anode, warranty and parts questions at once.
Vulcan Hot Water Repairs: Gas storage repairs by a licensed gas fitter. Pilot lights that will not hold, tripped cut-outs, failed gas control valves and anode checks on Vulcan tanks across Adelaide.
Aquamax Hot Water Repairs: Electric storage diagnosis on Aquamax tanks across Adelaide. Elements, thermostats, cut-outs, relief and tempering valves, plus a straight answer on stainless tanks and anodes.

Questions we get asked

Why does my Bosch go hot, then cold, then hot again?

If it happens when a tap is turned off and straight back on, that is the cold water sandwich and it is normal for every continuous flow unit. If it happens continuously while the tap runs, the cause is mechanical: a flow sensor reading intermittently, scale tripping the overheat cut-out, or a supply fluctuating around the minimum flow rate. Rainwater on a pressure pump is the common Hills version of the last one.

Do I need power for a Bosch hot water system?

It depends on the model. Hydropower units generate their own ignition current from the water passing through them and need no power connection, which is why they suit Hills properties and anywhere blackouts are common. Electronic ignition models need a 240V outlet and stop the moment power is lost. If yours has a plug and lead, check the outlet and breaker before assuming the appliance has failed.

Is it worth repairing a Bosch continuous flow unit or should I replace it?

Repair it if the fault is ignition, flame sensing, the flow sensor, the inlet filter, the fan or the hydro-generator, because those are serviceable and the unit returns to full output. Replace it if the heat exchanger has scaled past recovery or split, since that single part costs close to a whole new unit. Continuous flow units commonly reach 15 to 20 years, so age alone is a weak argument at 10.

My Bosch will not light on the bathroom tap but works fine in the kitchen.

That is a flow rate problem rather than an ignition problem. The burner needs a minimum flow before it fires, and a silted aerator, a restricted mixer cartridge or a partly closed isolation valve can hold one outlet below it while another sits above it. Clean the aerator first. If every outlet has faded together, the inlet filter on the unit is the likelier cause.

Can a Bosch continuous flow unit run off a rainwater tank?

Yes, and plenty of Hills properties do, but the pump has to deliver enough flow and steady enough pressure to hold the burner on. A cycling or undersized pump, a blocked tank outlet strainer or a filter overdue for a change pushes delivery under the minimum activation flow and gives hot water that comes and goes. Rainwater is soft, which is genuinely good news for the heat exchanger.

How long should a Bosch hot water system last in Adelaide?

Commonly 15 to 20 years. The 2 things that shorten it here are hard water scaling the heat exchanger and, near the coast, salt air corroding an externally mounted casing and flue terminal. Neither is inevitable on a unit installed with proper clearance and serviced when it shows a symptom rather than on a sales schedule.