How to Flush a Hot Water System, and When Not To

On an Australian mains pressure storage tank, flushing means draining the cylinder so sediment comes out with the water, and the manufacturers reserve draining for a qualified person. Rheem's owner's guide for its electric storage range says only a person qualified to install or service a water heater can drain it, if that is required. Solahart's thermosiphon solar owner's guide and Rheem's guide for its 565 series heat pump carry the same line.

That is the main gap in much of the flushing advice online, which is written for North American water heaters. Rheem's own draining procedure involves lifting the relief valve lever to release the pressure in the tank, then disconnecting the cold water connection at the inlet, and Rheem warns the water that comes out may be very hot. That is plumbing work on a pressurised, hot cylinder, so this guide does not give draining steps. It covers when a flush is worth asking for, which systems it does not apply to, and the one routine check an owner can do.

Flushing is a tank job, so it only applies to hot water systems that keep water in a tank. energy.gov.au splits systems into two groups: storage systems, where water is stored in a tank and kept hot, and instantaneous systems, where water is heated only as required and not stored in a tank.

Electric storage and gas storage heaters are storage systems. So are solar and heat pump systems: a solar system includes a storage tank on the roof or at ground level, and both Solahart's thermosiphon guide and Rheem's 565 heat pump guide list flushing the tank, if required, as part of the five-yearly major service.

A continuous flow unit has no tank to flush. energy.gov.au says continuous flow, also called instantaneous, is the most common type of gas water heater, so plenty of gas households have nothing to drain. If yours is continuous flow, the tank advice does not apply, and the owner's guide for your model sets its maintenance.

A flush earns its place when sediment is actually getting into the tank. Solahart says a solar storage tank may need draining or flushing during maintenance or if there is sediment in the water supply, and its five-yearly major service includes draining and flushing the tank of sediment build-up if required. Rheem's 565 heat pump guide lists flushing the tank, if required, in its major service too.

The water supply decides how likely that is. Rheem's 565 heat pump guide recommends a filter on the cold water line where the supply contains, or may come to contain, suspended solids, and says its warranty does not cover faults on the cylinder or other components caused by sludge or sediment settling in the tank. The QBCC adds that systems relying on bore water or rainwater may need servicing more regularly than those on town water. On a property with its own rainwater tank or bore, that is a good reason to ask about sediment at every service rather than wait for a fault.

A flush is aimed at sediment, and it is not the manufacturers' answer to every deposit. In a scaling water supply, Rheem explains, calcium carbonate comes out of the water onto any hot metal surface. Its major service for electric storage tanks checks the heating unit for excessive calcium build-up or corrosion and replaces it if necessary, so the remedy it lists for a scaled element is replacement, not a flush.

Rheem's major service list for that electric range covers replacing the relief valve, inspecting the anode, checking the heating unit, inspecting connections and a visual check for other problems. It does not list a routine tank flush, though it notes the heater may need to be drained during the service. So a yearly flush is not something Rheem asks for on that range. Whether a particular tank needs one is a judgement for the licensed plumber doing the service, based on the system, its age and the water supply it runs on.

The routine job an owner can do is not a flush. It is operating the easing lever on the temperature pressure relief valve. Rheem's guide recommends minor maintenance every six months and says the dwelling occupant can do it, and the lever is operated to clear any deposits and make sure the valve and its drain line are not blocked. The QBCC agrees that home owners and tenants can operate the valve themselves, and notes that doing it at regular intervals helps the valve keep working and removes metal ions that can build up in the water close to it.

Three cautions come with it. Raise and lower the lever gently, because Rheem warns that letting it snap back damages the valve seat. The water from the drain line will be hot, so stand clear of where it comes out. And make sure the valve has closed properly afterwards, as the QBCC asks. If water does not flow freely from the drain line when the lever is lifted, Rheem says the heater must be checked, so book a plumber.

Roof-mounted solar adds height to the risks. Solahart asks owners to operate the relief valve lever only if it is readily and safely accessible, and not to climb onto a non-trafficable roof to reach it. If it cannot be reached safely, Solahart says to contact your nearest Solahart dealer. A licensed plumber can also check it at a service.

Draining a solar tank has its own timing. Solahart says it should be done in the morning, within three hours of sunrise, when the collectors have had less time to heat up, because the water discharged may be very hot. It also warns that the drained water may contain sediment and should be diverted away from any section of roof that is part of a rainwater collection system. On a property that drinks from its rainwater tank, that is one more reason a solar flush belongs with a licensed plumber who can plan where the water goes.

In Queensland, owner maintenance on a hot water tank stops at the relief valve lever. The QBCC says most plumbing work in Queensland is regulated and needs a QBCC licensed plumber, and the unregulated jobs it gives as examples are things like replacing a shower head or a tap washer. Draining a mains pressure tank, disconnecting its cold water connection or replacing the relief valve is not on that list. The manufacturers say the same in their own terms: only a person qualified to install or service a water heater can drain it.

Gas storage heaters add a second licence. Business Queensland says a licence or authorisation is needed before any gas work in Queensland, whether the work is residential, commercial or industrial, so anything touching the gas side of a gas storage heater is a job for a licensed gas fitter. If you think your tank needs flushing, this is a job for a licensed plumber.

There is no single flushing interval for Australian hot water systems, because the manufacturer of each one sets the schedule. The QBCC tells home owners to follow the manufacturer's recommendations for servicing, and says it is generally recommended that hot water systems are serviced every five years by an appropriately licensed person. Rheem and Solahart both recommend a major service every five years by a qualified person, with six-monthly minor maintenance the occupant can do in between. Rheem's heat pump guide and Solahart's thermosiphon guide list a tank flush as 'if required', not as a fixed step.

So the practical answer is to find the owner's guide for your model, read its maintenance section, and book the major service it asks for. If your supply is bore water or rainwater, the QBCC says servicing may be needed more often. At that service, ask whether the tank needs draining and flushing. If the system is old and other faults are adding up, the service is also the point to weigh repairing against replacing.