Water quality, scale and bleed-off rates

Evaporative cooling is a water technology that happens to move air. Every litre that leaves the pads as vapour leaves its dissolved minerals behind. The tank then becomes a saucepan left to boil. Bleed-off is the controlled way of throwing some of that concentrating water away before it plates onto pads, pumps and tanks. Households that close the bleed to "save water" are saving the way a farmer saves rain by never opening a drain: the paddock still salts up.

The numbers that Australian utilities and consumer reporting have been willing to print are not marketing. Bleed-off on many existing systems sits around 11 to 20 litres an hour. Some newer designs get near 3 litres an hour. Water Corporation in Perth offers a test you can do with a bucket: half full in 30 minutes is about 11 litres an hour. If your hose beats that without trying, you are not bleeding. You are irrigating the roof.

Why bleed exists

How evaporative cooling works is evaporation. Evaporation is distillation. Pure water leaves. Everything else stays. In a closed tank the remaining water gets harder by the hour. Calcium and magnesium drop out as scale. Other salts stay dissolved until the brew is nasty enough to smell, to corrode fittings, and to glaze the pads so air cannot pass.

Bleed takes a fraction of the tank water to waste and lets fresh mains water make up the volume. The tank stays closer to the hardness of the supply instead of climbing towards a mineral slurry. That is the whole theory. It is also why a dump cycle on some cabinets — a periodic full or partial drain — is doing the same job in a batch instead of a trickle.

There is no clever setting that evaporates water without concentrating minerals. There is only how much you dump, how hard the incoming water is, and how often you clean what the dump did not catch.

11 to 20 litres an hour versus 3

The older, larger figure is what a lot of Australian roofs are still doing, sometimes by design and sometimes because a hose was left as found in 2004. Eleven to 20 litres an hour, over a six-hour afternoon, is 66 to 120 litres of bleed on top of whatever evaporated. That is real money in a metered city and a real volume in a town already watching the standpipe.

Three litres an hour is the newer claim: a tighter dump, often managed by a control that knows tank conductivity or at least a more miserly orifice. It is not magic. It still dumps. It just dumps less, which only remains safe if the incoming water is not a mineral soup and the tank still gets a proper flush when it needs one. A 3 litre setting on water that should have been at 15 will scale the pads in a season. A 20 litre setting on already-soft water will waste a resource for no gain.

Do not take the sticker as a measurement. Measure. The bucket test is crude and good. A known bucket, a watch, the cooler running as it would on a hot day, bleed hose into the bucket, not into a gutter where you cannot see it. Half full in 30 minutes ≈ 11 L/h. Scale from there. If you fill 10 litres in 10 minutes, you are at 60 litres an hour and you have a problem, a misadjusted valve, or a dump stuck open.

Water Corporation's Perth advice exists because Perth is the city where evaporative cooling, hot weather and a political water supply share a long history. Around 100 litres an hour can evaporate on a hard afternoon; at six hours a day for 50 days that is around 30,000 litres a year from evaporation alone. Bleed sits on top. The running costs in electricity and water are not a Perth-only story, but Perth is where the litres were counted in public. CHOICE, citing a 2009 study, reported 41 to 70 litres an hour in Hobart and 67 to 116 in Adelaide. NSW Planning's band of about 10 to 30 litres an hour describes milder or more humid running. Bleed is additional. Anyone quoting a single "the cooler uses X" without saying evaporation versus dump is not helping you.

Hard water, scale and the pads

Hard water is not a moral failing of a town. It is geology. It is also the reason two identical coolers, one in a soft-water pocket and one on a limestone bore, have different pad lives. Scale shows as a white or grey crust, a pad that rings stiff when you tap it, and airflow that has to find cracks. Evaporation falls because the wetted area has become a mineral plate.

Replacing cooler pads every one to three summers is common where the water is hard and the bleed has been timid. Softer supplies and a competent dump stretch that. No pad lasts if the tank is allowed to go brine. Rinsing delays the inevitable on light dusting; it does not reverse a glazed face. Once the media is a crust, replace it. Running a glazed pad because it is "still wet" is running a wet rock.

Bores, rainwater and mains are not interchangeable feeds. Rainwater is usually soft and kind to pads, then suddenly not if the tank is dirty or the first flush after a dry spell is full of roof grit. A bore can be very hard, or iron-rich, or both; iron stains and a metallic sludge are their own maintenance file. Mains water is whatever the utility sends, which in much of Australia is hard enough to matter. If you mix supplies, the tank still concentrates whichever minerals you gave it.

Dump cycles, continuous bleed and what to watch

Some cabinets trickle all the time the pump runs. Some dump a volume on a timer or a conductivity trigger, then refill. Some do both. A dump cycle that never fires is a continuous-bleed system with a stuck valve, or a "smart" control that has not been told the water is hard. A dump that fires every few minutes is a leak with a schedule.

Watch the overflow. Continuous overflow is not bleed; it is a float that is not closing or a dump that is not shutting. Watch the garden. A lawful, directed bleed can water something on purpose. An undirected one waters the eaves, the neighbour's fence and, eventually, a ceiling cavity.

Winter is the easy way to waste this whole arrangement. Leave the inlet live, the float weeping, the bleed dripping into a blocked gutter, and you have a four-month leak that has nothing to do with cooling. Isolate. Drain if the climate freezes or the tank will otherwise sit foul. Cover the unit. The water conversation starts again in spring, when you uncover and repeat the bucket test, because last year's orifice is not a contract.

Electricity remains the cheap meter on these machines. Water is the one that gets away. A well-bled cooler in a climate that suits it — Perth, Adelaide, inland Victoria, the New South Wales slopes, the Queensland west — still makes economic sense against refrigerated air on a dry scorcher. A cooler bleeding like a fire hose, or scaling because someone pinched the dump to zero, makes sense to nobody except the person selling new pads.

Set a bleed you can measure. Match it, roughly, to the hardness you actually have. Clean what the dump does not catch. Replace media that has become geology. That is water quality as a household practice, not as a brochure panel. The tank will still concentrate. The job is to stop it concentrating faster than the pads can survive.