How Solar Hot Water Works

A solar hot water system heats water with the sun's energy and keeps it in a tank. According to the Australian Government's Your Home guide, water is heated as it passes through roof collectors and then flows into an insulated storage tank.

The Clean Energy Regulator describes the parts as a storage tank, a heat exchanger, a controller and a backup heater. That backup heater is the booster, and it is the part most people forget about. The collectors do the bulk of the work on a sunny day; the booster covers what they cannot.

Your Home puts the ceiling at up to 90 per cent of a household's hot water coming from the sun, depending on climate. That figure is an upper limit, not a promise. Where a particular system lands depends on the roof, the shade, the household's habits and how the booster is controlled, which is what the rest of this guide works through.

Solar collectors come in two common types: flat plate panels and evacuated tubes. The Victorian Government's solar hot water buyers guide says evacuated tubes tend to work better through the cooler months in colder climates, and that in warmer climates either type performs well.

The Sunshine Coast sits at the warm end of that range. Bureau of Meteorology records for Sunshine Coast Airport put the average July minimum at 9.7 degrees, and July is the coldest month there. So on the coast itself, the choice between flat plate and tubes usually comes down to the product, the roof and the price rather than a big difference in winter output.

Collectors also differ in what runs through them. Your Home explains that in an open circuit system water flows straight through the collectors, while in a closed circuit system a separate fluid collects the heat and passes it to the tank through a heat exchanger. Closed circuit designs protect against frost, but that fluid has to be checked and replaced over time.

Solar hot water systems are either close-coupled, with the tank on the roof directly above the collectors, or split, with collectors on the roof and the tank on the ground. Your Home lists both, each available with an electric or gas booster.

The tank's position drives several other trade-offs. A roof tank is heavy: the Victorian guide notes that a 300 litre tank can weigh as much as 420 kilograms once the tank itself is counted, so the roof has to be inspected and its capacity assessed. A split system puts only the collectors on the roof, which is a light load.

Split systems carry their own costs. Your Home says pumped systems are usually more expensive to buy, need more maintenance and use some energy for pumping. The Victorian guide adds that the longer pipe runs between collectors and tank can lose heat. A close-coupled system keeps everything together, but servicing a tank on the roof is harder. The table below sets the two side by side.

On a close-coupled system the water circulates by itself. Your Home describes this as a passive, or thermosiphon, system: because the tank sits above the collectors, cooler water sinks down into the collectors, warms in the sun and rises back into the tank, creating a steady flow without a pump.

A split system cannot rely on that, because the tank is below the collectors. The Victorian guide explains that a temperature differential controller reads sensors at the top of the collectors and the bottom of the tank, and switches the pump on when there is heat to be gained from the roof.

That difference matters when something goes wrong. A thermosiphon system has fewer parts to fail. On a split system, a stopped pump or a faulty controller means the collectors stop contributing, and a thermostat-controlled booster ends up doing the heating instead. You can still have hot water, so the first sign may be a higher power or gas bill rather than a cold shower.

The Victorian buyers guide notes that virtually every system on the market has some form of boosting, and how often the booster runs is what sets the running cost. Your Home says most systems come with a gas or electric booster to provide hot water on cloudy days or when demand outstrips supply, and that boosters can be switched manually or controlled by a thermostat that cuts in when the tank cools below a set point.

Timing is the catch. Your Home points out that a household using most of its hot water in the morning may find the booster reheating the tank before the sun has had a chance, because the water has cooled overnight. That leads to higher bills.

The fixes Your Home lists include app controls on some newer models, a timer at the switchboard, an off-peak tariff, or switching the booster by hand. Fitting a timer or changing booster wiring is electrical work for a licensed electrician, so treat those as options to raise with your installer rather than jobs to attempt.

Solar hot water works at night because you are using water that was heated during the day and held in an insulated tank. The collectors stop contributing once the sun goes down, but the water already in the tank is still there to use.

Overnight, the tank slowly loses some heat. If the thermostat-controlled booster is set to keep the tank at temperature, it will cut in when the water drops below its setting. On a run of cloudy or rainy days the same thing happens on a bigger scale: the collectors deliver less, and the booster makes up the difference so the hot water does not run out.

With a working booster, the weather changes what your hot water costs more than whether you have it. The more of your use falls before the sun is up, and the more cloudy weather you get, the larger the booster's share of the heating.

Your Home recommends collectors face true north for peak performance, but they have room to move. Your Home says collectors can sit up to 45 degrees either side of north without significantly affecting efficiency, so a north-east or north-west roof is usually fine.

A west-facing roof can still work. The Victorian guide estimates a west-facing system with no shading may perform up to 20 per cent less efficiently than a north-facing one, and that a larger collector helps close the gap. That is a Victorian estimate, so read it as a guide to the direction of the effect rather than a figure for your roof.

For tilt, Your Home suggests an angle close to your latitude, while noting that in practice collectors usually follow the roof pitch, often 20 to 30 degrees. Shade matters more than tilt. Your Home warns against collectors being shaded by trees or buildings for long periods, particularly in winter when the sun sits low. On treed blocks, look at where the winter shadows fall before deciding.

A solar tank can get much hotter than an ordinary water heater, because the sun is a heat source nobody controls. The Victorian guide notes that in hot weather, with little hot water being used, a tank could reach boiling point.

Systems manage this with several mechanisms the same guide lists, including pressure and temperature relief valves, heat dumps, circulation control and tempering valves. So a relief valve releasing some water on a hot day with little hot water use can be the valve doing the job it is there for. Heavy discharge is different: Your Home's rule of thumb is that if a bucket placed under the valve fills in a day, the valve needs replacing.

Queensland also sets limits at the tap. The Queensland Government's guidance says stored hot water must be heated to at least 60 degrees to control Legionella, and that all new and replacement systems need a temperature control device so hot water for personal hygiene reaches the taps at no more than 50 degrees. Relief valves, tempering valves and anything on the pressurised side of the tank are a licensed plumber's job. Do not adjust, remove or cap them.

Solar hot water collectors and solar electricity panels look alike from the street but do different jobs. The Clean Energy Regulator puts it plainly: collectors generate heat, while solar panels generate electricity.

That distinction opens a second route to sun-heated water. Your Home notes that a heat pump running on rooftop solar power offers many of the same advantages as solar hot water and can sometimes be cheaper to install and operate. It adds that solar panels can power other appliances too, which may make them a better use of limited roof space.

Neither route wins everywhere. A clear north-facing roof with room for collectors suits solar hot water; a home that already has solar panels, or a roof with little spare space, may suit a heat pump better. The comparison guide sets out the factors for each system type, and the rebates guide covers Small-scale Technology Certificates, which the Clean Energy Regulator limits to models on its register.

In Queensland, a solar hot water system has to be installed by a hot water installer holding a solar and heat pump endorsement from the Queensland Building and Construction Commission. That requirement comes from the Queensland Government's hot water guidance. The QBCC also lists installing or replacing a hot water heater, solar included, as notifiable work that a licensed plumber lodges with it once the job is done.

The work also crosses trades. Valves and the tank are a plumber's job. Booster wiring and switchboard timers are electrical work, and Queensland's Electrical Safety Office says to always get a licensed electrician for any electrical work. A gas booster is a gas device, and the Queensland Government says working on one needs a gas work licence. What you can safely do yourself is limited to looking: check whether the booster switch or circuit is on, note any water from the relief valve drain, keep an eye on shading, and photograph the data plate before you call.

If you are weighing solar for your home, or an existing system's bills have crept up, talk to a licensed plumber who works on solar.