Solar hot water buyers guide

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If your old hot water system has given up the ghost, maybe it’s time to go solar. We show you how solar water heaters and heat pumps work,  what’s available and how to choose the one to best suit your needs.

There are many reasons to choose a solar hot water system or heat pump over a  conventional gas or electric water heater. With the rapid increase of energy prices in recent months, a solar or heat pump water heater can greatly reduce energy bills. Up to 30 per cent of household energy is used just to heat water, so anything that can reduce this energy use will save you a lot of money.

Another important benefit of such a system is that of greenhouse gas emission reduction. A solar water heater or heat pump can reduce the greenhouse emissions of an average family by as much as four tonnes of CO2 per year—the equivalent of taking a car off the road!

Most state governments  have recognised the advantages of solar and heat pump water heaters and offer incentives in the form of rebates. These vary from state to state, but can save you a great deal on the cost of a new water heater, making them more economically viable. The initial purchase price will probably still be higher than a similarly sized conventional water heater but the savings made in running costs will pay for this difference in less than 10 years—in as few as four years in some cases.

How does it work?

A solar hot water system usually consists of a hot water storage tank connected via pipework to solar collector panels. These collector panels are placed on a north facing roof and at an angle of no less than 15° to the horizontal. The tank can either be situated immediately above the panels on the roof (called a close coupled system), above and a small distance away from the panels within the roof cavity, or at ground level (a split system), in which case a pump and controller is required to circulate water through the panels.

As the sun shines on the collector panel(s) the water in the pipes inside the collectors becomes hot. This heated water rises through the panel and out through a pipe to the insulated storage tank. Cooler water from the bottom of the storage tank enters the panel at the bottom to replace the warmer water.

This is called the thermosyphon process, requires no pumps or other devices and is very simple and effective. However, it does require that the storage tank be situated above the collector panels. The collector panel is the driving force for the circulation, so due care must be taken with its mounting and orientation to get maximum benefit from it.

If the tank cannot be located above the collectors, a pump and a differential temperature controller must be used to provide water circulation. The controller also turns the pump on when the temperature drops to 5°C as a frost protection function.

Some systems don’t heat the water directly but instead heat a fluid similar to antifreeze used in vehicle cooling systems. This fluid flows through a closed loop system (through thermosyphon or pump action) and transfers the collected heat to the water in the tank via a heat exchanger.

There are pros and cons with each system. Close coupled systems require the roof support the full weight of the tank, but they are much simpler than split systems and little maintenance is required.

Split systems have a much slimmer roof profile and are more convenient should tank maintenance be required, but the added complexity of the pump and controller means that failures tend to be more common.

Read the full article, including tables with details on sizes and prices, in ReNew 114.

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