how does a solar hot water system work?

A (split system) solar hot water system works by water passing through solar collectors on the roof that absorb energy from the sun and transfer the heat into the water. This water heated in the collectors can reach extremely high temperatures on warm sunny days.

After the water is heated in the collector it is pumped back to a storage tank for later use while cold water from the bottom of the tank is circulated up to the collectors for heating. This cycle is continued until all of the water in the tank reaches a pre-set temperature.

gas boosted solar hot water

During the cold, cloudy and high demand periods there may not be enough solar gain the heat the water fast enough or hot enough, but with a gas booster you will have an uninterrupted supply.

This is because a gas booster is a continuous flow hot water unit that is capable of supplying enough hot water on its own without the preheat obtained from the solar collectors.

As a hot water tap is opened, water from the storage tank passes through the booster before reaching your taps. Sensors in the booster will detect the water temperature as it passes through. If the water is not hot enough the booster will ignite and bring the water to the correct temperature.

However, if the water is hot enough, it will pass through the booster without any additional heating.

Gas boosters are available in natural gas and propane (LPG) models.

electric boosted solar hot water

An electric system can’t heat the water on demand like a gas booster.  Just like a conventional electric storage unit, they heat the water with elements which takes considerably longer.

With a continuous electricity supply, the electric element will start heating the water as soon as the thermostat detects a drop in water temperature below a pre-set level. The element will continue to heat the water until it reaches temperature.

Off peak boosting will only occur in the evenings and on weekends whether you run out of hot water or not.  As off peak tariffs are lower, overnight boosting is a good option if you use most of your hot water in the morning. This way you allow the sun to re-heat all the water you used during the day.

However, if the majority of hot water usage is in the evening, the electric boost will heat the water overnight leaving little work for the sun to do the following day.

flat plate solar collector

A basic design of a flat plate collector consists of copper pipe, absorber plate and insulation all housed in a aluminium box with a glass cover.

As solar radiation penetrates the glass of a collector, heat is trapped inside much like within a glasshouse. The absorber, plate made from either aluminium or copper, soaks up much of the trapped heat inside the collector.

Bonded to the absorber plate is a series of copper tubes (or risers) with water flowing within.

Cold water enters the base of the collector and is heated as it travels through risers to the top of the collector where it exits the header tube as hot water.

The cover of the collector is manufactured with low iron tempered glass which is approximately 4 times as hard as regular (or annealed) glass. Toughened glass is used as it needs to withstand all weather conditions and damage from hail etc.

evacuated tube solar collector

Made from borosilicate glass, evacuated tubes comprise of an inner and outer glass tube separated by a vacuum to help retain heat. The same technology has been used for thermos flasks for many years.

Inside the glass tubes is a copper heat pipe that contains a small amount of purified water or glycol….which is also contained in a vacuum. The vacuum in the heat pipe allows the liquid within to boil at a much lower temperature.

As the sun heats the liquid (water or glycol), it begins to boil and the steam rises to the top of the heat pipe into a heat exchange inside the header assembly.

The cold water flowing through the header assembly its heated by the steam. As the water is heated, the steam at the top of the heat pipe cools and condenses before returning to the bottom of the pipe. This is an endless cycle of hot steam rising to heat the water and cool liquid falling back down the tube.

what are the maintenance requirements?

gas booster

  • Check air inlet and flue terminal are clear of obstructions every 6 months – by owner
  • Apply insecticide around the unit to protect from insect infestation as required – by owner
  • Clean cold water inlet filter if a reduction of water flow is detected – by a qualified technician
  • Check and clean the burner every 5 years by a qualified technician
  • Check gas pressure every 5 years by a qualified technician)
  • Check components and connections for possible issues every 5 years by a qualified technician


  • Flush the PTR (pressure & temperature relief) valve every 6 months – by owner
  • Flush the expansion control valve (if installed) every 6 months – by owner
  • Check and replace sacrificial anode every 5 years if required – by a qualified technician
  • Check and replace PTR valve every 5 years if required – by a qualified technician
  • Check and replace expansion control valve (if installed) every 5 years – by a qualified technician
  • Check and replace tempering valve (if installed) every 5 years – by a qualified technician
  • Flush the hot water tank every 5 years – by a qualified technician 


  • Check all connections for leaks
  • Check collectors are free from shading
  • Check the collector glass is clean