We receive quite a few enquiries regarding stainless steel water tanks compared to vitreous enamel lined hot water tanks when used in solar hot water applications. The following are a few points to help you in your decision as to which hot water tank suits your needs and situation
Solar rated tanks and glass lining
Solar rated glass lined tanks have a significantly thicker vitreous enamel lining than a standard mains pressure hot water heater, due to the high temperatures experienced from the return line. It can withstand very high to boiling water temperatures under pressure. Vitreous enamel in standard water heaters dissolves above 70 degrees.
Stainless steel tank strength
Stainless steel tanks are only as strong as the seam welds. However, they generally have a ten year warranty over standard five year warranty (minimum required by law) of glass lined tanks. They also cost on average $300-$400 more than a glass lined version.
The poorer the water quality, the stronger the argument to move away from stainless steel tanks, as impurities in the water can eat away at the silver in the weld, causing pinhole leaking and eventual failure. While stainless steel tanks have longer warranties, no manufacturer will honour a warranty if the TDS (Total Dissolved Solids) of the water is in excess of 600 ppm (parts per million).
Anodes and hydrogen production
Glass lined tanks have a magnesium sacrificial anode that ensures potable water quality (drinkable), by attracting the impurities away from the surrounding water supply. The anode can knock against the glass and electric element, causing “hot spots” on the element, or glass cracking/crazing, if transported lying down. It is vital that they are stored and transported standing upright.
The release of hydrogen gas by an anode can sometimes occur if the water is not used for a long period of time. Small quantities of hydrogen are produced that generally remains dissolved and flushes away as hot water is drawn from the tank. Depending on the water quality and the area where the water heater is installed, there may be a degree of hydrogen build up in the tank if the water heater hasn’t been used for two or more weeks.
However, even with a stainless steel tank, if water is left unused for two weeks, you will have some hydrogen build-up; which is indicated by the spurting of a hot water tap when it is first opened.
An aluminium anode can be used if your water supply is of poor water quality in order to extend the life of the tank. You can request a “hard water” aluminium anode to be fitted to Aquamax glass tanks as part of an Apricus solar hot water system; and if ordered through Energy Matters, there is no extra charge.
Anodes aren’t really an issue with rainwater tank water, but are definitely recommended for use with bore water, or poor water quality. Melbourne and Sydney water is of a good to very high quality. Adelaide is of very poor quality. Tasmanian water is regarded as the best in the world!
To combat hydrogen build-up, all water heaters have a pressure and temperature relief valve fitted to Australian Standards. You simply pull the lever and “purge” the tank for 30 seconds if you have been away on holidays for two weeks or more. This will dispel any hydrogen gas leeched from stagnant water.
If the system is used every day, the hydrogen does not separate from the oxygen in the water molecules, and this does not occur. Regardless, all tank manufacturers recommend you purge a system at least once every 6-12 months.
Do you have other questions about choosing a water heater tank best suited to your solar hot water needs? Contact one of our friendly team for no-obligation expert advice before you buy. Don’t forget, generous solar hot water rebates are still available!