Solar Water Heater Tank

Stainless Steel Solar Water Heater Tank

A major advantage that this solar water heater tank has over others is that it's made from stainless steel, so it should have a very long and hard working life resisting corrosion from the 142 litres (37.5 gal) of hot water inside it.

A pressurised SS tank like this is pretty expensive but because this one is vented to the atmosphere (non-pressurised) it can be manufactured for a lot less and the cost is pretty reasonable.

I didn't DIY this part of the SWH because I don't know how to bend and weld metal, so I designed it by trying to keep things as simple as possible and then sent the specifications to a company that spesialises in manufacturing stainless steel products.

This is how it came out...

Stainless Steel Tank Design

Stainless Steel Solar Water Heater Tank Two heat exchanger ports Stainless steel ball float valve inside the tank Hot water inlet ports
  1. Like I said, I tried to keep things very simple with this tank design so it's 1250 mm (49.2 inches) high and 390 mm (15.3 inches) in diameter, having a volume of 150 litres (39.6 gal) but only containing 142 litres (37.5 gal) of water as it doesn't get filled to the brim.

    The lid is held down with 8 stainless steel nuts and bolts and sealed using a round EPDM gasket. The vent pipe is 22mm (0.86 inch) stainless steel to alleviate any pressure buildup that might occur due to the heat.

  2. Those two ports on the side are the copper coil heat exchanger inlet and outlet. The 22 mm (0.86 inch) cold water inlet pipe is routed through the left port and connected to the 19 mm (0.74 inch) soft copper coil inside the tank and the end of the coil is then connected to another 22 mm (0.86 inch) copper pipe which exits at the right port.

    This pipe carries the water heated by the coil heat exchanger and is connected to the bottom of the electrically heated geyser to form a solar pre-heating loop.

  3. This ball float valve keeps the level of the water in the tank constant, working exactly the same as the mechanism in millions of toilet cisterns, except that this ones stem is stainless steel to handle the higher temperatures.
  4. The tank connections are stainless steel nipples, three at the top and three at the bottom, to ensure a fast flow of hot water from the copper solar water heater tank in the collector. They are angled at 25° to the horizontal so that thermosyphon flow can take place from the collector below.

EPDM Tank Cover

EPDM tank cover

This solar water heater tank has to live outside so it will be subject to some pretty extreme heat, cold and moisture. The cover's main function is to keep the insulation protected from the weather for as long as possible, so an ideal material to use to achieve that is EPDM because it's very robust and should last for up to 15 years.

Solar Water Heater Tank Insulation

Tank insulation

Insulation is a very important component of any solar tank because it's no use collecting a lot of heat from the sun only to lose it again when you store it overnight. There is yet another advantage of an unpressurised tank in that it allows you to increase the thickness of insulation very cost effectively in comparison to a pressurised tank.

The polyester fibre blanket is made from recycled PET bottles and the 100mm (3.9 inches) thickness provides a thermal resistance with a R value of 1.81.