A Thermosiphon or A Split Solar HW System?

(or where do you want to put the tank?)

The previous page explained the 2 main types of hot water panel and how they work.

Now, in order to have that solar powered shower we still need to:

a) get cold water in to to panel


b) get the hot water out of the panel and into the storage tank, ready and waiting for shower time

You have essentially 2 options here:

1) Put the tank on or in the roof, and use the fact that hot water rises to move the water through the panel without any need for an externally powered pump. This is called a thermosiphon system because the water is siphoned into the tank 100% thermally.


2) Put the tank at ground level, which means the water has to travel all the way up to the roof, which means you’re gonna need to give it a helping hand with an electric pump. This is called a split system.

Lets look at each configuration in more detail:

1) Thermosiphon systems.

Trust geeky engineers to come up with such a complex sounding name for such a simple device. This is the style of panel that is most popular in Australia, mainly because it is cheap, effective and very low maintenance. The feature that gives them away is the big tank at the top of the panel:

How it works:

a) Cold, mains pressure water is connected to the bottom of the panel.

b) The sun shines on the panel and the water gets hotter.

c) Just as hot air rises, so does the hot water, all the way into the tank at the top of the panel.

d) As the hot water moves up the panel and into the top of the tank it pushes cold water out of the tank, down to the bottom of the panel.

e) The sun shines on the panel and the water gets hotter. The whole cycle repeats as long as the sun is shining.

The result is a tank full of nice, hot water.

If the sun isn’t strong enough and the water in the tank doesn’t get hot enough, then the water temperature will be boosted by either electricity or gas.

If it is an electric boost then an electric element in the tank will provide a ‘heat boost’ to get it to temperature.

If it is a gas boost, then whenever a tap is switched on, the warm water in the tank goes down from the roof in an insulated pipe and through an instant gas heater where it is heated on demand.

The gas boosting is a lot more efficient.

Advantages of this configuration

  • Cheap(ish) (approx $1500-$5000 installed depending on rebates)
  • Simple and reliable.
  • No pump or electricity required to move the water.


  • Your roof needs to be strong enough to hold it (typically 500kg when full for a 300l system).
  • If the system overheats – it will dump hot water down the drain to protect the panels and tank. This can be hundreds of litres of water on a very hot day.

If your roof is not strong enough to hold the tank, you can still have a thermosiphon system in theory, by placing your tank in the roof. As long as the bottom of your tank is more than 300mm higher than the top of the panel, you’ll get the thermosiphon effect. But in Australia this is very uncommon. Why? Because modern hot water cylinders are a lot more flimsy, and can spring leaks after a decade or so. If this happens to a tank in your roof – it’s gonna be messy!

2) Split system.

If you don’t want a tank on or in your roof, then you are gonna have to have one at ground level and pump the water up to the roof with a small pump.

Here’s how it works.

You might not like the thought of using electricity to pump the water, but the good news is that you only need a very small pump rated at about 30 Watts. This uses very little electricity (about $10 per year worth) so its not really worth worrying about.


  • Easy access to tank.
  • Easier to super-insulate the tank.
  • More flexible boosting choices (instant gas, gas storage, electric, or electric heat pump).
  • Don’t need to worry about how strong your roof is.


  • Needs electricity to pump the water.
  • Longer pipe runs can mean more heat loss (make sure the hot pipes are super well insulated!)
  • Slightly more expensive / more complex.
  • More things to go wrong than thermosiphon. More maintenance required. The pump will fail after a few years, believe me.

OK: So you (or your installer) have now decided whether you want a thermosiphon or a split system, the next decision is if you want 1 tank or 2…

Read next: One tank or two?