Warm water basics

Warm water basics

How about warm water ?

Warm water can be “won” from solar energy just like electricity. This is my first attempt to put all the findings to an decent article, so feel free to add information.

We seperate different systems each with it’s own special features that should be adapted to our own specific needs and location. It is possible to use the solar power to just heat water to replace or support the water heater, but also heat a pool or even a building.

A complete system looks a lot like a central heating installation but with different specifications. For example an expansion tank may look the same, but in a glycol filled system the volume is a bit more and the maximum pressure needs to be up to 150PSI or 10 Bar.

Integrated systems:


Integrated system
Integrated systems

In this system everything is intergrated in one system that is installed on the roof or just a sunny spot in the garden.

Thermosiphon causes the hot water to rise and be replaced by colder water inside the tank. The water water connector is on the top of the tank, cold water at the bottom. As soon as a value or faucet is openend, warm water is pushed out of the tank since new cold water is running in. This works great in warmer climats, but in colder climates it not the best solution.


A split solar heater:

Solar warm water
A split water heater – Solar warm water

A small warm water system consists of following parts:

  • Collector(s), either flat plate or vacuum tubes with heatpipes.
  • A pump station to pump fluids around.
  • A storage tank simular to a water heater, usually with one or more heat exchangers.

In this drawing the pump is just drawn as a single item, usually this part consist out of many different parts and is sold as a “solar pump station” which can contain a differential controller, or can be connected to a external differential controller.


Mechanical parts

Solar pump station
Solar pump station
  • Pump
  • Flow measurement
  • Deaerator
  • Temperature gauges
  • Pressure gauge
  • Pressure control value
  • Fill / Drain connection(s)
  • Insulation
  • Connection for the expansion tank.

Regardless of the brand, this will mostly be the layout. This example includes a double connection to the collector, so in- and out-temperature are shown and it also includes a deaerator to seperate the air from the fluid.



Solar Differential Controller
Solar Differential Controller

To make sure everything work according to plan a differential controller is needed, this controller has a few temperature sensors connected and can switch the pump on and off depending on the settings made by the user.

Besides the many extra (security-) features in these type of controllers, the main thing is that the measure the temperature in at least 2 but mostly 3 different places and switch on and off the pump accordingly:

  • Top of the collector (array)
  • Bottom of the storage tank
  • Top of the storage tank


Solar warm water heater with temperate sensors
Solar warm water heater with temperate sensors

The pump is switched on, if the difference in temperature between the collector (T1) and the storage tank (T2) has reached the (pre-) set temperature difference.

The pump is switched off again, as soon as the difference in temperature drops below this (pre-) set temperature difference or in case of 3 sensors, as soon as the temperature of the storage tank (T3) reaches the maximum (pre-) set temperature.

The first case would mean the collector is actually not warm enough to extract heat since the storange tank has a higher temperature. The latter case would mean all the water is heated enough.

Most controllers also have an output to switch on an auxillery heater incase the storage tank never reaches the set temperature in a (pre-) set time-limit.