A solar water heating controller is a device which measures the temperature of the solar panel and hot water tank in a solar water heating system and uses that information to decide when the circulation pump should be on (solar panel hotter than hot water tank contents) and when it should be off (tank contents hotter than panel).
We have a few articles on putting together your own simple solar water heating pump controller, and we also sell more advanced micro-controller based pump controllers in the REUK Shop. While these are simple to operate, easy to install, and cheap to make/purchase, they do not offer the monitoring capabilities (e.g. graphical display) and user programmability of commercial versions.
In this article we will introduce the Steca TR 0301 temperature difference controller which is a very effective solar water heating pump controller.
Steca TR 0301 Temperature Difference Controller
The Steca TR 0301 is a controller designed to monitor and control solar water heating systems. As the image above shows, this controller has a graphical display which presents system status and measured temperatures clearly to the user.
Below is a schematic of how the Steca TR 0301 can be integrated in a solar water heating system.
A total of three temperature sensors are used – one for the top of the storage (hot water) tank, one for the bottom of the storage tank, and one at the exit (i.e. hottest point) of the solar thermal panel.
When the exit of the solar panel is measured to be 8 degrees Celcius (or more) above that measured at the bottom of the storage tank, the pump is switched on circulating fluid around the system. When the temperature difference falls below 4 degrees Celcius, the pump is automatically switched off.
Advanced Functions of Steca TR 0301
The Steca TR 0301 has a selection of advanced features and functions not typically found on DIY solar water heating pump controllers. These features are introduced below:
Maximum Storage Tank Temperature
If the temperature of the water at the bottom in the storage tank gets too hot (e.g. 60 degrees Celcius) the pump is switched off. It will only switch back on again when that temperature has fallen by 3 degrees.
Maximum Collector Temperature
Similarly if the temperature of the solar collector (panel) gets so hot that the heat transfer fluid could boil (e.g. 130 degrees Celcius) the pump is switched off until it has cooled to below 127 degrees Celcius.
Tube Collector Function
For evacuated tube solar collectors (where immersion temperature sensors cannot be used) it is difficult to get an accurate temperature reading. Therefore, this controller will automatically turn on the pump for 30 seconds twice per hour so that the temperature of the tube collectors can be measured.
If you go away on holiday and leave the solar water heating system heating water in a well insulated storage tank, it can get too hot since no hot water is drawn off. In order to bring the temperature of the stored water down, if the holiday function is switched on, the pump will be switched on at night whenever the water in the tank is within 10 degrees Celcius of the maximum. This circulates the hot water so that it is cooled in the collector.
Whenever the temperature of the solar collector falls below 5 degrees Celcius, the pump will be switched on until the solar collector temperature reaches 7 degrees Celcius. Obviously this cools the water in the storage tank, but prevents the collector and pipes from freezing.
More Steca TR0301 Information
The Steca TR 0301 is a wall-mounted 230 VAC (mains) powered device which can switch pumps rated at up to 800 Watts. The controller itself draws less than 1 Watt of power and will therefore consume 8 kWh (units) or less per year of electricity.
Temperatures are measured using PT1000 temperature sensors. There are immersion thermistors which have a resistance which changes with temperature in a known manner – e.g. at 20 degree Celcius the resistance of a PT1000 sensor is 1078 Ohms, and at 40 degrees Celcius it is 1155 Ohms.
Other Steca TR Range Products
In addition to the TR 0301 with its 3 temperature inputs and 1 pump output, there is also the TR 0201 – 2 inputs, 1 output, TR 0502 – 5 inputs and 2 speed controlled pump outputs for multiple tanks and collectors, TR 0603 – 6 inputs and up to 3 speed controlled outputs, and the programmable TR 0704 with 7 inputs and up to 4 speed controlled outputs.