STDC Temperature Difference Controller

For the last five years we have been selling our simple (and economical) solar water heating pump controller which measures the temperature difference between a solar water heating panel and a hot water storage tank, and runs the pump when appropriate.

Solar water heating with storage tank - temperature difference controller

While our solar controller is more than adequate for small scale DIY installations, for more expensive solar water heating systems a more complex controller is required with a digital display to give the user the ability to accurately track and record temperature data. It is also desirable to be able to easily configure the controller according to personal (and often changing) requirements, and to provide protective functions such as frost, over-heating, and legionella protection.

We are often asked to recommend an affordable temperature difference controller with advanced functionality, and this article introduces that product: the Sorel STDC.

Sorel STDC

Sorel STDC temperature difference controller for solar water heating systems

The STDC (pictured above) is a temperature difference controller designed primarily for use with solar water heating systems with a hot water storage tank, but also to be used for solar swimming pool heating, for solid fuel boilers heating water, or even as a standard thermostat. It can also be used to control storage transfer (moving hot water from one tank to another), as a stop valve, or as a universal temperature difference controller.

A mixture of simple graphics and text are displayed on the screen during general operation and set up showing temperatures, pump status, and system configuration. Setting up the STDC after first installation takes a matter of minutes, and everything is very intuitive and easy.

Solar temperature difference pump controller

Temperatures are measured using excellent Pt1000 temperature sensors. Two of these are supplied with the STDC pre-fitted with 2m long cables (which can be extended to up to 30 metres), and a third temperature sensor can be purchased separately if for example you would like to measure the temperature in two places in the hot water tank in addition to the solar collector.

There are two operating modesautomatic, when the pump is run as and when required according to user-set parameters, and manual when the pump can be turned on and off manually.

The STDC stores a selection of useful data which can be viewed on the digital screen. The average temperature difference, and operating hours of the solar pump are recorded and can be displayed as bar graphs for various time ranges.

Protective Functions

A big selection of protective functions are included to protect both the users from the danger and expensive equipment from damage. These are detailed below.

Seizing Protection: turns the pump on for (at least) five seconds every day or week so that the pump (or valve if used) does not seize up.

Frost Protection: used primarily with direct solar heating in which the water to be heated travels itself through the solar panel rather than anti-freeze (glycol) going through the panel and heating the water via a coil in the tank (indirect heating).
If direct water heating is used, then the danger of the water freezing in the panel and external pipework can be avoided using this function. In stage 1 the pump is swithed on for 1 minute every hour when the solar collector temperature is below 7 degrees Celcius (by default – a value from -25 and +10 degrees can be selected by the user). In stage 2 when the temperature has fallen further (to 5 degrees by default, but again user can set from -25 to +8 degrees) the pump is turned on continuously until the collector temperature is 2 degrees over the stage 2 threshold. Obviously this results in the solar collector panel acting as a radiator, but it is better to lose some heat than to have burst pipes and a damaged collector.

System Protection: prevents excessively hot water being pumped into the hot water tank, by default at 120 degrees Celcius. User can set any value from 60-150 degrees.

Solar Collector Protection: works the other way, and turns on the pump if the solar collector gets too hot (default 110 degrees Celcius), pumping the cooler water from the tank (or swimming pool etc) through the collector.

Collector Alarm: sounds if the solar collector temperature exceeds a set value (default 150 degrees Celcius – can be set from 60 to 300 degrees).

Anti-Legionella: is designed to ensure that legionella bacteria are killed. When this is selected, the hot water tank can be heated higher than normal every 1 to 28 days (default 7 days) – as long as there is sufficient sunshine of course. 70 degrees Celcius is the default temperature setting for this function.

More Information

This temperature difference controller offers a lot more features than can be detailed in this article, so for more information the full STDC Installation and Operation Instructions (PDF) can be downloaded here.


Sorel’s TDC range of temperature difference controllers are designed to meet all possible needs, but for the vast majority of systems the ‘Small’ ‘S‘TDC will do everything and more than you will ever want.

The STDC is sold in the UK typically for under £90-100, and a third Pt1000 temperature sensor can be purchased for around £15 if desired.

The STDC is very well made and designed, operates reliably, and is easy to set up and use. The instructions are clear and understandable, and the statistics collected give a good picture of what is going on.

The only bad point is that there is no battery back-up for the internal clock and so if there is a brief power cut, the time is reset which can mess up saved statistics.