Solar Tips


If the day is bright with blue skies your hot water cylinder will receive good solar thermal gain by noon. It is recommended, therefore, that you choose to utilise this hot water (e.g. after lunch) for washing up and/or bathing if possible. In this way you will ‘free up’ extra capacity in the cylinder which will absorb more solar energy for the evening.
If the day is cloudy you will still receive good solar thermal gain. The boiler and/or immersion heater will have less work to do in order to bring the water in your hot water cylinder up to temperature.

Legionella Bacteria

Legionella Bacteria grow in water at temperatures between 20°C to 46°C. They are killed quickly at 60°C. It is therefore important to ensure that solar pre-heated water (which can vary in temperature from 5°C in winter to 85°C in summer) passes through an auxilliary heat source capable of heating to at least 60°C before distribution to the domestic hot water system.

We recommend your hot water cylinder is regularly sterilised, ideally at the end of the day and that this is accurately controlled by time and temperature. If there is any doubt please seek specialist advice.

Temperature Controls

During hot weather there is a risk of scalding from solar heated water. We have ensured that your solar system is capable of controlling the temperature of stored water to 60°C by the use of one or several of the following:

  • thermostatic mixing valve on the hot water distribution
  • cylinder thermostat
  • pump control on the solar primary circuit


Scale deposition tends to accelerate rapidly above a temperature of 60°C, therefore it is important to ensure that this temperature is not exceeded. We suggest that your hot water cylinder thermostat is set to a maximum of 60°C. We have ensured that the solar differential temperature controller store maximum temperature (SMX) is set at 60°C.


Try to programme your central heating/hot water timer so that backup heating is not left on 24 hours a day. This would be wasteful, especially in the summer. In the mornings minimise the duration of your backup heating to provide just enough hot water for your needs. In this way the bottom of the cylinder will be cool and ready to receive solar energy. Avoid lunch time backup heating if possible and allow the sun to do its work. In the evening try to set the timer to bring the water up to 60°C after the sun has done its work and before the main bath/shower time in the house. This will ensure the bottom section of the cylinder remains cool overnight ready for another good solar day. In simple terms re-programme your boiler programmer for hot water as follows: ONCE per day, 6pm, for 1 hour!!

Going away for holidays etc.

If you are not going to be using hot water for 2 days or more between March and September we suggest switching off your backup heating altogether until you return. This will save energy and control overheating. It is good practice to leave the airing cupboard door open. Upon your return switch the boiler back on and heat the water to at least 60°C for an hour before using it.


If we have installed a drainback system this will not be a concern since the solar fluid will drain out of the collector(s) if/when the cylinder is up to temperature.


We have ensured that the solar system pipework is highly insulated with high temperature closed-cell elastomeric insulation. It is also important to ensure that all hot pipes (especially vent pipes in the loft space) are lagged. Lagging should be at least as thick as the pipe it covers and should run at least a meter, where possible, from the cylinder.

Freeze control

As mentioned above, protection from freeze damage is ensured by the use of antifreeze as the heat transfer fluid. This is propylene glycol (Tyfocor LS) which is low in toxicity and biodegradable. It has frost protection down to -28°C.

Additionally, the collector sensor is sometimes set to automatically run the solar pump when the temperature drops below a set temperature.

The quality and quantity of antifreeze in the solar system should be checked at least every five years by a specialist. With the use of a refractometer and pH strips it is possible to check glycol concentration and acidity.