Back-up heating is the supplementary heat source that brings solar pre-heated water up to the required temperature. A solar thermal system for domestic hot water covers roughly half to sixty per cent of the annual demand in northern European conditions; the rest — at night, in winter, on a wet day — comes from the back-up. Without it, a solar water heater is not a reliable hot water supply.
How it works
The collector on the roof heats a fluid that gives up its heat to the cylinder through a coil. Cold mains water enters at the bottom, hot water is drawn from the top. If the stored water is not hot enough, the back-up takes over. There are three usual arrangements:
- From the boiler — the upper part of the cylinder is heated through a second coil, or the pre-heated water feeds a combination boiler. The most common retrofit
- From an immersion heater in the cylinder, typical where there is no gas supply, or as a reserve
- From a heat pump, with solar heat pre-warming the incoming water and the heat pump doing the rest
Why temperature matters
Legionella bacteria multiply in stagnant water between roughly 25 and 50 °C. A solar cylinder holding pre-heated water sits squarely in that band. The cylinder therefore has to be raised to a high temperature regularly by the back-up. In the UK, HSE guidance for hot water systems sets out storage and distribution temperatures; comparable rules apply elsewhere. Ask the installer explicitly how the reheat cycle is configured — this is a health matter, not a detail.
At the tap, by contrast, the water should not arrive scalding. A thermostatic mixing valve limits the outlet temperature and prevents scalds, particularly for children and older people.
Points to watch
- The boiler must accept pre-heated feed water. Not every combination boiler modulates cleanly on a 45 °C inlet; the result is a shower that swings hot and cold
- Insulate every pipe run. Heat lost between cylinder and appliance eats the solar gain
- Do not switch the back-up off in summer. The saving is small; the legionella risk is not
- Service annually: check the pressure and antifreeze concentration in the collector circuit, and the expansion vessel
- Have the saving calculated on your actual hot water use before investing, rather than relying on headline figures
Related terms
- Central heating boiler
- Energy label
- Condensation
- Grid congestion
- Green roof
- Ecological building
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.