Solar Thermal Hot Water System Not Working
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Do not attempt to work on the solar thermal fluid circuit — the propylene glycol fluid operates at high temperature and pressure. Only a qualified plumber should work on the sealed collector circuit.
Solar thermal hot water systems are generally very reliable but can develop problems with the pump, controller, expansion vessel, or fluid pressure. Most diagnostic checks can be done by the homeowner; repairs to the sealed circuit require a qualified plumber.
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Most likely cause & what to check
Check the solar controller display — it should show collector temperature (the panel temperature) and cylinder temperature. If the controller is blank, check the power supply and fuse.
Check that the pump is running on a sunny day. The pump should operate whenever the collector temperature is significantly higher than the cylinder temperature (typically 5–8°C differential). Listen for pump noise or feel for vibration on the pump body.
Check the system pressure gauge — solar thermal systems typically operate at 1.5–3 bar. Below 1 bar means fluid has been lost from the circuit (either leaked or through the pressure relief valve). A qualified plumber must refill the circuit with the correct glycol mix.
If the pump runs but no heat is being delivered, check the non-return valve on the collector circuit — a stuck non-return valve prevents hot fluid from reaching the cylinder coil.
Check the immersion heater backup — most solar systems have an electric immersion as backup. If the solar is not working in winter or cloudy periods, ensure the immersion is operational.
If the controller shows an overheating fault, the system may have stagnated in high summer (common when hot water is not used for a week, e.g. holidays). This is a temporary condition — the system usually recovers when hot water demand resumes.
Annual maintenance: a qualified solar thermal engineer should check the glycol concentration and pressure every 3–5 years. The glycol degrades over time and loses freeze protection.
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