Solar Thermal FAQ

Yes — solar thermal is a mature technology with more than 500 GWth of capacity installed worldwide (IEA SHC). A domestic system typically supplies 50–60% of a northern European household's annual hot water and 70–90% in southern Europe and the Mediterranean, with a boiler or immersion covering the remainder. Performance is measured to [ISO 9806](knowledge/glossary/iso-9806) and certified under [Solar Keymark](knowledge/glossary/solar-keymark), so declared yields are comparable between manufacturers.

On a clear day a 4 m² array delivers roughly 2 kW of useful heat around midday, which raises a 200 L cylinder by about 30 °C in three to four hours. Small thermosiphon systems in sunny climates usually reach usable temperature by late morning. In thin winter sun the same array may lift the store only 5–15 °C across a whole day, which is why a backup heat source stays in the system.

Yes, at reduced output. Collectors use diffuse as well as direct radiation, and on an overcast day irradiance falls to roughly 100–300 W/m² against about 1,000 W/m² in clear midday sun — so you get a fraction of a sunny-day yield rather than nothing at all. It normally takes two or three consecutive dull days to bring the backup heater in, not a single cloudy afternoon.

Yes. No domestic solar thermal system is sized to cover 100% of hot water year-round, because doing so would mean an oversized array that spends the summer in [stagnation](knowledge/glossary/stagnation-temperature). The standard design keeps the existing boiler or immersion as backup, connected to the upper part of a twin-coil cylinder so it only tops up the temperature the sun has not already reached.

Around 4–5 m² for a typical family system, in an unshaded position (Energy Saving Trust). The underlying rule is 1–1.5 m² of flat plate [aperture area](knowledge/glossary/aperture-area) per person, or 0.8–1.2 m² of evacuated tube. You also need indoor space for a larger solar-compatible cylinder — in a retrofit the tank, not the roof, is usually the real constraint.

Roughly 6–8 m² of flat plate collector, or 5–7 m² of evacuated tube, with a store of about 350–450 L. The store sizing follows from two checks: 1.25–1.5 times daily hot water demand, and 50–70 L per m² of collector. Above about 500 L it is usually better to split the volume across two vessels than to fit one very tall cylinder.

About 50 L per m² for flat plate collectors and about 70 L per m² for evacuated tubes, measured against [aperture area](knowledge/glossary/aperture-area) rather than gross panel size. Too little store volume is what drives an array into [stagnation](knowledge/glossary/stagnation-temperature) on summer afternoons.

Only if the cylinder already has a spare coil mounted low in the tank and dedicated to solar. A standard single-coil cylinder has nowhere to put solar heat, and heating from the boiler coil at the top destroys the temperature difference the collectors need. The usual solutions are replacing it with a twin-coil solar cylinder, or adding a pre-heat vessel upstream so mains water is warmed before it reaches the existing tank.

Yes to all three — wall mounting, ballasted or framed flat-roof mounting and ground mounting are all standard; the trade-off is yield, not feasibility. For balanced annual output the collector wants a tilt of roughly 30–45° in most of Europe, close to the site latitude, so a truly horizontal collector is normally raised on a frame. Flat mounting also sheds rain, dust and snow poorly, which costs further output.

An on-roof or in-roof collector adds roughly 20–30 kg/m², plus 5–10 kg/m² for the mounting frame — comparable to a PV array and within the capacity of most sound roofs. A [thermosiphon system](knowledge/glossary/thermosiphon-system) is different: a filled 200 L roof tank adds over 200 kg concentrated on a small area and needs a structural check. Split systems with the cylinder indoors avoid the issue entirely.

Three things: an unshaded roof or wall area of about 4–5 m² facing anywhere from east through south to west; space for a cylinder with a dedicated solar coil low in the tank; and a pipe route between the two. A [forced circulation system](knowledge/glossary/forced-circulation-system) also needs a mains socket for the pump station and controller. An existing single-coil cylinder normally has to be replaced with a twin-coil model.

Close to what it was the previous night. A modern 300 L cylinder has a standing loss of about 2 kWh per 24 hours — roughly 5–7 °C per day — so an overnight drop of 2–3 °C is normal. Losing 10 °C or more overnight points to uninsulated pipework, a failed [check valve](knowledge/glossary/check-valve) letting the solar circuit siphon heat back up to the collector, or a gravity circulation loop on the boiler side.

Yes. The store is the buffer: heat collected during the day sits in the cylinder and is drawn in the evening, losing only a couple of degrees. Evening-heavy use actually suits solar thermal well, because the tank is at its hottest at the end of the day. Morning-heavy households benefit more from a larger store or a tank designed for good [stratification](knowledge/glossary/stratification).

No more than with any other stored-water system. A solar cylinder behaves like any cylinder — hot water leaves the top while cold mains enters the bottom — and good [stratification](knowledge/glossary/stratification) keeps that top layer hot until roughly two thirds of the tank has been drawn off. Solar systems often feel more generous because in summer the store frequently sits well above the 60 °C a boiler would target.

Yes — a solar cylinder can exceed 80 °C on a clear summer day, and water at 60 °C scalds in a few seconds. This is why a [thermostatic mixing valve](knowledge/glossary/thermostatic-mixing-valve) is required on solar systems in most European codes; UK Approved Document G requires one wherever stored water could exceed 80 °C and limits bath outlets to 48 °C. Specify a valve rated for solar duty, because standard TMVs are not built for 90–100 °C inlets.

Every stored-water system needs two independent layers of protection: the control thermostat, plus a separate manual-reset high-limit thermostat that cuts power to the immersion element — with a [thermostatic mixing valve](knowledge/glossary/thermostatic-mixing-valve) at the outlets as the final defence. On solar and PV-diverter systems the controller must also have a maximum store temperature setting, typically 60–75 °C, that stops the pump or diverter. A single-thermostat immersion with no high limit is the classic cause of a boiled cylinder.

A visual check once a year — system pressure, pump operation, controller readings and any sign of leakage — plus a full service every five years when the heat transfer fluid is tested or replaced and the circuit is flushed (Energy Saving Trust). Well-maintained systems commonly last 20–25 years or more; Solimpeks designs its collectors for a 20–25-year service life. Skipping the fluid change is the most common cause of premature failure.

Two tests on a drawn sample: a refractometer calibrated for [propylene glycol](knowledge/glossary/propylene-glycol) to confirm the freeze protection concentration, and a pH strip or meter. Healthy inhibited solar fluid sits around pH 8–10; once it falls below roughly 7.5 the corrosion inhibitors are exhausted, the fluid turns acidic and it begins to attack copper and solder. Dark brown or black fluid with a burnt smell has been cooked in [stagnation](knowledge/glossary/stagnation-temperature) and must be replaced.

The [stagnation temperature](knowledge/glossary/stagnation-temperature) — the equilibrium a collector reaches with no flow — is typically 150–200 °C for a glazed flat plate and 250–300 °C for evacuated tubes, measured under [ISO 9806](knowledge/glossary/iso-9806) at 1,000 W/m² irradiance and 30 °C ambient. Every datasheet states the specific figure. The whole solar circuit, including the expansion vessel, insulation and seals, has to be rated for it.

With no hot water being drawn the store reaches its maximum, the controller stops the pump, and the collector goes into [stagnation](knowledge/glossary/stagnation-temperature): the fluid vaporises out of the absorber into the [expansion vessel](knowledge/glossary/expansion-vessel) and the circuit sits dry and hot until it cools. A correctly sized expansion vessel makes that a normal, survivable event; an undersized one dumps fluid through the safety valve. Repeated stagnation is what degrades glycol fastest.

Not in an indirect system, which is the European standard: the collector loop is a sealed glycol circuit separated from the potable water by a coil inside the tank, so mains water never enters the absorber. Direct systems, where drinking water circulates through the collector, do scale in hard water — calcium carbonate precipitates fastest above about 60 °C and can block the narrow riser tubes. In hard water areas, specify an indirect system.

On a tilted glazed flat plate, snow usually slides off within a day or two because the glass warms above freezing even in weak sunlight. Evacuated tubes behave the opposite way: their insulation is so effective that almost no heat reaches the outer glass, so snow can sit on them for days. In snowy regions this is a practical argument for flat plates, or for a steeper tilt of 45° or more.

No. The absorber's dark surface is a [selective coating](knowledge/glossary/selective-coating) — typically a sputtered layer with solar absorptance around 95% and thermal emittance around 5% — and ordinary paint destroys that ratio, cutting output sharply and charring at [stagnation](knowledge/glossary/stagnation-temperature) temperature. The outer casing and mounting frame can be repainted; the glazing and the absorber cannot.

It depends strongly on climate. In Mediterranean and MENA climates solar commonly covers 50–70% of winter hot water demand; in northern Europe it is around 25% in winter against roughly 90% in summer, averaging about half the annual total (Energy Saving Trust). The winter balance comes from the backup — a boiler, an immersion or, ideally, a heat pump charging the upper coil of the same cylinder. At high delivery temperatures in cold weather evacuated tubes lose less heat, while for normal hot water temperatures flat plates remain the value choice.

A glazed collector converts roughly 55–75% of the solar energy striking it into usable heat at domestic hot water temperatures — several times the 15–22% electrical efficiency of a PV panel of the same area. The exact figure comes from the [collector efficiency curve](knowledge/glossary/collector-efficiency-curve) on the Solar Keymark certificate: zero-loss efficiency for a good flat plate is typically 0.75–0.83, falling as the gap between collector and ambient temperature widens.

It is a photovoltaic sizing rule of thumb, not a solar thermal one: size a PV array roughly 20% above measured electricity consumption to allow for shading, panel degradation and demand growth. Solar thermal has no equivalent 20% rule — the corresponding rules are 1–1.5 m² of collector per person and about 50 L of store per m² of collector. Seeing the term applied to a hot water system is a sign that PV advice is being reused.

Solar pool collectors typically hold a pool 4–7 °C above the temperature it would otherwise sit at, which in practice extends the swimming season by two to three months in a temperate climate rather than making an unheated pool tropical. Sizing is by pool surface area: about 50% of it in warm climates, 75–100% in cooler ones. Unglazed polymer absorbers are used because the target is only 26–28 °C.

Yes — the entire flow and return, indoors and out, and the exposed sections need UV- and temperature-resistant insulation rated to at least 150 °C, because ordinary foam melts at [stagnation](knowledge/glossary/stagnation-temperature) temperature. Domestic circuits normally use 15 mm or 18 mm copper, or DN16 corrugated stainless, for arrays up to about 6 m². Flow is set to roughly 15–50 L/h per m² of collector, depending on whether the system is designed low-flow or high-flow.