A forced-circulation solar system uses an electric pump, commanded by a differential temperature controller, to move heat-transfer fluid between the collectors and a storage tank that can stand anywhere in the building. The pump typically starts when the collector runs 5–8 K hotter than the tank. It is the standard solar water heating configuration in central and northern Europe.
How does a forced-circulation solar system work?
A controller watches two temperature sensors — one at the collector outlet, one in the lower part of the store — and switches the pump whenever collecting is worthwhile. When the collector runs typically 5–8 K hotter than the tank, the pump starts and pushes a water–glycol mixture through the collector array; the fluid surrenders its heat through a coil heat exchanger in the tank and returns. When the difference falls to 2–3 K, the pump stops. Because an electric pump provides the driving force, the tank no longer needs to sit above the collectors — the defining constraint of a thermosiphon system — so collectors go on the roof and the store goes in the plant room.
Forced circulation or thermosiphon: which do you need?
Choose by climate and building. A thermosiphon is simpler and cheaper where frost is rare and a roof tank is acceptable; forced circulation is the answer for freezing climates (the glycol loop is fully frost-proof), for combi systems that support space heating, for larger collector fields, and wherever the tank belongs indoors — which describes most of central and northern Europe, and it is the configuration behind most of Germany's installed solar thermal base. The pump adds components and a small electricity demand; in exchange the designer gains complete freedom in tank placement, storage size and system topology.
What components make up the solar circuit?
Beyond collectors and store, the closed loop needs a defined set of parts, usually delivered pre-assembled as a solar pump station: the circulation pump, a check valve to block reverse thermosiphoning at night, flow meter, safety valve, pressure gauge and fill-and-drain cocks. Around it sit the expansion vessel, sized to swallow fluid expansion plus the collector content as steam during stagnation, the solar controller with its sensors, and an air separator with vents at the high points. Every component is specified for stagnation conditions — a power cut on a summer day sends the collectors to 160–200 °C, and the circuit must shrug that off as a normal operating state.
High-flow, low-flow or matched-flow: what flow rate should the pump deliver?
| Strategy | Specific flow rate | Best for |
|---|---|---|
| High-flow | 30 – 50 L/h per m² of collector | Small hot water systems; maximum collector efficiency |
| Low-flow | 10 – 20 L/h per m² | Larger systems; fast, strongly stratified tank charging |
| Matched-flow | Variable, PWM-controlled | Modern controllers adapting flow to irradiance |
High-flow keeps the collector cool and efficient but returns lukewarm water to the store; low-flow produces immediately usable temperatures in one pass and charges a stratified store top-down. Matched-flow lets the controller modulate the pump continuously and has become standard with PWM-capable high-efficiency pumps.
How much electricity does the pump use?
Little — modern high-efficiency circulators draw roughly 3–20 W in solar duty. EU Ecodesign Regulation 641/2009 caps circulator consumption at an Energy Efficiency Index of 0.23 for pumps placed on the market since 2015, which retired the old 40–80 W constant-speed pumps from new installations. Over a year the pump of a family-sized system consumes a few tens of kilowatt-hours while moving one to two thousand kilowatt-hours of solar heat — an electrical overhead of one to two percent. Solimpeks forced-circulation packages pair Wunder flat-plate collectors with coil-equipped storage tanks from its Konya factory, covering single-family hot water through large commercial fields with the same loop principle.
Frequently asked questions
What happens to a forced-circulation system during a power cut?
Circulation stops and the collectors climb toward stagnation temperature while the sun shines. A correctly designed circuit tolerates this routinely: the expansion vessel absorbs the steam volume and the system resumes normal operation when power returns.
Can a forced-circulation system run without antifreeze?
Yes, as a drain-back system: the collectors empty into a reservoir whenever the pump stops, so plain water can be used and both frost and stagnation lose their sting. Classic pressurised systems in frost-prone climates use a propylene glycol mixture instead.
What are typical controller settings?
Switch-on at a collector–tank difference of 5–8 K, switch-off at 2–3 K, and a tank maximum limit commonly set between 60 and 90 °C. The hysteresis gap prevents the pump from rapid cycling around dawn and dusk.
Does the solar pump run at night?
No — the differential condition is never met in darkness, and the check valve blocks reverse circulation that would cool the tank through the collectors. Some controllers offer a deliberate nighttime-cooling function to dump excess summer heat, which only runs if enabled.