A check valve (non-return valve) lets fluid flow in one direction only and closes automatically against reverse flow. In solar circuits its key job is stopping night-time reverse thermosiphoning, in which warm tank water circulates backwards to the cold collector and radiates stored heat to the sky. Solar gravity brakes open at roughly 20 mbar of forward pressure.
What does a check valve do in a heating or solar circuit?
A check valve permits flow in its marked direction and shuts against any attempt to flow backwards — no actuator, no power, just a flap, poppet or plate moved by the fluid itself. In closed heating and solar circuits its purpose is rarely accident protection and almost always the prevention of unwanted natural circulation: warm water is lighter than cold, and wherever a warm vessel connects to cooler pipework, gravity will quietly drive a circulation loop the designer never intended. In potable installations, spring check valves are also the working element of backflow protection under EN 1717, keeping heated or treated water out of the drinking water supply.
Why does a solar circuit lose heat overnight without one?
Because the day's flow pattern reverses itself. After sunset the collector becomes the coldest point of the circuit while the tank stays warm — exactly the density arrangement that drives a thermosiphon system, only backwards. Warm tank water rises into the return pipe, cools in the collector, sinks back down the flow pipe, and the collector spends the night as a radiator aimed at the sky. The effect is silent and shows up only as a tank several kelvin colder each morning; over a season it can erase a meaningful share of the harvest. That is why every solar pump station carries gravity brakes in both lines — one line alone still allows a creeping single-pipe circulation loop.
Which types of check valve exist?
| Type | How it closes | Typical use |
|---|---|---|
| Swing (flap) valve | Gravity swings a hinged disc shut | Larger pipes, lowest pressure drop |
| Spring-loaded in-line valve | Spring presses a poppet onto its seat, works in any orientation | Pump discharge, DHW circulation |
| Gravity brake | Weighted plate opens at ≈ 20 mbar forward pressure | Solar stations, anti-thermosiphon duty |
| Double check valve | Two spring stages in series | Potable backflow protection (EN 1717) |
The gravity brake deserves the closer look: its opening pressure of roughly 20–50 mbar is deliberately higher than any density-driven pressure difference, so natural circulation cannot lift it, while the pump opens it effortlessly. Most designs have a service position — turning the cap or slot puts the valve permanently open for filling, flushing and complete draining.
Where are check valves installed in solar and heating systems?
Four places cover most systems: in the flow and return lines of the solar circuit (usually integrated in the pump station); in the domestic hot water circulation loop, so the circulation pump cannot push backwards through the tank; after each generator in multi-source systems, so a running heat pump does not circulate through an idle boiler or solar exchanger; and in the cold water inlet group of pressurised cylinders as part of the safety assembly. In every position the valve must match the medium — glycol-tolerant seals for solar fluid — and its opening pressure must be included in pump sizing.
When is a check valve the wrong component?
Whenever flow must be able to reverse or drain. A drain-back system relies on water falling back out of the collectors when the pump stops — a check valve in that path would defeat the entire freeze-protection concept. A thermosiphon solar water heater is powered by natural circulation, so a brake in its loop stops the system dead. And during filling, flushing and fluid changes, gravity brakes must be set to their open service position, then closed again afterwards — a step that commissioning checklists exist to catch.
Frequently asked questions
How do I know my solar system needs a new check valve?
The classic sign is a tank that cools unusually fast overnight while the collector or its piping feels warm before dawn. A stuck-open or fouled gravity brake lets reverse circulation run; replacing or cleaning the cartridge restores the barrier.
Do I need one gravity brake or two?
Two — one in the flow line and one in the return. Reverse thermosiphoning can establish itself through a single pipe as an internal convection loop, so twin-line solar pump stations fit a brake in each line as standard.
Does a check valve add pressure drop?
Yes — the opening pressure, roughly 20–50 mbar for solar gravity brakes, acts as a permanent head the pump must overcome. It is accounted for in pump station design and is negligible for correctly sized circulators.
Is a gravity brake the same as a check valve?
A gravity brake is a light-duty check valve tuned for anti-thermosiphon service: low opening pressure, glycol-resistant seals and a manual open position for filling and draining. Every gravity brake is a check valve; not every check valve makes a good gravity brake.