An anti-thermosiphon valve is fitted after the generator, in the flow direction marked on the body, and introduces a pressure drop of 5 to 15 mmWC, enough to break natural circulation when the circulator is at rest. The valve becomes essential as soon as a tank sits higher than the generator or a loop connects two floors, the configuration where hot water rises on its own and the tank loses degrees with no draw-off at all. A valve fitted the wrong way round blocks the flow and makes the pump strain, a valve fouled with sludge or swarf no longer closes: flushing the network, or fitting a filter, therefore comes before commissioning. The check is made with the circulator at rest, comparing upstream and downstream temperatures after a few minutes.
Understanding thermosiphon and parasitic circulation in your networks
Thermosiphon: why water circulates on its own even with the circulator off
Thermosiphon is natural circulation. Hot water, being lighter, rises through the pipes. Cooler water sinks back down. If the flow and return create a continuous path, the network starts "pumping" itself, even with the circulator switched off. An anti-thermosiphon valve, or a layout with a high point and a pressure drop, breaks this loop.
Parasitic circulation: signs on site (lukewarm radiators, a tank that cools down)
You spot it when a circuit heats up when it shouldn't. Typical examples. Lukewarm radiators in the shoulder seasons, a DHW recirculation loop that stays hot at night, a tank losing degrees with no draw-off. It's often linked to a height difference, a "leaky" 3-way valve, or the absence of a check valve on a branch connection.
The highest-risk situations: DHW loops, heating, mixed systems
The cases to watch first.
- Long DHW loops, rising columns, a recirculation return that's too "direct".
- Heating with parallel circuits and a permanent bypass.
- Mixed systems (heat pump, buffer tank, DHW) with short connections and low pressure drops.
Choosing the right anti-thermosiphon valve for your installation
Check valve vs. anti-thermosiphon valve: what's the difference in practice?
A check valve simply blocks flow reversal, full stop. An anti-thermosiphon valve is mainly designed to prevent natural circulation when the circulator is off, typically parasitic feedback between a tank, a boiler, or two zones. On site, you recognise it by its firmer opening pressure or a design meant to stay well sealed under convection while opening freely once the pump pushes.
Selection criteria: diameter, pressure drop, mounting direction, temperature
Choose the valve's DN to match the pipework, otherwise you create a bottleneck. Check the pressure drop and the opening pressure, especially on small pumps and underfloor heating. Respect the arrow and the intended orientation (vertical or horizontal). On temperature, allow a margin against your actual operating regime. Heating and domestic hot water don't always call for the same limits.
Where to install it: flow/return, proximity to the circulator, mixed zones
Installation is often on the return, near the circulator, to cut convection when it stops, while staying accessible. In mixed zones (radiators, underfloor, tank backup), fit a valve on every branch that could start "circulating on its own". Also think about isolation valves for maintenance, to be priced with the valve in your plumbing quote.
Installing a valve properly to block circulation without creating failures
Installation steps: identifying direction, sealing, bleeding, and recommissioning
Locate the arrow on the valve body. It must follow the actual direction of flow. Shut down the system, drain the section concerned, then fit the valve on clean, well-aligned pipework. Take care with the seal, using the correct gasket. Refill, bleed at the high point, then bring the system back into service gradually while monitoring pressure and leaks.
Common mistakes: valve fitted backwards, fouling, noise and vibration
Fitting it backwards blocks everything and strains the pump. Another classic: sludge or shavings prevent the valve from closing. Plan for flushing or a filter if the network is dirty. Abnormal noise or vibration often comes from too high a flow rate, an undersized valve, or one fitted too close to a circulator.
Quick checks after installation: temperatures, flow, a valve that doesn't seat properly
After a few minutes, compare the upstream and downstream temperatures. With the circulator off, check that thermosiphon has stopped. Check the flow rate, and listen. If the valve "doesn't seat properly", suspect a foreign object, a damaged seat, or a non-compliant installation position.
Real-world cases: valves and comfort in energy renovation (2026)
Heat pump and backup: preventing hot water returning to the heat pump
In renovation, a heat pump may operate alongside a backup unit (boiler, resistance heater, hydraulic stove). Without a check valve, water heated by the backup can flow back into the idle heat pump. The result: unnecessary cycles, a temperature that yo-yos, and sometimes control errors. A properly blocked return flow, in the right place, sends the flow back in the right direction.
DHW tank and recirculation loop: limiting losses and stabilising temperature
On a DHW recirculation loop, the valve prevents reverse circulation and thermosiphon when the pump stops. You cut losses and keep a stable temperature at the draw-off points. Also think about insulating the loops and fine-tuning the flow rate, otherwise the recirculation loop mainly ends up heating the service duct.
Collective/individual heating: reducing overheating and imbalances
In collective systems with rising columns, or in a mixed collective/individual network, a valve on certain branches stops water taking shortcuts when valves close. Fewer scorching radiators on one side, more heating network balancing on the other. To be validated at commissioning, with flow and return-temperature checks.
Maintenance and troubleshooting: making your anti-thermosiphon valve last
Simple upkeep: flushing, filtering, preventing scale and sludge
A valve holds up better over time when the water circulating stays clean. Flushing the circuit and checking the filter or sludge trap limits blockages. In hard-water areas, keep an eye on scaling on the tank side and on deposits. Water that's too heavily loaded eventually slows down closing over the long run.
Diagnosing a stuck valve: symptoms, tests, and solutions
Typical symptoms. Heat feeding back when the pump is off, lukewarm radiators outside demand, a knocking noise, or reversed circulation. Simple test. Switch off the pump, let it cool, then feel the upstream and downstream sections. If heat gets through, the valve is no longer closing. Solution. Removal, cleaning, checking the direction, then refilling. If it happens again, plan for system flushingif needed.
When to replace: wear, a tired spring, a valve that won't seal
Replace it if you find a leak, corrosion, a weak spring, or a lack of tightness despite cleaning. A valve that "scrapes" when operating, or that lets flow through permanently, wastes heat and can disrupt control on a daily basis.
