Boiler Flow Switches: Function, Faults, and When to Replace Them
There are many components in a modern combi boiler; the flow switch is one of the unsung heroes. Found in combi and system boilers, the flow switch helps regulate hot water delivery and keeps your system safe by ensuring correct operation before ignition. When it fails, the results are immediate: no hot water, error codes, and a puzzled homeowner. This guide explores what boiler flow switches do, how they fail, and how to maintain or replace them effectively.
What Is a Boiler Flow Switch?
A flow switch is a safety and control component that detects water movement within the boiler. It plays two key roles:
- In hot water mode: It senses when a hot tap is turned on, prompting the boiler to fire up and provide hot water.
- In heating mode, It ensures the pump is circulating water before ignition is allowed.
Flow switches come in different styles—paddle switches, turbine sensors, reed switches—and are typically found near the domestic hot water heat exchanger or on the return flow.
Without a functioning flow switch, the boiler won’t fire up because it cannot confirm safe flow conditions.
Why Flow Switches Matter to System Operation
A boiler needs to know when water is moving to activate safely. The flow switch prevents:
- Dry firing (igniting with no water flow, which can damage the heat exchanger)
- Unnecessary cycling or burner activation
- Water wastage or overheating due to inaccurate flow detection
Depending on demand, the flow switch in a combi boiler works with the diverter valve to direct heat to hot water or radiators. If the flow switch doesn’t trigger correctly, you may get:
- Hot water delay or no hot water at all
- Boiler ignition failures
- Radiators are not heating as expected
- Constant boiler cycling or error codes
Typical Signs of a Faulty Flow Switch
- Taps run cold even though the boiler shows demand
- The boiler fires briefly and shuts down
- Fault codes referencing flow sensor, ignition failure, or DHW faults
- Inconsistent hot water temperatures
- Visible corrosion or scaling on the switch housing
Flow switches can also stick due to sludge, limescale, or debris in the system. This is particularly common in older or poorly maintained boilers.
Causes of Flow Switch Failure
- Debris or sludge obstructing internal sensors
- Hard water scaling around moving parts
- Worn-out internal components after years of service
- Loose electrical connections are causing intermittent faults
- Incorrectly rated or fitted replacement parts
If a flow switch is suspected, it should be tested with a multimeter or diagnostic tool before replacement.
Benefits of Reliable Flow Switches
- ✅ Ensures accurate hot water delivery on demand
- ✅ Prevents dry firing and component damage
- ✅ Contributes to energy efficiency by reducing unnecessary burner activation
- ✅ Key safety control in combi systems
- ✅ Available as OEM and BSI-compliant refurbished parts
- ✅ 12-month warranty included with NBS refurbished switches
See our full range of flow switches for top brands and fast dispatch.
Expert Insight
“Flow switches are critical for DHW delivery. If the boiler isn’t responding when a tap is opened, we always check the switch before diving deeper.” — Senior Engineer, Sheffield
“We see a lot of stuck switches due to magnetised sludge in older systems. A good power flush or inhibitor can prevent recurring faults.” — Technical Advisor, NBS
When to Replace vs When to Clean
Replace If:
- The flow switch no longer clicks or moves under flow
- There’s no continuity during testing
- The switch triggers errors despite clear pipework
- It shows signs of cracking, scaling, or corrosion
Clean If:
- It’s only lightly scaled or has minor debris
- You’re conducting annual servicing
- The system is suffering from intermittent flow issues
Cleaning involves gentle descaling, checking movement, and clearing any sludge build-up—always follow the manufacturer’s guidance.
Maintenance Tips
- 🔄 Add an inhibitor to the system to reduce limescale and sludge formation
- 🧼 Clean flow switches during annual boiler servicing
- 🔍 Visually inspect for corrosion, seal wear, or cracks
- 🧰 Always test with a multimeter after cleaning or before replacing
- 🔧 Check for proper wiring and connection tightness
FAQs
1/ How long does a boiler flow switch last?
Typically 5–8 years depending on water quality and usage.
2/ Can I test a flow switch myself?
You’ll need a multimeter and the ability to safely access the part. If unsure, consult a Gas Safe engineer.
3/ Do all boilers use the same type of flow switch?
No. Flow switches are model-specific—always check your boiler model and match accordingly.
4/ Are refurbished flow switches reliable?
Yes. Our NBS refurbished switches are tested to BSI standards and have a 12-month warranty.
5/ Why does my boiler keep clicking but not lighting?
It could be that the flow switch is not triggering correctly. It’s a common fault in combi boilers.