W5
Universal HVAC System
Severity: ModerateWhat Does This Error Mean?
The W5 error means the water flow sensor has detected a problem with water moving through your system. This applies to hydronic systems — heating and cooling systems that use water pipes instead of air ducts. If water is not flowing correctly, the system shuts down to prevent damage.
Affected Models
- Hydronic heating systems (boilers)
- Chilled water cooling systems
- Water-source heat pumps
- Radiant floor heating systems
Common Causes
- Water circulator pump has failed and water is not moving through the pipes
- Zone valve is closed or stuck, blocking water flow to a section of the system
- Air pocket trapped in the pipes is blocking water circulation
- Water flow sensor itself is faulty or has mineral scale buildup
- System water pressure is too low — not enough water in the system
How to Fix It
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Check the system pressure gauge on your boiler or hydronic unit.
Most hydronic systems should show 12–25 PSI. If it reads very low or zero, the system may need water added. Do not add water to a hot system.
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Listen near the circulation pump for any hum or vibration that indicates it is running.
A completely silent pump when the system should be heating or cooling is a sign the pump has failed.
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Check that all zone valves are in the open position.
Zone valves are motorized valves that open and close to direct water. A stuck closed valve blocks flow entirely to that zone.
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Bleed the system to remove trapped air pockets if you hear gurgling sounds in the pipes.
Most hydronic systems have bleed valves at high points in the pipework. Consult your manual — improper bleeding can drain the system.
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If the above steps do not resolve the error, call a technician to test the flow sensor and circulation pump.
A faulty flow sensor gives false readings. A failed pump needs replacement. Both are repairs for a qualified technician.
When to Call a Professional
Hydronic systems involve pressurized water and can be complex to diagnose. Call an HVAC technician or plumber experienced with hydronic systems.
Frequently Asked Questions
What is a hydronic HVAC system?
A hydronic system uses hot or chilled water pumped through pipes to heat or cool your home. Instead of air ducts, it uses radiators, fan coil units, or radiant floor tubing.
What does a water flow sensor do in an HVAC system?
It measures whether water is actually moving through the pipes at the right rate. If flow is too low or stops completely, the sensor triggers an error to protect the boiler or chiller.
How do I know if my circulation pump has failed?
Put your hand on the pump body when the system should be running — it should be warm and you should feel a slight vibration. A cold, silent pump is a sign it has stopped working.