f4
Moderate
Rheem · HVAC Systems & Water Heaters
Thermal Fuse Activated (Tankless Water Heater)
Quick Answer
Error code F4 on your Rheem HVAC Systems & Water Heaters means: Thermal Fuse Activated (Tankless Water Heater). This is usually fixable — see the step-by-step guide below.
Likely Causes
How to Fix It
1
Check the exhaust vent for blockages including bird nests, ice, or vent termination restrictions
2
Verify the combustion air intake is not blocked or restricted
3
Descale the heat exchanger using a vinegar flush or commercial descaling solution and a pump
4
Check the thermal fuse with a multimeter for continuity (no continuity means it has tripped)
5
Thermal fuse replacement and heat exchanger descaling should be performed by a licensed plumbing or HVAC technician
Pro Tip
This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.
Parts You May Need
Thermal fuse
Descaling pump and hoses
Descaling solution (white vinegar or commercial)
Vent pipe components (if blocked)
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