Furnace displays E8, burner shuts down due to low water or system pressure (for hydronic furnace/air handler combinations using Danfoss controls)

Safety first: Turn off power and allow the system to cool before opening any water components. Hot water under pressure can cause burns.

Steps:

• Check system pressure gauge: Locate the pressure gauge on the hydronic loop. Typical cold pressure is around 1–1.5 bar (15–22 psi), but follow your system's specification.
• Top up system (if allowed): If pressure is low, slowly open the filling loop valve and watch the gauge rise to the recommended value. Close the valve securely afterward.
• Bleed air: Air in the system can cause false low-flow readings. Bleed radiators or air vents starting from the highest point, topping up pressure as needed.
• Inspect for leaks: Check around the expansion vessel, safety valve discharge, and visible piping for signs of water leakage. Repair leaks before repeatedly topping up.
• Check pressure sensor: Locate the Danfoss pressure sensor on the return line. Inspect wiring and connector. If the gauge shows normal pressure but E8 persists, the sensor may be faulty and require replacement.