F22 displayed, furnace will not start, low water pressure warning

Safety first: Allow system to cool before refilling.

• Step 1 – Check pressure: Confirm the gauge reading. If below 0.8 bar, refill via the filling loop to about 1.2–1.5 bar.
• Step 2 – Check expansion vessel: If pressure rises rapidly when heating and then drops to zero when cool, the expansion vessel may be flat or ruptured. This requires a technician to recharge or replace.
• Step 3 – Leak inspection: Look for visible leaks around the Protherm furnace, safety valve discharge pipe, and system radiators.
• Step 4 – Sensor check: If pressure is normal but F22 persists, test and replace the pressure sensor as needed.