Control shows E07 and the burner does not start; may occur immediately on power‑up.

Safety: Allow the system to cool before adding water. Overfilling a hot system can cause thermal shock.

• Check pressure gauge: Look at the system pressure gauge on the Viadrus furnace or nearby pipework. For most systems, cold pressure should be around 1.0–1.5 bar (check your manual).
• Top up system: Locate the filling loop (flexible hose with two valves) connected between the mains cold water and the heating return. Open both valves slowly and watch the gauge rise. Close both valves when the correct pressure is reached. Remove the hose if required by local regulations.
• Inspect for leaks: Walk around the property and check radiators, valves, and visible pipes for signs of water. Repair any leaks before repeatedly topping up.
• Check expansion vessel: If pressure rises quickly when heating and then drops to zero when cold, the expansion vessel on the Viadrus furnace or external vessel may have lost its air charge or bladder. This requires a technician to test and recharge or replace the vessel.
• Pressure sensor: If the gauge shows normal pressure but E07 persists, the electronic pressure sensor or mechanical pressure switch may be faulty and should be replaced.