Error 78

Safety precautions:

1) If you are not confident working around gas or mains electric appliances, stop and call a Gas Safe engineer. Do not attempt internal electrical or gas repairs.

2) Before doing any checks that involve the boiler controls or casing, switch the boiler off at the programmer and isolate the mains power at the fused spur or switch. Allow the boiler to cool if it has been running.

3) Never tamper with gas components, the burner compartment or sealed parts.

Initial checks a homeowner can do:

1) Look at the pressure gauge. Many ATAG boilers show pressure on the front display or via an analogue gauge under the lower panel. Typical target pressure when cold is about 1.0–1.5 bar (check your manual for the exact target). Note the reading and whether it is low (<1.0 bar), very low (<0.5 bar) or high (>2.5–3.0 bar).

2) Visually inspect visible pipework, radiators, valves and around the boiler for obvious leaks or water staining. Check the floor under the boiler and any visible joints.

3) Check whether the PRV discharge pipe is dripping or has recently discharged water to outside — this indicates overpressure or a PRV issue.

4) If pressure reading is low, locate the filling loop (usually a flexible hose or two valves under the boiler). If you cannot find it, stop and call an engineer.

Specific diagnostic and fix steps you can attempt safely:

1) If pressure is low and you are comfortable doing so: with the boiler isolated from the mains electric and cool, follow your boiler manual to open the filling loop valves slowly. Open quarter-turn and listen for water entering the system. Watch the pressure gauge and close the valves when pressure reaches the recommended cold pressure (commonly about 1.0–1.5 bar). Ensure the filling loop is closed and any removable filling hose is re-secured.

2) Switch the boiler back on and press the reset if the boiler shows a lockout. Many codes clear once correct pressure is detected. If the display shows how to access pressure readout (some models use service menu A6), use that to confirm.

3) If pressure rises above the recommended maximum: do not keep adding or operate the boiler. To reduce pressure you can carefully bleed a radiator using a radiator key until the pressure drops to a safe level, watching the gauge. If the pressure remains high or the PRV is discharging water, isolate the mains water to the boiler (if you know how) and call an engineer.

4) If you refill the system but the pressure falls again over a short period, this suggests a leak or a failed expansion vessel/PRV. Check again for leaks; if none visible, do not continue topping up—call an engineer.

5) If the pressure gauge reading is erratic or the boiler continues to show 78 despite correct/normal pressure, the fault may be a bad pressure sensor, loose connection or earth short. Do not open the sealed gas area or PCB if you are not qualified. You may safely turn the boiler off and call a Gas Safe engineer and describe that code 78 persisted despite correct pressure and you checked for leaks and filling loop operation.

When to call a professional:

1) Call a Gas Safe registered heating engineer if you find any gas smells, persistent leaks, the pressure will not stay up or keeps rising, the PRV is discharging, or if the fault returns after you reset the boiler.

2) Call an engineer for any sensor, wiring, expansion vessel or PRV repairs, or if you are not confident performing the filling loop/refill procedure.

3) If the boiler shows additional error codes, or if there are repeated lockouts or other symptoms (no heating, no hot water, strange noises), book a professional inspection.

Final notes:

1) After any pressure adjustment and successful reset, monitor the system over the next 24–48 hours for leaks or pressure loss.

2) Regular annual servicing by a Gas Safe engineer helps prevent pressure-related faults and ensures safety and reliability.

3) Do not attempt internal electrical or gas component repairs yourself; these must be performed by a qualified engineer.