Error S.85

Safety precautions:

1. Before doing anything, read the boiler manual. If you are unsure, stop and call a Gas Safe engineer. Do not work on gas components or internal electrical parts. If you must isolate power, use the boiler on/off switch or the fused spur; if you isolate electricity to check components, be aware it will reset the boiler and controls.

2. Keep children and pets away. Beware of hot surfaces and scalding hot water when bleeding radiators or working near the boiler.

3. If you smell gas, leave the building immediately and call the gas emergency number — do not operate electrical switches or the boiler.

Initial homeowner checks (safe, do these first):

1. Note any other fault codes or messages on the display and the exact behaviour (no heating, no hot water, intermittent). This information is useful for the engineer.

2. Check the system pressure on the boiler gauge. Target pressure for most sealed systems is about 1.0–1.5 bar when cold. If pressure is low (below about 0.8–1.0 bar), top up the system using the filling loop following the boiler manual. After topping up, bleed radiators to remove trapped air and re-check pressure.

3. Bleed one or two radiators (start with those closest to the boiler) to remove airlocks. After bleeding, re-check system pressure and re-pressurise if necessary.

4. Put the thermostat or programmer to demand heat (raise set temperature) and observe the boiler. Listen for the pump running — you should hear a steady hum or whir. Note any unusual noises: loud banging, clunking or silence when the boiler should be circulating.

5. Check radiator valves and TRVs are open and any isolation/service valves around the boiler and pump are in the open position (two small gate/lever valves either side of the pump are usually open when parallel with the pipework).

6. Check accessible external filters/magnetic filters (if you have one fitted on the system) for debris and empty the removable strainer if you are comfortable doing so and the system manual allows it. Always isolate power and follow the manual before removing filters.

Specific diagnostic steps and fixes (if comfortable and safe to proceed):

1. Try a boiler reset: switch the boiler off at the control, wait 5–10 minutes, then restart. If S.85 was an intermittent condition it may clear; if it returns, further checks are required.

2. If system pressure is low: after repressurising and bleeding radiators, demand heat and watch whether flow is restored. If pressure repeatedly drops, there is a leak in the system that needs investigation.

3. If pressure is OK but there is still poor circulation: check pump operation by listening when the boiler is calling for heat. If the pump is completely silent or just humming, it could be seized or electrically faulty. Do not try to dismantle the pump unless you are qualified. Note the symptom and call an engineer.

4. Check for airlocks: after bleeding radiators and topping up, some systems need a pump-priming or purge procedure. Some Vaillant boilers have a pump purge/program (refer to the boiler P-codes and manual). If you are comfortable, follow the manual’s pump purge instructions; otherwise call an engineer.

5. Check for blockages: closed isolations, blocked strainers on the DHW heat exchanger, or heavy sludge in the system can restrict flow. If you have a magnetic filter fitted and can access it safely, isolate and clean it. If you suspect internal blockage or heavy debris, a powerflush or chemical clean performed by a professional is likely needed.

6. Sensors and wiring: S.85 can be related to incorrect sensor readings (flow/return NTCs) or wiring faults. Checking sensor wiring and replacing thermistors requires opening the boiler and electrical testing — do not attempt unless qualified. Record any sensor-type fault codes to pass on to the engineer.

7. If you have an in-line mass-flow or impeller sensor (some models) check for wet/blocked filter caps as described in the manual; again, isolate power and follow manufacturer guidance before touching these parts.

When to call a professional and what to tell them:

1. Call a Gas Safe registered engineer when: the fault persists after the initial checks and reset, the pump does not appear to be running or is noisy, you cannot restore or maintain system pressure, you suspect blockages or internal scale/debris, the boiler reports other fault codes, or you are not comfortable performing any of the checks above.

2. When you call, describe the boiler model (Vaillant EcoTEC 65kW), the S.85 message, any other displayed codes, what you have already done (pressure checks, bleeding radiators, reset), pump noise/behaviour, and whether hot water or heating is affected. This helps the engineer bring correct parts and diagnostic tools.

Final notes:

- Do not attempt repairs on gas valves, internal pump electronics, PCB, or replace sensors unless you are a Gas Safe qualified engineer. Working on gas appliances without the correct qualification is dangerous and illegal.

- S.85 is a circulation-related service message that can sometimes be fixed by simple homeowner checks (pressure, bleeding, open valves), but persistent issues require professional diagnosis to avoid damage or unsafe conditions.