Error D.70

Safety precautions

1. Never open the boiler casing or work on gas or sealed parts. Only a Gas Safe engineer should access internal components. 2. Isolate electrical supply to the boiler and any external controls before touching wiring terminals. 3. Do not tamper with gas pipes, burners or the flue. 4. If you smell gas or suspect a leak, turn off the gas supply at the meter, evacuate the property and call the emergency gas number.

Initial homeowner checks (safe, no-entry checks)

1. Check the boiler display and confirm the message is D.70 and note any other messages or status codes. 2. Check central heating and hot water programmer schedules (VRC 430): ensure DHW and CH times are set as you expect. 3. Put the boiler into manual CH mode and then call for hot water; see how the boiler responds. Put it back to auto. 4. Listen at the boiler while calling for DHW and while calling for CH: can you hear the diverter motor move or the zone valve motors operate? 5. Check the cylinder thermostat and room thermostat settings and temperatures to confirm they are actually calling for heat when you expect them to. 6. Check central heating pressure and confirm boiler has normal operating pressure. Top up if low following the boiler manual (do not open the internal casing).

Specific diagnostic checks you can do without opening the boiler

1. Observe zone valve heads or wired actuator indicators on the VR65 and on the hot water cylinder valve. If fitted, the indicator should show the valve position. When DHW calls, the DHW valve should move to the DHW position. When both are called and simultaneous operation is intended, you should see CH valve and DHW demand signals present. 2. Verify wiring at the VR65 controller: confirm CH and DHW end-switch wires go to the correct VR65 terminals and that the VR65 outputs to the boiler are connected to the boiler's CH and HW demand inputs (power off before inspecting terminals). If you are not confident with wiring, photograph the terminals and wiring for the engineer and do not disconnect anything live. 3. Check the cylinder NTC probe wiring and plug are securely connected. A faulty or disconnected NTC can stop correct charging logic. 4. If the boiler is set to allow simultaneous charging in diagnostics (d70 = 1 or mid-position), note that the system controller (VR65/VRC430) may still enforce priority. Observe whether the boiler enters s24 (charging) then rapidly goes to s28 (anti-cycling). That suggests the boiler is not seeing sustained demand from the valve end switches or sensors.

Steps to narrow the fault (do not open boiler)

1. Replicate the fault: set DHW programmer to ON and lower cylinder setpoint so cylinder calls for heat. While DHW is in its programmed ON window, force CH on from the controller (manual) and see whether the boiler runs both. If forcing CH makes the boiler sustain firing and charge the cylinder, the problem is signalling or priority rather than a mechanically stuck valve. 2. If the boiler only runs when you force CH to manual, check end-switch continuity on zone valves (with power isolated) or confirm the VR65 end-switch outputs change state when valves move. 3. If you hear no diverter movement when DHW or CH demand changes, the diverter actuator may be faulty or stuck. If you hear it try to move but the valve position does not change, the valve may be seized or mechanically blocked (limescale or debris) or the actuator coupling is stripped. 4. If wiring and end-switch behaviour look correct but the boiler still behaves as if only one demand can be satisfied, the issue may be an internal boiler setting or a software/firmware/priority interaction between the VR65 and the boiler. This requires installer-level diagnostic access and possibly changing the d70 setting or other manufacturer parameters.

When to call a professional and what to ask them to check

1. Call a Gas Safe registered engineer if any work inside the boiler is needed, for valve actuator replacement, diverter valve replacement, internal wiring, PCB or eBUS faults, or if you do not understand wiring checks. 2. Ask the engineer to: check diverter actuator operation and valve travel, verify zone valve end switches and VR65 outputs, check cylinder NTC and wiring, inspect for limescale or seized diverter and clean or replace as required, and check and if necessary correct the boiler diagnostic setting d70 or other installer parameters so they match the system arrangement (S-Plan with VR65 and VRC430). 3. If the system was recently retrofitted or reconfigured, ask the installer to verify wiring diagrams and that the VR65 has been commissioned correctly with the boiler and Unistor cylinder. 4. Do not let non-registered personnel attempt internal repairs. Any replacement of diverter motor, valve or electronic modules must be done by a Gas Safe engineer.

Summary

D.70 points to diverter valve position/settings or interactions between valve, controller and boiler. Start with safe observation and basic checks of schedules, thermostats, valve movement and wiring at the controller. If signals, end-switches or valve travel are wrong, or if you need to change installer-level parameters, get a Gas Safe engineer to diagnose and repair. Do not open the boiler or attempt internal repairs yourself.