Error D.70

Safety precautions

- Do not work on gas components, internal PCB wiring or replace gas-carrying parts unless you are a Gas Safe registered engineer. If in doubt, switch the boiler off at the fused isolator and call your installer.

- Before any visual checks, turn off the boiler electrical supply at the isolator if you will be touching wiring or connectors. Do not isolate gas unless instructed by a qualified engineer.

- Take care with hot surfaces and moving parts; allow the system to cool if recently firing.

Initial homeowner checks (safe, non-invasive)

1. Note the symptoms and codes: when the problem happens record the display codes (D.70 and any S or F codes like S24, S28, S4) and what you are calling for (CH on, DHW on, both scheduled). This information helps the engineer.

2. Confirm room and cylinder thermostats and programmers: temporarily force heating on (manual) and force hot water demand (if possible) to see how the boiler behaves. Observe whether the boiler goes to heating (S4) or cylinder charge (S24) and whether the diverter moves (you may hear the actuator motor or feel the valve head moving).

3. Check wiring and LEDs on VR65/zone controller: visually inspect the VR65 and controller for indicator LEDs showing end-switch/valve positions and correct zone operation. Ensure the VR65 and VRC430 are powered and that their status LEDs look normal (no obvious fault LEDs).

4. Confirm cylinder sensor NTC connection: check that the cylinder thermostat/sensor wiring to the VR65/boiler is connected and not loose. A missing or shorted sensor can confuse the control logic.

Specific diagnostic checks you can do without opening the boiler

1. Verify boiler parameter for diverter position: the installer referenced D.70 (diverter valve position) and P6 (mid position option). Ask the installer to confirm D.70 is set to enable mid-position (often D.070 = 1 for mid). If you have documentation for your model, ask the installer to show you this setting rather than attempting to change it yourself.

2. Exercise the valve with simple commands: with heating forced on manually, observe valve behavior. Then force DHW on and observe. Finally force both demands (if your controllers allow) and see if the valve moves to a mid position and the boiler attempts to heat both circuits. If the valve does not move to the expected positions or the boiler still only serves one demand, the problem is mechanical, wiring or control logic.

3. Listen and watch: a working actuator will usually make a soft motor sound when moving and the valve head will shift. If there is no movement but the boiler is trying to serve both, suspect a stuck valve or failed actuator. If the actuator moves but flow does not change, suspect internal valve mechanism seized or incorrectly fitted.

What to check or ask your installer/engineer to do (professional work)

1. Confirm wiring and end-switch behaviour on the VR65: the VR65 must provide the correct volt-free end-switch signal(s) to the boiler for CH and DHW. Incorrect wiring or a wired-or arrangement may prevent true simultaneous demand. Ask the installer to check the schematic and that CH and DHW end-switch outputs are wired to the correct boiler terminals.

2. Confirm eBUS/VRC430 interactions: if a VRC430 weather compensator is on the eBUS, it can alter boiler requests. The engineer should check that the controller mode and priorities are set to allow simultaneous cylinder charging and CH demand when the diverter is in mid; confirm the VRC430 is not forcing priority to DHW only.

3. Check and test the diverter actuator and valve mechanism: the engineer should verify the actuator motor, limit switches and linkage are functioning and that the valve can reach the mid position. If the actuator is faulty or the valve body seized, it may need replacement.

4. Verify boiler parameter settings: the engineer should confirm D.070 and any related parameters (such as the stored gas valve offsets or valve assembly IDs) are correct for a regular/system boiler with unistor and VR65 arrangement and that P6/P settings used to force mid-position are applied properly.

5. Check cylinder NTC and sensor wiring: incorrect temperature readings can cause the boiler to stop charging. The engineer should test sensor resistance and wiring integrity.

6. Log S/F codes during tests: the engineer should run tests while watching the S-state transitions (S24, S28 anti-cycling, S4 etc.) to see why the boiler drops charging. That log helps pinpoint whether the boiler itself is cancelling charge due to temperature, flow detection or a diverter feedback signal.

When to call a professional

- If the diverter does not move when commanded, or you cannot get both circuits served when you know the boiler is set to allow mid-position, call the installer or a Gas Safe engineer. Replacing the actuator, servicing the valve or changing wiring/parameters requires a qualified engineer.

- If you see additional fault (F) codes or safety shut-downs, switch the boiler off and call a professional.

Notes and final advice

- Do not attempt to dismantle or replace the diverter valve, actuator or internal wiring unless you are qualified. Incorrect work can create gas leaks, electrical hazards or further damage.

- Provide your installer with recorded observations (which demand was active, what S codes appeared, whether you heard/seen the valve move and the D.70 setting). That will speed diagnosis and resolution.