Error D.22

Safety first

- If you smell gas: do not operate electrical switches or the boiler. Turn off the gas at the meter if safe to do so, ventilate the area and call the emergency gas number immediately.

- If you are unsure or uncomfortable with any step, stop and call a Gas Safe engineer. Do not attempt internal repairs to gas or electrical components.

Initial quick checks a homeowner can do

1. Look at the boiler display: note D.22 and check for any additional fault codes (F-codes). If only D.22 appears while you have a hot tap open, that is a normal DHW demand signal.

2. Open a hot tap and see whether the boiler fires and hot water arrives. Listen for the boiler firing and the pump running.

3. Check the boiler pressure gauge on the front panel. Normal pressure is about 1.0–1.5 bar when cold. If pressure is below about 0.5–0.8 bar the boiler may lock out with an F22 (low pressure).

4. If the boiler shows other fault codes, write them down — they point to the real problem and may require an engineer.

If the problem is low system pressure (common cause of no hot water and F22, and a common related check when seeing D.22 but no hot water)

1. Only proceed if you are confident and your boiler has a visible/fixed filling loop. Locate the filling loop — usually a silver flexible hose or two small valves underneath the boiler. If you have an external filling loop that must be connected, do not improvise one; call an engineer.

2. With the boiler powered on and the pressure gauge in view, slowly open the filling loop valves (usually two taps) and watch the pressure gauge. Keep your hand on the valve(s) and listen — when the pump starts the pressure can rise quickly.

3. Increase pressure to about 1.0–1.5 bar. Close both valves fully and make sure they are returned to the closed position. Wait for the pump to stop and watch the gauge settle (it can drop slightly after the pump stops).

4. If the pressure stays up and the boiler clears its fault after a reset, the immediate issue is solved. If pressure continues to drop repeatedly, you likely have a leak, faulty filling loop, or expansion vessel issue and must call an engineer.

Resetting the boiler

- For many Vaillant units the power button also acts as a reset. A common method is to press and hold the power button for about 5 seconds to reset. If a fault returns after reset, further diagnosis is required.

If D.22 is shown but there is no hot water and pressure is correct

- Check hot water temperature setting on the boiler and any pre-heat or timer settings. Ensure domestic hot water mode is enabled.

- If you have a system boiler with a hot water cylinder, check the cylinder thermostat and electrics to the cylinder immersion/stat if accessible; call an engineer if unsure.

- If the boiler fires but no hot water comes, possible causes include a stuck/failed diverter valve (on combi or system boilers with motorised valves), pump faults, or sensor issues. These are not user-repair items and need a Gas Safe engineer.

When to call a professional

- Persistent or repeating pressure loss after topping up.

- Any ignition, flame or gas-related fault codes (F27, F29, F61, etc.) or if the boiler locks out and will not reset.

- Suspected leaks, failed diverter valve, failed pump, faulty sensors, expansion vessel issues, or any internal component replacement.

- If you smell gas or see signs of combustion problems or smoke.

Final notes

- D.22 by itself usually just means the boiler is responding to a hot water demand. If the boiler is not delivering hot water or other fault codes appear, follow the checks above and call a Gas Safe registered heating engineer to diagnose and repair any gas, ignition, pump, diverter valve or sensor faults. Avoid attempting gas or internal boiler repairs yourself.