Remeha Boiler Error Code H.01.21
H.01.21 means the temperature rises too quickly while hot water is being used. The cause is usually in the flow. Check the pressure and bleeding; if it returns, an installer must check the hot water circuit.
Also shown as: H01.21
Applies to: Remeha Ace platform (Tzerra Ace, Calenta Ace, Avanta Ace)
Common causes
- Temperature rising too quickly because of insufficient flow on the hot water side
- Pump or flow in the hot water circuit not in order
What to check, in order
- Check the water pressure and bleed the system.
- Reset the boiler once and test whether the hot water works normally again.
- If the code returns, call an installer.
Clear the code and test
- Fix what the checks above found first — pressure topped up, a radiator bled, the thermostat set right. Resetting without fixing the cause only brings the code back.
- Make sure the boiler has power and the gas tap at the boiler is open.
- If the water pressure is below 1.5 bar, top it up to between 1.5 and 2 bar with the filling loop, as your manual shows.
- Press the reset button once and watch a full cycle: the boiler should start, heat the radiators or the tap water, and keep running without the code coming back.
When to call a professional
Most cases are solved by the checks above. Book a technician if any of these apply:
- The code comes back after you have run the checks and a test cycle.
- You smell burning, see sparks or scorch marks, or the breaker trips.
- Water is leaking and you cannot stop it.
- The fix needs panels opened beyond what your manual describes, or a part tested with a meter, and you are not set up for that.
When you call, give the brand, the code H.01.21, your model number (on a label on the type plate, usually at the bottom or on the side of the boiler) and the checks you already ran. It shortens the visit.
Parts that may be involved
Circulation pump
Parts are model-specific. Find your model number first — usually on a label on the type plate, usually at the bottom or on the side of the boiler — and match the part to that number before ordering.
Questions people ask about H.01.21
What does H.01.21 mean on a Remeha boiler?
H.01.21 means: The flow temperature rises too quickly during hot water operation (maximum DHW temperature gradient exceeded). H.01.21 means the temperature rises too quickly while hot water is being used. The cause is usually in the flow. Check the pressure and bleeding; if it returns, an installer must check the hot water circuit.
Can I fix Remeha H.01.21 myself?
Start yourself: the checks on this page cover the causes you can reach without opening the machine up. If they point to a faulty part, testing or replacing it is usually a technician job.
How do I clear Remeha H.01.21?
Fix what the checks above found first — pressure topped up, a radiator bled, the thermostat set right. Resetting without fixing the cause only brings the code back. Make sure the boiler has power and the gas tap at the boiler is open. If the water pressure is below 1.5 bar, top it up to between 1.5 and 2 bar with the filling loop, as your manual shows.
Sources checked
The meaning and checks for this code were checked against the sources above in November 2026. Codes vary by model and region; if your manual lists a different meaning for your exact model, follow the manual.