The error message

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This error occurs when Hysopt is unable to select a 2 or 3-way valve, because the calculated volume flow/pressure drop is out of range of a suitable selection. In the example above, you can see that the filled in capacity is 1W at 70/50°C, which calculates a nearly zero m³/hr flow. This was meant to be 1kW but is filled in incorrectly. Hysopt is unable to calculate a valve and hence sends this error message.