How to set a differential thermostat on DesignBuilder

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Hi everyone,

On design-builder I have a zone conditioned by a pthp (packaged terminal heat pump) with a winter set point temperature of 21°C. I want to insert a differential thermostat so that, for example, the pump switches off at 21.5°C and switches on again at 20.5°C. Now, however, it is always on to maintain a constant temperature of 21°C which in reality does not happen, what can I do?

Thank you

Marco