how to simulate without thermostat schedule?

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At a minimum, it seems like you could specify a thermostat with a very high setpoint so that it always calls for heating.

Most buildings with radiators do not have thermostats as your building, but occupants control the temperature by opening or closing the valves in the radiators. You can assume that the occupants will try to keep the spaces at around 20-22oC, (Depends on the country too). By setting that as the thermostat temperature the results will grossly represent reality.

Also, in Design Builder in particular if you don't want to even use thermostats and just want to keep temperatures "at a certain setpoint all year" go to: Model options-Data-HVAC-Set the slider at "Advanced HVAC"-Set the drop window to "1-Simple HVAC data". This will make the HVAC use the simple HVAC data window for heating and cooling setpoints, simply keeping the zones at those temperatures (if the availability schedule allows for it).