When a system is designed it is is designed for max load conditions. This is usually -2 or -3 outside temperature and a target room temperature of 21°c. From the calculations you will determine the heat loss in watts per room.
Now say a room has a window or door left open, this will massively increase the heat loss, so much so it's unlikely that room will ever reach target temperature, even on a mild day. It will also cause what's called micro zoning, whereby Any adjacent rooms will struggle as well.
If the system was designed correctly then the output of the heat source will provide enough power to maintain target temperature at maximum load. If in milder weather the boiler could potentially reach and increased target temperature but leaving windows open will impede that chance.