How can I tell when my steam boiler needs water?
Questions  Is the floor a radiant system with the piping in the floor like concrete or under the floor (tubing running under the floor between the joists). It makes a difference. You said the system was steam but if is that would be odd.  I am going to say that you mean hot water. One note; a radiant floor heating system has a large lag time, the house does not respond quickly to a thermostat change of a quick weather change. Also if it is a radiant slab system in concrete the slab water temp is rarely above 140 degrees F. I think you need to call a heating expert.( D. George)