This sounds like an air operated thermostat or pnuematic T-stat in the trade. This T-stat is connected to a valve in your ceiling or radiator and is a valve that also works on air pressure. Because heating valves fail to the open position the office is getting hotter because the rubber diaphram in the valve or air line going to or from the valve is leaking air and is allowing the valve to always stay a bit open. The hiss that you hear from the T-stat is the stat trying to make up the the leaking air.
WHAT I AM SAYING IS YOU NEED A PLUMBING & HEATING TECH.