Follow the call for heat
A thermostat, zone valve, relay or circulator may affect whether hot water is sent to a zone. The technician can confirm where the sequence stops.
Air and water flow matter
Air in the loop, closed valves, low pressure, a weak circulator or a restriction can reduce heat delivery. Purging or adding water without understanding the system can create new problems.
Distribution can hide the cause
Radiator valves, baseboard condition and balancing influence room comfort. Report which emitters are cold and whether the issue follows one floor, loop or thermostat.