The heating is not working.
ri-scal-da-MEN-to — five syllables, stress on the fourth.
Report this immediately to reception, especially in winter. Do not suffer in silence — heating problems can always be fixed.
'Funzionare' (to work/function) is the key verb for reporting broken items. 'Non funziona' (it does not work) is your most useful phrase for any malfunctioning equipment. 'Il riscaldamento' = the heating system.
Il termosifone è freddo.
The radiator is cold.
Specify what you have checked to help staff diagnose the problem
Non riesco ad alzare la temperatura.
I cannot raise the temperature.
Say this when the thermostat seems unresponsive
La camera è gelida.
The room is freezing.
A stronger way to emphasise the urgency of the problem
Italian hotels in many regions only activate central heating between November 1 and April 15, as regulated by local energy laws ('Decreto termoregolazione'). If you arrive outside these dates and the room is cold, ask for a portable heater ('stufa portatile') or extra blankets ('coperte extra').