Some central heating systems will stop working if the system pressure becomes too low. You will know if you have a pressurised system by the pressure gauge on the front or underneath of the boiler. The pressure should not fall below 1/2 bar. To increase pressure, look for a silver braided hose that will need connecting (if not already) between 2 valves: this is the link pipe between mains water and the heating system. Slowly turn one or both (if fitted) valves to depressurise. Follow your particular boiler guide on how to top up with the silver braided hose, or the white link key for a Worcester Bosch boiler. These are the most common but there are other boilers and scenarios where this information will not apply.