The now cold walls will drag all the warm air into them until its warm and only then will the church begin to feel warm which can take as much as 2 hours longer running time. By leaving the thermostat on a low level temperature it will circulate warm water when required but not let the fabric get cold. Once the system is then in demand the warm air will not take as long to heat the church. This will then shut off the maximum thermostat control shutting off the boiler and saving cost on running the system. This will also protect the building from damp and condensation which is a minimum amount of requirement needed in any listed building no matter what the grade.
Check out our church heating advice page here for more helpful tips.