The Efergy transmitter will work up to around 230ft (40m) within the home. This is dependent on the amount and type of walls. It works on the 433.5MHz frequency range, which is well suited for multi-story home use.
The transmission range will depend upon the environment in which it is working. If the building contains ironwork in the walls, thick concrete walls or floors and other wireless technology such as weather stations and wireless thermostats or other interference, then this can greatly reduce transmission ranges and possible cause the monitor to drop its link. If you have problems receiving a signal from the transmitter try moving the monitor around the building to find the optimum hi signal areas.