Continuing the discussion from Zigbee devices flaky – anyone else?:
thinking of doing this to prevent this from happening again but i am curious of the range of these and if putting one in the room with the hub will do any good or harm. seems like the last time i had this event one of the sensors was under 10’ away from the hub.
See the following. It should answer most of your questions. 
http://thingsthataresmart.wiki/index.php?title=Is_this_device_a_repeater%3F
The hub itself is a repeater, and it is common to have other devices that are also repeaters in the same room, since light switches, pocket sockets, in wall relays, and plug-in sensors are all repeaters. So you don’t have to worry about that part at all.
A zigbee repeater is not like a Wi-Fi booster. It doesn’t change the signal at all. It just passes the message on to another device that is farther away, which helps extend the range. Again, details in the wiki article.