I have a concrete ceiling with an old-style electrical box that has a stud for a hickey, but does not have the traditional mounting holes for fixture screws, like the box in this diagram:
Can I install a ceiling fan using a hickey and crossbar kit to provide a mounting location for the ceiling fan bracket?
The box is embedded in concrete so replacing it is not practical. I'm confident it will support the weight of the fan but I'm worried the crossbar might not provide enough stability.
You have two issues:
Strength of the hickey - Some hickeys are cast from cheap pot metal and would be sketchy. Some are steel but bent in the shape of a U with one open side, also problematic. Heavier cast iron ones with support on both sides wold probably be better.
Strength of the crossbar - Also an issue. consider a heavier duty type or even a full round. Then consider doubling the cross bar up with small bolts joining the pair.
Supplement - In line with other answers and comments, there are real risks installing a fan with other than a properly rated fan box. Perhaps most important is the security of the box itself. The fan depends on the plate, the plate on the hickey, the hickey on the stud and the stud on the box. Is the box well enough attached that it can't be loosened by fan vibration? Also, if the plate is not pulled tight against the box, the fan is balanced on the single point of the extension coming from the hickey.
The primary consideration is not the static load, but the dynamic load from a heavy, rotating load.
That's why there are electrical boxes specifically designed and designated by code for ceiling fan loads.
See http://homerepair.about.com/od/electricalrepair/ss/elec_box_ltg.htm#step5
