heartbeat timeout doesn't work

[MrAsanjar .]

In my namenode's hdfs-site.xml file, I set the following values to reduce
datanodes heartbeat timeout value to less then a minute.
  dfs.heartbeat.interval = 3
  dfs.namenode.stale.datanode.interval = 15
However it is still takes 10 to 15 minutes before it timesout..
What am I doing wrong here?