Watch everything:
$ mosquitto_sub -t /house/#
Watch settings:
$ mosquitto_sub -t /house/heating/state/#
Test:
$ mosquitto_pub -t /house/heating/settings/test -m 2
a*x + b:
$ mosquitto_pub -t /house/heating/settings/a -m ... $ mosquitto_pub -t /house/heating/settings/b -m ...