UTH = under tank heat (can be a pad or heat tape). The reason for the dimmer or a thermostat is to control the heating device. The dimmer will adjust the wattage down and thus decrease the amount of heat the pad will emit. A thermostat can be set to turn your UTH off on your hot side at a certain temp (i.e. 93) and on at another temp (i.e. 89). Usually the thermostats are more effective in maintaining proper temps as it has a probe to monitor the temp while dimmers are set by you after figuring out the best setting to maintain your temps. Good luck!