(An example of this would be adding a 1.5 Amp 180 watt Towel Warmer onto a Floor warming circuit of 10 amps / 1200 watts).
In the past when other items have been added to the circuit the floor heating system is connected to, there have been some minor issues. It has caused unnecessary GFCI trips on all of Warm Your Floor's thermostats. A dedicated circuit for other devices and the floor heating are preferred, no matter what the other item is.
We would suggest that if you were to try connecting something to your SunTouch control thermostat, you should use a snubber on your relay to diminish the liklihood of damage to the control and false GFCI trips. A snubber provides an alternative path for the current in the Relay to allow for a safer switch between the two items wired into the circuit. This could possibly damage the thermostats circuit board or other components, causing a failure.