[jacquesm]

Yes, what happens is that the power draw is large enough to cause the heater to see that as a demand for heat. There are little converters for this that ensure that the thermostat has enough power to run and the heater only sees a potential free switch when there is actual demand. Google 'Nest Thermostat C-wire adapter'. This is especially convenient if you'd otherwise have to run new wiring which can require a lot of work.