Hi there, it's Jack at Dyson.
AM04 has an intelligent thermostat which you can set to the degree. The machine will then monitor the room temperature and cycle on and off as needed to maintain the temperature you've chosen. If the heater doesn't seem to be running, it may be because the temperature you've selected is below the current room temperature. When the room temperature drops below the temperature you've picked, the heater should come on.
For additional troubleshooting to check your machine is working correctly, you might like to visit our Support page:http://www.dyson.co.uk/support/am04/am04-whsil
I hope this helps, but if you'd like any further advice, please give us a call. Our Helpline numbers are 0800 298 0298 (UK), or 1-866-693-9766 (US).
Dyson Online Customer Supporthttp://www.dyson.com