FBH only kicks in when the temperature drops below 5 deg c (I think). Its a great peice of kit taking a small amount of fuel and burning it in a heater hense fbh (sirry couldnt resist).

The fbh warms the engine water jacket and as a result, also produces warm air into the cabin long before the normal engine heater would be up to speed.

If the fbh hasnt been used for a while, you get clouds of smoke out of the front near side bumper with lots of noise which carries on going long after you have turned the engine off.