Low fermentation temperature...brrrr

I checked on the AK this morning and found it bubbling away slowly but the temperature is a little low.  The little 25w vivarium heater stuck to the side of the FV is barely able to keep the fermenting beer above 17°C.  Originally I fitted two heaters but one melted.  I must get around to replacing it with a new one.  Anyway, currently it's at 17.2°C despite being set to 19°C.  This is OK and I would expect the fermentation to start creating its own heat soon but it's just a bit slower than I wanted.  Still, it should make a nice, clean beer and stop the 1.75kg of sugar causing nasty flavours.