It should work as a heat source depending on the wattage. Im no expert by any means, but red light has a longer wavelength and would prob have less affect on fementation than say a white light. That being said, its still light and I would avoid light interacting with fermentation if possible. Could you cover the bulb by chance? I have a mini fridge with a basic 150 watt soft white lamp in it. Modified a duct cap (coffee can might work depending on size) to fit over the lamp and base. The thin metal heats up no problem to raise temps very quickly and off course blocks all the light. Starts raising the ambient temp in minutes when the fridge is at room temp. Theres plenty of air between the lamp and duct work so the lamp has been fine. Cheap, effective and a clean looking install. Just a thought