We had 150 egg Gloucester incubator and converted it to electric by putting a dull emitter bulb in the chimney, still using the capsule as the thermostat. It had felt sheets in the bottom which were removed as the embryos developed to increase the air flow as their oxygen requirement increased. It worked well, especially with ducks as I remember, but needed to be full and all the eggs a similar size and set on the same day to work best.We had a "farmyard" flock ages back , and we used to hatch eggs from them - hybrids ? The incubators were in an old 12 x 8 poultry cabin with no insulation at all . They were paraffin heated "Gloster Glevum (I think ) and every night's job was to turn the eggs , fill with paraffin and trim wicks . My uncle who did all this said that the incubators were "air circulation " types and needed the cabin to have a free flow of air to work satisfactorily - a bit draughty in other words . I don't know how true that was but he always managed a very high percentage hatch .
They used to say that to run a still air incubator was an art, but a forced air machine was a science.