A heat lamp is the most expensive and inefficient means possible to heat your water. Heat RISES, remember, so you have to run a lot of wattage thru a lamp to keep your water warm, as compared to if the heat was being produced *under* the waterer.
Heat lamps are also one of the more frequent causes of barn fires. You might be surprised what accidents can occur to people who were sure nothing bad would ever happen. Heat lamps should probably only be used if there is really no alternative, and then, they should be hung by two separate suspensions, going to two separate points on the lamp and two separate attachment points on the coop ceiling, and make sure the heat lamp's guard is in place (it does not necessarily prevent fire if the lamp should fall onto bedding, but it gives you a little longer to find the fire while it's still just smoldering; also reduces incidence of chickens flying into heatlamp and burning selves etc)
There is a LOT to be said for a good heated waterer base. Other alternatives exist too (heated buckets or heated dogbowls, the 3 gal waterer with the built-in heater in its bottom, manually replacing the water as many times a day as required, etc).
Good luck,
Pat
