Heat lamps are a documented cause of coop fires.
There are much safer ways to keep your water liquid during cold spells.
Heat lamp fire!
Another heat lamp fire!
Yet another issue, and fires!
yes heat lamps can cause fire, so can a waterer heater or an incandescent light bulb or really any electrical connection. I think most all coop electrical fire are caused be negligent use and or wrong type or faulty equipment. We have used heat lamps for years and never had a fire. Here are some things to consider about heat lamps,
1: don't use the clamp type moveable light fixtures unless you have no choice and if you do be sure to screw them on or tie them with wire or zip ties so they cannot fall, and use a decent quality one with a ceramic socket not plastic
2: wire your coop as you would a home or garage, install actual ceramic light fixtures of the type used in utility rooms and basements, screw your lamp or light bulb into this, it is impossible for this setup to fall and burn. I also installed an outlet in the ceiling so I can plug into a timer and a outlet on the wall so I can plug in a waterer heater. I then have a wire coming out the bottom of the coop which I just plug into a quality extension cord, 1 cord runs the entire coop
3: size your heat lamp accordingly, don't put a 250w bulb in a small rabbit hutch type coop it will likely be far too hot for the birds and likely be to close to combustible materials, use a lower wattage bulb where necessary. I have a walk in shed type coop and the light fixture is in the center so the larger bulb works OK. Always keep your heat source a safe distance from combustibles.
Not trying to lecture but a little common sense goes a long way, if done properly heating a coop can be safe.