If you can, your safest option is to not heat it at all. There is always a risk of fire. Also, if the chickens are not allowed to acclimate they may be in trouble if you have a power outage, assuming you really do have cold weather. What is cold to us is not necessarily cold to chickens. That's why we are questioning whether heat is required at all, there is a risk in heating.
I'll link a thread from someone that does experience really cold weather. You might find it interesting.
Cold Weather Poultry Housing and Care | BackYard Chickens - Learn How to Raise Chickens
Other than extreme temperatures one risk if cold weather is frostbite. Any time the temperature is below freezing there is some danger of their combs or wattles freezing and becoming frostbitten. That's tied to moisture. If the moisture level is really high so is the frostbite danger. The moisture we are worrying about is mostly from their breathing, their poop, and any open water in the coop. If you have good ventilation you can get rid of most of that moisture. With poor ventilation moisture can build up. Most chickens, even those with large combs and wattles, can handle temperatures below zero Fahrenheit (-18 C) if the air is dry.
The other risk is a cold breeze hitting them strong enough to ruffle feathers. Chickens have tiny air pockets trapped in their feathers and down that insulate them. If their feathers are ruffled and those air pockets escape they can get cold. The way the feathers lay on a chicken if they face into a light breeze feathers are unlikely to get ruffled. But we need to provide them a safe place to roost where they are protected from strong breezes.
Another risk having nothing to do with the cold is ammonia. When their poop breaks down it generates ammonia gas. This gas is lighter than air so if you have a ventilation hole above their heads it will safely escape. If you don't have an escape hole it might build up and poison them. This really doesn't happen much, it doesn't take much of a hole for the ammonia to escape.
The way most of us provide good ventilation and protect them from breezes is to have ventilation holes above their heads when they are on the roosts.
I don't heat my coop so I'm not going to recommend anything. I don't have the experience. Warm air does rise so if you have any ventilation up high most of the heat will escape but you can get some benefit from it.
Good luck!