What is a landlord's obligation to provide heat?

I'm assuming you're talking about a residential property where you live in Pennsylvania. If so, you could take the position that your landlord is obligated to adequately heat your home because of the doctrine of "implied habitability". Generally speaking, what this means is that whether your lease says it or not, you Landlord has a duty to make the property "habitable". This does not mean that you aren't responsible for the cost of heat to your home, but it would mean that your Landlord is responsible to make it possible that you could have heat.