As a previous thread on 'room temperature' showed, people have different ideas on what is a reasonable heat. My brother-in-law expects to be able to wander around the house in a T-shirt in the middle of winter: I'll be in 4 layers before I even consider turning the heating on.
If the house is safe and can be heated as-is, it's not your responsibility to keep them in the manner they prefer. If they want heaters because they like it warm, that's up to them. I assume they are paying the energy bills!
I'd consider the furniture request, if you are willing to re-let unfurnished next time. I wouldn't have though this would disadvantage you. People who already have furniture won't consider furnished properties, but people without will often consider unfurnished. We've all done the IKEA / Argos trip to get a mattress for the first night! That is, unless you are aiming for a particular type of tenant who may not be able to afford this e.g. student.
Plus, the government are changing the tax rules from April 2016, so there is no longer a difference between unfurnished and furnished in that respect. You get a deduction for actual spend, not the 10% allowance you used to get with furnished. Letting furnished commits you to replacing furniture when it breaks.
All the unfurnished places I have rented have had white goods.