@Olena51
I have frequently had problems with guests saying they are too cold. It's not even always guests who come from a warm climate. It often just depends on what they are used to at home. Some people expect it to always be hot enough that they can walk around barefoot in a t-shirt and shorts, or want the heating on and the windows open.
There's not much you can do about that, other than write something into your listing description and house rules about the heating but, in my experience, that doesn't help much either.
If you haven't done so already, I would follow @Anonymous 's advice and investigate if there's some sort of insulation issue. As the thermostat is in your living room, perhaps the temperature in their unit is not the same.
However, if it's truly over 30 degrees in there, it's more likely that these guests are simply the type who are never going to be warm enough. All I can suggest in that case is that you politely stress to them the safety concerns and how you are worried about all this causing an electrical fire.