I consider a winter where the coldest it gets is 0F a mild winter. The important thing isn't average or normal it is the worst case. I've personally seen -25F here in the last 10 years - it was only one time and lasted about a week, but that means the HVAC system needs to work down to at least -25F just in case.
I don't know what the climate is like in Poland. Maybe 0F is as cold as you ever get and you are okay. Maybe your system will work to -20F even though you haven't tested it. But your might have a system like mine that while it can deliver heat at 0F, it is sized such that below 30F it can't deliver enough heat (I have the backup system for those colder days)
I don't know what the climate is like in Poland. Maybe 0F is as cold as you ever get and you are okay. Maybe your system will work to -20F even though you haven't tested it. But your might have a system like mine that while it can deliver heat at 0F, it is sized such that below 30F it can't deliver enough heat (I have the backup system for those colder days)