They can be tough to get in some areas though, if you want to use a local supplier. I bought the best modern model available from HVAC companies in my area, and

They can be tough to get in some areas though, if you want to use a local supplier. I bought the best modern model available from HVAC companies in my area, and it resorts to backup electrical heat around 15F. But that's still fine for my climate (northeast US).