The rule of thumb is that your heat pump will not produce heat or much heat once it gets below 40o. Principle of how a heat pump works. All it does is move heat. in cooling it moves the heat from in the house and discharges it outside, hence the warm air coming out of the top of your air conditioner. In heating it takes the heat from outside air and discharges it inside your home. Once it gets to 40o there isn't much heat outside to discharge inside your home, so all or most heat pumps have emergency heating strip to run when temperatures reach below 40o. To compensate for the lack of hot air outside to move inside you should have aux. heat. So for example its 20o outside and you're running your heat pump in heating. It will run, won't produce much heat, but it will run. If you look at the condenser outside it it will begin to frost up. Every heat pump has defrost cycle for low temp days. What your HP will do is switch to cooling, stop your outside fan motor, and then run hot refrigerant through your outside coils to defrost. Once it defrost your outside coils it will automatically switch back to heating and begin try to heat the house. You'll know when it begins the defrost cycle because you will hear a loud swooshing sound outside. Your outdoor condenser will be running but without the fan motor running. Probably more detail than you wanted to know, however the lockout feature on the Nest is so no one can change the preset temperatures you have set. So if you don't want to run your system no cooler thatn 70o, and no higher than 80o, then if you set the lock, those will be the highest and lowest temps the thermostat will be able to be set at. I'm in Florida so heating isn't much of an issue, however once in a blue moon we may have a 30o morning. the HP will handle it without issue. The reason heat pumps are pushed so hard is because it is cheaper(electric cost) to run your compressor to heat than electric heat strips. Utility companies push heat pumps real hard specifically because of this. Believe me they're not looking to save you money, they're looking to not have to produce more power to cover the demand, and why they offer rebates for installing heat pumps. Remember they make money selling you power, it cost them money of they have to go on the open market and by power to sell you if they don't have enough to handle the demand.......
