This is no longer correct. We have heat pumps that can be more than 100% efficient, even air-sourced heat pumps in -30° weather. There are still many places where this will still be more expensive than a gas furnace.
I don’t know how you’re measuring efficiency, but a heat pump with greater than 100% efficiency lets you build a perpetual motion machine. That’s not possible.
You just change the definition of your isolated system. It takes less energy to move heat from one place to another than it does to excite matter to release energy. For resistive or combustion heating, the isolated system is your house, plus the gas if using gas heat. For heat pumps, you include the rest of the world.
As an aside, heat pumps are generally considered good when they reach 300% efficiency, i.e., when every watt of energy expended adds 3 watts of heat to your home.
This is no longer correct. We have heat pumps that can be more than 100% efficient, even air-sourced heat pumps in -30° weather. There are still many places where this will still be more expensive than a gas furnace.
I don’t know how you’re measuring efficiency, but a heat pump with greater than 100% efficiency lets you build a perpetual motion machine. That’s not possible.
You just change the definition of your isolated system. It takes less energy to move heat from one place to another than it does to excite matter to release energy. For resistive or combustion heating, the isolated system is your house, plus the gas if using gas heat. For heat pumps, you include the rest of the world.
As an aside, heat pumps are generally considered good when they reach 300% efficiency, i.e., when every watt of energy expended adds 3 watts of heat to your home.