Emily Waltz: Thanks. I’m glad to be here.

Cass: So first off, when we talk about heat pumps, I think one of two pictures form in people’s minds. One is a geothermal system in which pipes are buried in the ground outside a home and the ground acts as a heat reservoir where you can dump heat during the summer and then you extract it during the winter. But today we’re going to be focusing on the other type of heat pump, the air source heat pump. Can you sketch out how that works?

Waltz: Yeah. So what’s great about heat pumps is that they transfer heat rather than generate it. And that’s part of what makes them more energy-efficient than other sources of heating. They can both heat and cool a home. And I’ll describe how they work in heating mode. So in heating mode, what they do is they pull ambient heat from outside air and compress it and then release it into the home. And there’s an outdoor unit, which from the exterior looks like a big box with a fan. And then there’s some connection lines and then an indoor unit. And so what happens is the air gets drawn into the system in the outdoor unit. It passes over a heat exchanger, which contains a refrigerant that has a very low boiling point. So the most common refrigerant is called R410A, and it has a boiling point at about negative 48 degrees Celsius. So it may be 0 degrees outside, but when that air passes over the refrigerant, the refrigerant boils. So the refrigerant boils, and then it evaporates into a vapor. And then the compressor increases the temperature and pressure so that it becomes this superheated vapor. And the superheated vapor moves to an indoor unit and goes over through a set of coils. And there a fan blows across it, and it moves the heat into the home. So the heat is distributed through the home, usually through ductwork, but there are ways to do it without ductwork too. And then in the summer, the system works in reverse. It pulls warm air out of the home and moves in cooler air.