# How a Residential Heat Pump Works How a home heat pump moves heat from cold outdoor air into a room, using refrigerant, a compressor and an expansion valve, and how it reverses for summer cooling\. By Dhanur Grandhi Topic: Machines [Open the interactive Jig](https://ask.jig.com/j/RI2kqYhLymA4) ## Transcript ### 1. Moving heat, not making it It is snowing outside, yet this living room is cosy\. Nothing is burning: a heat pump gathers heat from the cold outdoor air and carries it inside\. Let's follow it around the loop\. Watch the pipes: blue glow means cold refrigerant, orange means hot\. ### 2. Cold air still holds heat Even freezing air carries some heat\. The fan pulls outdoor air through the coil, a long bent tube wrapped in thin metal fins\. A fluid called refrigerant flows inside, colder than the air, so heat moves into it and it boils into a gas\. A common refrigerant, R\-410A, boils at about −48 °C \(−55 °F\) at normal air pressure\. ### 3. The compressor squeezes The gas flows to the compressor, the pump at the heart of the outdoor unit\. Squeezing a gas into a smaller space makes it much hotter, the way a bicycle pump warms up\. This is where most of the electricity goes\. The air leaving the outdoor fan is colder than the air going in: its heat stayed behind in the refrigerant\. ### 4. Heat pours out indoors The hot gas travels through the thick, insulated pipe to the indoor unit\. A fan blows room air across the indoor coil and the air carries the heat away\. As it gives up heat, the refrigerant turns back into a liquid\. Heat\-pump air feels warm rather than scorching, often about 35–45 °C\. ### 5. The expansion valve The warm liquid returns through the thin copper pipe and squeezes through a tiny opening called the expansion valve\. Its pressure drops suddenly and it becomes very cold again, ready to soak up more heat outside\. The same refrigerant goes round and round and is never used up\. ### 6. Run it backwards for summer A part called the reversing valve can send the hot gas the other way\. Then the indoor coil soaks up heat from your rooms and the outdoor unit releases it outside\. One machine heats in winter and cools in summer\. Try both modes\. ### 7. More heat than electricity Because it moves heat instead of making it, a heat pump is very efficient\. For every 1 unit of electricity, it can deliver about 3 to 5 units of heat into your home\. Compared with plain electric heaters, that can cut heating electricity by up to 75%\. The yellow glow is electricity; the orange glows are heat\. One plus three from the air makes four indoors\. ### 8. One sealed loop, all year Just two copper pipes join the outdoor and indoor units, with the refrigerant sealed inside\. Gather heat outside, squeeze it hotter, release it indoors, let it expand and cool, and repeat\. That is the whole trick\. A puff of steam from the outdoor unit on a frosty day is normal: it is briefly melting ice off its coil\.