How Does a Heater Work? Heat Pumps and Furnaces for Kids
Most Austin homes warm up in one of two ways. A heat pump moves heat from outside into your house, and a gas furnace makes heat by burning gas inside a closed metal box.
When it gets cold, your house loses heat through the walls, windows and roof. A heater puts heat back in so your rooms stay comfy. The warm air then travels the same path the cool air does in summer: through a fan, through the ducts and out of the vents.
The heat pump: an AC that runs backwards
Rookie: In summer, an air conditioner carries heat from inside your house to outside. A heat pump can do that too. But in winter, it flips around. It picks up heat from the outdoor air and carries it inside.
Wait. How can cold outdoor air have heat in it? Here is the trick: even cold air has some heat. Only something at the coldest temperature possible would have none, and nothing on Earth is that cold. The heat pump’s outside coil gets even colder than the winter air, so heat flows from the air into the coil. Then the heat pump carries that heat inside.
Pro level: how does it flip?
A heat pump has a part called a reversing valve. It is like a railroad switch for the refrigerant. In summer it sends the refrigerant one way, so the inside coil gets cold. In winter it sends the refrigerant the other way, so the inside coil gets warm and the outside coil gets cold.
Expert level: frost, steam and backup heat
In winter, the outside coil is so cold that frost can form on it, the same way frost forms on grass. A heat pump knows how to melt it. It switches into defrost for a few minutes, and the frost melts off. Sometimes this makes a cloud of steam rise from the outside unit and a whooshing sound. It looks surprising, but it is just frost melting.
On the very coldest nights, the heat pump may need help. Many heat pumps have backup electric heaters inside the indoor unit. They work a bit like the coils in a toaster.
The gas furnace: a hot box for air
Rookie: Some homes have a gas furnace instead. It is a metal cabinet, often in the attic, a closet or the garage. Inside, a small, controlled flame heats up a metal box called the heat exchanger. A big fan blows house air across the outside of that hot metal. The air warms up and goes through the ducts to your rooms.
The flame and the air you feel never mix. The flame stays inside the heat exchanger. The smoke from it goes out a pipe called the flue, up and out of the house.
Pro level: the furnace start-up, step by step
- The thermostat says, “It’s too cold in here!”
- A small fan starts to push air through the burner area and up the flue pipe.
- The furnace lights the gas.
- The heat exchanger warms up for a little while.
- The big blower fan turns on and sends warm air to your rooms.
That is why you may hear a few clicks and a whoosh before you feel warm air.
Expert level: why the heat exchanger matters
The heat exchanger is a wall between the burning gas and your house air. Heat can pass through the metal, but the smoke cannot. Technicians look at it carefully during a check-up, because it must stay in one piece to do its job.
What heat pumps and furnaces share
Thermostat
The boss on the wall. It feels the room’s temperature and tells the heater when to start and stop.
Filter
Catches dust and fur before the air reaches the fan and the hot parts.
Blower
The big fan that pushes warm air through the ducts.
Ducts and vents
The air’s hallways and front doors to each room.
Heat pump or furnace: a quick table
| Question | Heat pump | Gas furnace |
|---|---|---|
| Where the heat comes from | Moved in from the outdoor air | Made by burning gas in a closed box |
| Does it cool in summer too? | Yes, the same machine | No, it teams up with a separate air conditioner |
| Part outside | The outside unit runs in winter and summer | The outside unit is only for summer cooling |
| What you might notice | Steam from the outside unit on cold days | Clicks and a whoosh when it starts |
Why does warm air feel so nice?
Your body makes its own heat all day long. When the air around you is cold, that heat leaves you fast, and you feel chilly. When the air is warm, your heat leaves more slowly, and you feel cozy. A heater does not really put heat into you. It slows down how fast your own heat leaves.
Try it: the warm-air hunt
- On a cold day when the heater is on, stand under a ceiling vent with a grown-up.
- Can you feel warm air coming down? Warm air likes to rise, so it may feel gentler than cool summer air.
- Now find a spot near a window. Does it feel cooler there?
- Guess why. (Hint: windows let heat sneak out faster than walls.)
Wait, but why…?
- How does an air conditioner work?
- Does hot air really rise? Try the lab.
- What are all those clicks and whooshes?
- Why does Austin get blue northers?
- The Why Bank, Heat shelf
Next stop on the Air Trail: Hero Words, or see the whole Air Trail.