How do ventilation systems replace stale air?

1. In a closed room, every breath uses a little oxygen and adds carbon dioxide and moisture. After a while the air feels stuffy and people get sleepy.
2. Opening a window lets fresh air in and stale air out. In winter it also lets the heat out, so the room turns cold and the heater has to work hard.
3. A ventilation system does the job gently, all day. One fan pulls stale air from the kitchen and blows it outside. A second fan brings the same amount of fresh air to the bedrooms.
4. Before the fresh air goes any farther it passes through a filter: a folded sheet of fine fibers that traps dust, pollen and insects.
5. Inside the box the two airstreams pass through a stack of thin plates. They never touch or mix, but heat flows through the plates from the warm outgoing air into the cold incoming air.
6. The fresh air arrives clean and already warmed, and the stale air leaves with most of its heat taken back. The house breathes all day without getting cold.

Tap each part of the box. Follow the fresh air and the stale air as they cross.

• Fresh air intake. This duct comes from a hood on the outside wall. Cold, fresh outdoor air is pulled in through it.
• Filter. The folded filter traps dust, pollen and bugs before the air reaches the rest of the box. It needs cleaning or changing a few times a year.
• Heat exchanger core. A stack of thin plates. Stale air flows through every other gap and fresh air through the gaps between. Heat crosses the plates, but the air never mixes.
• Supply fan. This fan pulls fresh air through the filter and the core, then pushes it along ducts to the bedrooms and living room.
• Stale air duct. This duct brings warm, damp, stale air from the kitchen and bathroom, where most steam and smells are made.
• Exhaust fan. This fan pulls the stale air through the core, where it gives up its heat, and then blows it outside.
• Exhaust outlet. The stale air leaves through this duct to a hood outside. It has already given most of its heat to the fresh air.

Tap the fans to switch them on. Then tap the filter and the heat-exchanger core to take them out and put them back. What changes?

Experiment: It is freezing outside and the fans are on. What does the heat-exchanger core do to the fresh air?

1. Tap both fans. Watch the purple stale air and the green fresh air.
2. Look at the color of the duct that carries fresh air to the bedroom.
3. Tap the core to pull it out. Look at that duct again, and at Ada.

What we found: With the core in place, the fresh air arrived warm, because heat crossed the thin plates from the outgoing air. With the core out, the fresh air came in icy cold. The two airstreams never mixed.

Turn on JavaScript to try it yourself.

Four ducts need connecting. Tap a duct end, then tap a port on the box. Fresh air must reach the bedroom and kitchen air must go outside. Then switch on.

Every time you breathe out, you add a little carbon dioxide and water to the air. Cooking, showers, wet laundry, paint and furniture add steam, smells and dust. In a home that is sealed up tight to save heat, all of that stays inside.

Ventilation means swapping indoor air for outdoor air. The simplest way is to open a window, and on a mild day that works well. In winter, though, an open window throws away the heat you paid for.

A heat-recovery ventilator solves this with two fans and a heat exchanger. One fan sends stale air out, the other brings fresh air in, and the two streams pass each other inside a stack of thin plates. Heat always flows from warm things to cold things, so it crosses the plates into the cold fresh air. The air itself stays on its own side, so smells do not come back in.

The scarf comparison is not perfect. With a scarf, your breath goes in and out through the same cloth, one after the other. In the ventilator both streams flow at once, side by side, which is why it needs two fans and two sets of ducts.

Key words

Ventilation

Replacing indoor air with outdoor air.

Stale air

Indoor air that has collected breath, steam, smells and dust.

Heat exchanger

A part that passes heat from one stream of air to another without mixing them.

Carbon dioxide

A gas that people and animals breathe out. A high level indoors shows that the air needs changing.

Duct

A tube or channel that carries air around a building.

Efficiency

How much of the heat the box manages to save.

Check your thinking

1. Why does a ventilation system have two fans?

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   One sends stale air out and one brings fresh air in. Air that leaves has to be replaced, so one fan handles each direction.

2. What is the filter for?

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   Trapping dust and pollen in the fresh air. Its fine fibers trap particles so the air that reaches the rooms is clean.

3. In the heat exchanger, what passes from the stale air to the fresh air?

   Show the answer

   Only heat. The plates let heat through but keep the two airstreams apart.

4. It is winter. What happens if you take the core out but leave the fans on?

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   The fresh air comes in cold and the room cools down. Without the core there is nothing to pass the heat across, so the outdoor air arrives as cold as it started.

5. Four people share a room. Which airflow keeps carbon dioxide near 920 ppm (outdoors is 420 ppm)?

   Show the answer

   About 40 L/s. 420 + 4 × 5,000 ÷ 40 = 920 ppm. Less airflow leaves much more carbon dioxide in the room.

6. Why are the stale-air ducts usually in the kitchen and bathroom?

   Show the answer

   That is where most steam and smells are made. Taking air out where it is dampest and smelliest stops it from spreading, and draws fresh air through the rest of the home.