Furnace (central heating)

A furnace makes warm air.

It stays in your home. It keeps us cozy in winter. It helps us stay warm.

Many furnaces use gas to make heat. Some use oil or wood. Some even use electricity. These machines work to heat a building.

Old furnaces used tall chimneys. Warm air rises, so it moved up. This helped heat the rooms.

New furnaces use a fan. The fan pushes warm air through pipes. This is called forced-air heat. It can also cool a home.

Some new kinds are very good. They save a lot of fuel. They keep your home very warm.

A furnace is a machine that makes hot air.

It helps keep buildings warm. Most furnaces use natural gas for fuel. Some use oil, wood, or coal.

Old furnaces were called natural draft systems. They used big chimneys to let heat out. These worked because warm air rises. They did not have fans to move the air.

Later, people made forced-air furnaces. These use a blower, which is a large fan. The fan pushes warm air through pipes called ducts. This makes it easy to add air conditioning too.

Newer models are very smart. They are called condensing furnaces. These are high-efficiency, meaning they do not waste much fuel. They can be 98% efficient. They use a second part to catch extra heat. This part captures heat from the waste gas. This helps save a lot of power. Some furnaces have one speed. Others have two speeds or can change speeds constantly. This helps save even more energy.

A furnace is a machine used to make hot air.

It helps keep buildings warm and cozy. Most modern furnaces in North America and Europe use natural gas. Other types use fuel oil, wood, or liquefied petroleum gas. Some even use coal or electricity. A furnace is a key part of a central heating system. It works to change fuel into heat for a whole house.

How a furnace works depends on its design. Old natural draft systems worked because warm air rises. They used big pipes to move air into rooms. Modern forced-air furnaces use a blower to push air.

This blower moves air through metal or plastic ducts. The air goes through a heat exchanger to get warm. Then, the fan blows it into every room. This method makes it easy to add air conditioning later.

History shows how these machines have changed over time. Early furnaces were built with brick or steel shells. They used large chimneys to let waste gas out.

During the 1950s and 1960s, people began using forced-air styles. These replaced the very large natural draft systems. They were smaller and used belt-driven blowers. These older machines were still quite bulky compared to today.

Today, we have very smart condensing furnaces.

These are high-efficiency machines. They can be up to 98% efficient. They use a secondary heat exchanger to catch extra heat. This part captures heat from the waste gases. This process can even turn water vapor into liquid. Because they are so smart, they save a lot of fuel. They often use PVC pipes for venting to prevent rust.

Different furnaces also control their heat in different ways. A single-stage furnace is either fully on or fully off. This can be noisy because it always runs at high speed. A two-stage furnace can run at a lower speed.

This helps keep the house at a steady temperature. A modulating furnace is the most advanced. It can change its heat and air speed constantly. It only works as much as the house needs. This saves the most energy of all.

A furnace is an appliance used to generate hot air for buildings. It serves as the primary component in many central heating systems.

Most modern furnaces in North America and Europe use natural gas as their fuel. Other common fuel sources include liquefied petroleum gas (LPG), fuel oil, and wood. In rare cases, coal is used as a fuel. Some areas use electrical resistance heating, especially where electricity is cheap. Efficiency is a key measure for these machines. This is called Annual Fuel Utilization Efficiency, or AFUE. A typical gas furnace is about 80% efficient. However, modern high-efficiency models can reach up to 98% efficiency.

To understand how a furnace works, we must look at the heat exchanger. This is a part where fuel is burned to create heat. The heat is transferred to air without the fuel and air mixing. In older systems, air moved through the house using natural draft. This worked because warm air is less dense and rises.

These systems used large pipes to channel warm air into floor or wall vents. Modern systems use forced-air technology. A blower or fan pushes heated air through a network of ducts. This fan easily overcomes the resistance of the ductwork. This allows for much more flexible duct arrangements in a building.

Furnaces are classified into four main categories based on their design. The first is the natural draft or atmospheric burner furnace. These used cast-iron or steel heat exchangers inside brick or steel shells. They required large masonry chimneys to vent waste gases. The second category is the forced-air furnace. These became popular during the 1950s and 1960s to replace bulky natural draft systems. They used belt-driven blowers to move air. Their efficiency ranged from just over 50% to about 65% AFUE.

These were often designed to work with air-conditioning systems.

The third category is the forced draft furnace. These are mid-efficiency models with steel heat exchangers. They are much more compact than older styles. They use combustion air blowers to pull air through the heat exchanger. This process increases fuel efficiency and allows the unit to be smaller. The fourth category is the condensing furnace. These are the most efficient models available today. They typically achieve between 90% and 98% AFUE.

They use a specialized process to capture extra energy.

A condensing furnace works through a unique mechanism. It includes a sealed combustion area and a secondary heat exchanger. The primary way it gains efficiency is by capturing latent heat. This is heat from the exhaust gases. The secondary heat exchanger removes most of this heat energy. This process actually condenses water vapor and other chemicals into a liquid. Because these chemicals can form a mild acid, these furnaces often use PVC pipes for venting. Metal pipes might corrode from the condensation. A draft inducer allows these vent pipes to exit through a side wall or the roof.

Furnaces also differ in how they control their heat output. A single-stage furnace has only one level of operation. It is either completely on or completely off. This makes them inexpensive to install because the technology is simple. However, they can be noisy because they always run at the highest speed. A two-stage furnace is more advanced. It can run at both full speed and a reduced speed. This allows it to maintain a steady temperature more quietly. Finally, a modulating furnace can change its heat and air velocity nearly continuously. It only works as much as is necessary to save energy.

Heat distribution has also evolved significantly over time. Older systems used a passive method called gravity-fed circulation.

In these "octopus" systems, cooler air sinks into the furnace through floor registers. The warmer, lighter air then rises into the ductwork. Modern forced-air systems are much more efficient at moving air. They pull cool air back to the furnace through separate ducts. The air passes through a filter and a blower before being heated. This setup makes it very easy to add a cooling coil for air conditioning. This turns the heating system into a complete climate control system.