• SkillCat Team

A complete guide to Heating Systems in HVAC

Updated: Jul 5


Heating Systems: Chapter 2


Furnaces


In this module, we will explore the components of furnaces. We will also go over the different types of furnaces and why we use them. Skip to quiz!


Introduction to Furnaces


Recall that a furnace is a heating unit that uses combustion to heat entire homes or buildings. Furnaces provide warmth by sending hot air to different spaces that we want to be heated. They are the most popular way to heat homes in the United States!


Furnaces mix fuel with air in the burner system to start combustion. Combustion heats the heat exchanger. Cool air flows over the hot heat exchanger and absorbs its heat. This warms up the air around the heat exchanger, and ductwork then distributes this hot air to our rooms.


Recall that natural gas furnaces use gases like natural gas, propane, and butane as fuels for combustion. Burning these fuels produces heat, and this heat is used to warm the air in your home. Natural gas furnaces receive the gas they burn from pipelines.


Like natural gas furnaces, oil furnaces rely on combustion. However, instead of burning gases, they burn oil. Recall that oil furnaces store oil in tanks that must be refilled manually.


Electric furnaces have metal coils. We pass electricity through these coils, heating them up. Cool air from your home then passes over these coils, absorbing their heat. This hot air warms your home! Ovens are examples of electric furnaces, but they cook food instead of heating homes.


Industrial applications like chemical plants, cement factories, and metalworking industries require furnaces. These furnaces must be large to produce all the heat required by these huge factories!


Components


The main components of a gas furnace are the:

  • Burner system,

  • Blower fan,

  • Heat exchanger, and

  • Exhaust pipe

The burner system is the section of the furnace where fuel mixes with air. Burners mix fuel and air, then apply a spark of heat using the igniter. This starts the combustion reaction that provides our furnaces with the heat they used to warm our homes.


Fuel first enters the burner through an inlet pipe called the gas manifold. The gas manifold has holes for the fuel to flow through, and a burner is attached to the end of each hole.


The burner system also has a gas valve. The gas valve opens and closes, allowing fuel from the gas manifold to enter the burners. If the gas valve is closed, no fuel will enter the burners. When it is opened, fuel passes into the burners.


At their inlets, the burners have a small hole called an orifice. This hole sucks outside air into the burner, where it mixes with the fuel from the gas manifold. Recall that air is required for the gas to combust completely and avoid producing carbon monoxide!


This gas and air mixture flows through a small outlet hole in the burner and is lit by the ignitor.

We use a small outlet hole so that the ignitor does not light too much fuel, as this would create a large, dangerous flame.


The blower fan blows the hot air from the burner system up and into the heat exchanger. Recall, the heat exchanger is the component that transfers heat from the burning fuel to the air.


It is best to place the heat exchanger above the burner system. This way, the heat and smoke created by the burner rise into the heat exchanger. After the heat exchanger warms the air, ductwork distributes this hot air to our rooms to keep us warm and cozy!


Finally, we need a way to remove the smoke created by combustion. This is where the exhaust pipe comes in. It takes the smoke produced in the burner system and moves it outside our homes. This way, dangerous gases like carbon monoxide cannot get in the air inside our homes and harm us.



Types of Furnaces


There are three different types of furnaces based on the directions they are oriented in:

  • Horizontal gas furnace,

  • Vertical upflow gas furnace, and

  • Vertical downflow gas furnace.

Horizontal gas furnaces are designed to be placed on their side. They take cold air in one end and exhaust hot air out the other.

Horizontal gas furnaces are usually used in small spaces that are not tall enough to fit a traditional furnace. One common location for horizontal furnaces is in attics, as they are typically small and cramped spaces.


Upflow gas furnaces stand upright. They take cold air from below the unit, heat it in the heat exchanger, then release warm air out of the top. Upflow furnaces use the blower fan to blow hot air from the heat exchanger up out of the furnace and into our rooms.


Like upflow furnaces, downflow furnaces stand upright. However, they take cold air in through the top and release warm air out of the bottom. Downflow furnaces use the blower fan to blow hot air from the heat exchanger down out of the furnace and into our rooms.


Because upflow furnaces blow hot air upwards, it is best to place them somewhere below the rooms we want heated, like in a basement. Since downflow furnaces blow hot air downwards, it is best to place them somewhere above the rooms we want heated, like in the attic. Recall that if the attic is too short, we can use a horizontal furnace instead.


Upflow furnaces blow hot air up, while downflow furnaces pull hot air down. Because downflow furnaces move heat in the opposite direction of its natural flow, they are less efficient. Horizontal furnaces are like up and down furnaces, but they are placed sideways to fit in tight spaces.


Furnaces can run on natural gas, oil, or electricity. Furnaces have many components, but the one that creates heat is the burner. We use different types of furnaces — horizontal, upflow, or downflow — in different types of spaces.




Boilers


In this topic, we will discuss how boilers use the steam cycle to create heat. We will also explore the different kinds of boilers and why we use them. Skip to quiz!


Steam Cycle

Recall, the refrigeration cycle harnesses the phase changes of a refrigerant to transfer heat. The air conditioner and refrigerator in your home rely on the refrigeration cycle to keep you and your food cool!


The refrigeration cycle increases the refrigerant gas’ pressure with a compressor (4). The condenser (1) then cools this gas until it condenses into a liquid. The pressure of the liquid is reduced with an expansion valve (2). An evaporator (3) then heats this liquid until it boils into a gas.


The Rankine cycle is the basis of all different heating and cooling cycles! The refrigeration cycle is a type of Rankine cycle that uses a refrigerant to transfer heat.


When we use water or steam to transfer heat, we call this type of Rankine cycle the steam cycle.


Boilers use the steam cycle to heat our homes. The steam cycle relies on various components that change the pressure or temperature of water. This causes the water to change phases, and the heat from these phase changes heats our homes!


The Steam Cycle Process

The component that transfers heat in the steam cycle is the working fluid. As you might have guessed, the working fluid in the steam cycle is either steam or water! Steam is just another word for water vapor, the gaseous form of liquid water.


The first step in the steam cycle is to move the fluid from low to high pressure. We use compressors to do this in the refrigeration cycle, but in the steam cycle, we use pumps. Pumps move the water so that it reaches all of the different components of the heating system.


Next, we increase the temperature of the fluid using a heat source. In the steam cycle, we use gas or oil boilers to heat the fluid. This increase in temperature causes the liquid water to boil, creating steam. Recall that in the refrigeration cycle, we instead used the evaporator to heat the refrigerant.


The steam then enters the radiator. The radiator carries the hot steam to our cool rooms and heats them up! After transferring its heat to our rooms, the steam cools down and once again becomes liquid. Recall that the refrigeration cycle instead uses the condenser to cool the refrigerant into its liquid state.


Also unlike the refrigeration cycle, the steam cycle does not use a metering device to reduce pressure. Instead, the pressure of the fluid decreases naturally as it flows through the cycle because of friction.


To understand this, think of driving a car. If you remove your foot from the gas pedal, the car will eventually come to a stop. This is because of the friction between the car’s tires and the road! In the same way that friction lowers the speed of your car, it lowers the pressure of the working fluid!


Properties of Boilers


Boilers heat liquid water with burners until it boils into steam. Heat exchangers then distribute this heat through pipes and into our homes. Basically, the purpose of a boiler is to produce steam that heats our homes.


Recall that furnaces heat air and send this hot air to different rooms in our homes and buildings. Boilers do the same thing, but with hot water. They also distribute heat differently — boilers use pipes, while furnaces use ductwork.


Because boilers heat water, they are considered hydronic heaters. Hydronic heaters use water or steam to provide heat. Hydronic heating systems have several advantages over furnaces.


Hydronic heaters are more efficient than furnaces because moving hot water with pumps requires less energy than moving hot air with fans. There are also other uses for the hot water produced by these systems. It can be used for cooking, showers, or other day-to-day tasks so it does not go to waste!


We can see examples of boilers both in our homes and in nature. When you boil water on your stovetop, the pot is acting as a boiler. Geysers are natural boilers. Hot lava underneath the ground heats up water, boiling it. Steam then shoots out of the ground in the form of a geyser!


Components of Boilers

The main components of a boiler are categorized into:

  • Heat Generator Components

  • Heat Distributor Components

  • Controls

The components used in generating heat are:

  • Fuel line assembly,

  • Burner, and

  • Combustion chamber

The components used in distributing heat are:

  • Heat exchanger,

  • Supply and return pipes,

  • Circulator pumps, and

  • Exhaust pipes

Let us have a look at each of the boiler components.


The fuel line assembly provides the boiler with the fuel it needs to create heat. It carries fuel to the burner.


In the burner, fuel mixes with air and is ignited, starting the combustion reaction. This creates the flame that heats up the water in the boiler!


The burners reside within the combustion chamber. As burning fuels can reach very high temperatures, the combustion chamber is made of heavy-duty metals like steel or cast iron.

These metals can handle this high heat without melting or deforming.


Recall that the heat exchanger is the component that makes the transfer of heat from burning fuel to the air possible. It is best to place the heat exchanger above the burner. This way, the heat and smoke created by the burner rise into the heat exchanger.


Supply lines send heated water or steam from the boiler to radiators in your home, providing you with heat. This water or steam cools down after transferring its heat to the air in your room. Return lines then bring this cool water or steam back to the boiler, where it is reheated and the cycle repeats!


The supply and return lines use circulator pumps to push the water or steam to where we need it. Recall that pumps move the fluid from low to high pressure.


The exhaust pipe, also known as a chimney, vent, or flue, removes the smoke created by combustion. This way, dangerous gases like carbon monoxide cannot enter our homes.


The controls are the brain of the boiler. They allow us to regulate the water temperature and pressure, as well as the amount of fuel we burn. The controls keep us safe by preventing the temperature or pressure from getting too high!


Types of Boilers

Hot water boilers are very common. As you can probably guess from their name, they heat up water. This water can be used for many purposes — heating, showering, cooking, etc. Hot water boilers heat water at temperatures below its boiling point of 100℃.


Steam boilers heat water to temperatures above its boiling point, converting it into steam. Because they must heat water to higher temperatures, steam boilers require more energy.

However, they still produce the same amount of heat as water boilers, so steam boilers are less efficient.


Superheat steam boilers heat water to temperatures as high as 500℃. For reference, this is almost as hot as molten lava! These types of boilers are used for industrial purposes where large amounts of heat are required.


The steam cycle is a type of Rankine cycle that changes the pressure and temperature on steam or water to transfer heat to our homes. Boilers are hydronic heaters that rely on the steam cycle. Boilers have several components that allow them to provide heat to our homes, business, and industries.




Complete Heating System


In this module, we will discuss when and where we use the different types of heating systems. Skip to quiz!


Overview

Different heating systems exist for two reasons — different heating needs and fuel availability.


Different heating systems can accomplish different heating needs. For example, if you only need heat in one room, then a wood-burning fireplace will work just fine. However, if you want heat distributed to all of the rooms in your home, you would use a more advanced heating system like an oil or gas heater.


Firewood was one of the first fuels that humans discovered, so we mostly used wood-burning fireplaces to heat our homes in the past. When we found out oil could be used as fuel, oil boilers gained popularity. In the U.S. today, natural gas and electricity are readily available, so they have gained popularity.


We can heat our homes more efficiently with gas heaters. However, when we leave our homes and go outdoors, gas heaters are not available! That’s why when you go camping, you bring firewood — it is what’s available.


The most popular fuels for heating homes in the U.S. are shown in this pie chart:


Gas heaters are so common in the U.S. because gas pipelines make this fuel easily available.

Electric heaters are popular because they are small and easy to install.


Oil heaters need to be manually re-filled with oil. This inconvenience, paired with the availability of gas and electricity, has led gas and electric heaters to dominate the U.S. market.


Gas Furnaces

Recall that furnaces rely on burners, heat exchangers, and ductwork. Burners mix fuel and air to create a flame. This heats up the heat exchanger, which transfers this heat to the air. Ductwork then distributes this hot air to our rooms!


Gas furnaces burn fossil fuels like natural gas, propane, and butane. Burning these fuels produces more heat than electric heaters. However, gas furnaces produce smoke that is harmful to people and the planet.


Because gas heaters can reach such high temperatures, they can be used in very cold regions. Therefore, they are often used in the cold, northern states of the U.S. More than half of the residential houses and commercial buildings in the U.S. rely on gas heating systems!


Electric Furnaces

Recall electric heaters have metallic coils. We pass electricity through these coils, heating them up. Cool air from your home then passes over these coils, absorbing their heat. This hot air warms your home! Ovens are an example of an electric heater, but they cook food instead of heating our homes.


Electric heaters are smaller than oil or gas heaters. This is because they do not combust fuels, so they do not get as hot as oil or gas heaters. They require less space around them and can be smaller.


Because electric furnaces are so compact, they are common in smaller spaces like apartments. They also require no special installation, so they can be used anywhere there is access to electricity.


Oil Boilers

Like gas furnaces, oil boilers combust fuel in a heat exchanger. However, instead of ductwork, they use pipes and pumps to distribute this heat to our homes. This is because oil is a liquid, which is heavier than gas, so we need stronger materials to transport it.


Boilers are more efficient than furnaces because moving hot liquid with pumps requires less energy than moving hot air with fans. However, because we must manually refill them, they are inconvenient.


Oil boilers were very common in the U.S. in the 1970s. However, because of the convenience of gas and electric heaters, their use has declined. Oil boilers are still common in areas of the U.S., like the Northeast, where there are no gas pipelines.


Different heating systems exist for two reasons — different heating needs and fuel availability.

Gas heating is the most common in the U.S. Oil heaters are mostly found in areas with no gas pipelines. Electric heaters are the second most common in the U.S., and they work well in small spaces.



Heat Pumps


In this topic, we will explore the properties, advantages, and disadvantages of heat pumps. Skip to quiz!


Properties of Heat Pumps

Unlike furnaces and boilers, heat pumps do not burn fuels like natural gas or propane. This means that your gas bill at the end of each month will be considerably less expensive. The cost of heating a home with a gas furnace is $850 per year, but it only costs $500 per year to heat a home with a heat pump!


Burning fossil fuels releases gases that reduce our air quality and contribute to climate change. Climate change poses a threat to all of Earth’s species, including humans. Because heat pumps do not burn fossil fuels, they are better for the planet than heating systems like furnaces or boilers.


On top of the lower cost and environmental benefits of heat pumps, they are also safer to use.

Recall that burning gases in furnaces can produce carbon monoxide, a dangerous gas that can kill humans. Because heat pumps do not burn anything, they produce no smoke and are much safer than furnaces!


One disadvantage of heat pumps is that they can not produce as hot of air as gas heaters. This means that they cannot be used in extremely cold climates where the temperatures fall below freezing.


Heat Pumps vs. Other Heaters

Heat pumps do not burn fuels like gas or oil heaters. Instead, they transfer heat using a substance called the refrigerant. Also unlike gas or oil heaters, heat pumps can be used to heat or cool our homes.


Both electric heaters and heat pumps rely on electricity instead of burning fossil fuels. However, heat pumps use much less electricity. Once again, heat pumps, unlike electric heaters, can be used to heat or cool our homes.


Recall heat pumps are better for the environment than gas or oil heaters. This is because they do not burn fossil fuels that release harmful gases into the atmosphere.


Heat pumps consume less electricity than electric heaters to produce the same amount of heat, so they are more efficient. This means they are more cost-effective and more sustainable. Sustainable practices help protect the planet for future generations.


Types of Heat Pumps


The three main types of heat pumps are:

  • Air source heat pumps,

  • Ground source heat pumps, and

  • Water source heat pumps

These heat pumps differ in the source that they exchange heat with.


Air-source heat pumps exchange heat with air. The condenser exchanges heat with the outside air, while the evaporator exchanges heat with the inside air.


Ground source heat pumps use the heat stored in the ground. The soil below ground is slightly warmer than the air above ground.


Ground source heat pumps make use of this temperature difference by pumping cold water underground. This cold water absorbs heat from the warmer soil. The water, which is now warm, can be used to heat homes!


Water source heat pumps work like ground source ones, but they exchange heat with water instead of the ground. The water in lakes or ponds is warmer than the air. Water source heat pumps make use of this by pumping cold water into a lake or pond, where it absorbs heat and can then be used to heat homes.


Water source heat pumps make use of this by pumping cold water into a lake or pond. This cold water absorbs heat from the warmer pond water and can then be used to heat homes.

Water source heat pumps can only be used in homes that are near bodies of water, like lakes or ponds.


The basic principle of how these heat pumps work is the same — they move a fluid from a cold place to a hot one, where it absorbs heat and can then be used to heat our homes. The difference is where the fluid absorbs heat from — air, earth, or water.



Common Issues with Heat Pumps


Some of the most common issues heat pumps encounter are as follows:

  • Power loss,

  • Capacitor malfunction,

  • Stuck reversing valve,

  • Dirty air filters, and

  • Thermostat problems.

We will now go over each of these in more detail.

Heat pumps rely on electricity. So, if there is a loss of power, they will fail.


Recall that capacitors help the blower motor in the heat pump run smoothly. Capacitors often fail after a while, causing the blower motor to fail. In this event, the capacitor must be repaired or replaced by a trained HVAC technician.


Recall reversing valves allow us to reverse the flow of refrigerant so heat pumps can be used to heat or cool our homes. Sometimes the reversing valve can get stuck in one position. This causes the heat pump to only work as a heater or a cooler, instead of both.


Heat pumps work year-round to heat and cool our homes. Because of this, their air filters can get very dirty and must be replaced more frequently.


Thermostat issues are common with heat pumps. Old thermostats often read the temperature wrong and must be repaired or replaced.


The root of most of these issues is that heat pumps are used for cooling and heating. This means that unlike furnaces or boilers, which are mostly used in the winter, heat pumps work year-round. This causes some components to face issues more frequently.


Another reason that heat pumps encounter problems is their reliance on electricity. Components often get damaged by large amounts of electricity and must then be repaired or replaced.


Heat pumps are cheaper and better for people and the planet than gas or oil heaters. Air, ground, and water source heat pumps absorb heat from the air, ground, and water, respectively. Because they rely on electricity for year-round heating and cooling, heat pumps can encounter various issues.



Question #1: Which of the following is NOT a type of furnace?

  1. Natural gas

  2. Oil

  3. Electric

  4. Heat pump

Scroll down for the answer...










Answer: Heat pump

The types of furnaces are natural gas, oil, electric, and industrial. A heat pump is not a furnace.


Question #2: When the gas valve is closed, fuel flows from the gas manifold into the burner system.

  1. True

  2. False

Scroll down for the answer...












Answer: False

When the gas valve is open, fuel flows from the gas manifold into the burner system. The question statement is false.


Question #3: Fuel flows from the _____________ to the inlet of the burner, where it is mixed with air from the orifice.

  1. Gas manifold

  2. Ignitor

  3. Heat exchanger

  4. Blower fan

Scroll down for the answer...








Answer: Gas manifold

Fuel first enters the burner through an inlet pipe called the gas manifold. The burner orifice sucks in outside air, which mixes with the fuel from the gas manifold. This mixture is then lit by the ignitor to start combustion!


Question #4: Which component makes the transfer of heat from the burning fuel to the air possible?

  1. Burner

  2. Heat exchanger

  3. Blower fan

  4. Combustion chamber

Scroll down for the answer...








Answer: Heat exchanger

The heat exchanger is the component that makes the transfer of heat from burning fuel to the air possible.


Question #5: Horizontal gas furnaces are usually used in _______ places.

  1. Small

  2. Large

  3. Cold

  4. Hot

Scroll down for the answer...












Answer: Small

Horizontal gas furnaces are usually used in small spaces that are not tall enough to fit a traditional furnace.


Question #6: Which type of furnace would NOT function well in an attic?

  1. Downflow

  2. Upflow

  3. Horizontal

  4. All would function well in an attic.

Scroll down for the answer...












Answer: Upflow

Upflow furnaces use the blower fan to blow hot air from the heat exchanger up out of the furnace and into our rooms. Therefore, it is best to place upflow somewhere below our rooms, like in a basement.


Question #7: The steam and refrigeration cycles are both Rankine cycles, but rely on different substances to transfer heat.

  1. True

  2. False

Scroll down for the answer...












Answer: True

The refrigeration cycle is a type of Rankine cycle that uses a refrigerant to transfer heat. When we use water or steam to transfer heat, we call this type of Rankine cycle the steam cycle. The question statement is true.


Question #8: Instead of an evaporator, we use ________ to heat the working fluid in the steam cycle.

  1. Furnaces

  2. Heat pumps

  3. Wood fires

  4. Boilers

Scroll down for the answer...













Answer: Boilers

In the steam cycle, we use gas or oil boilers to heat the working fluid.


Question #9: We use a ________ to transfer heat from the working fluid to our rooms in the steam cycle.

  1. Pump

  2. Radiator

  3. Compressor

  4. Boiler

Scroll down for the answer...












Answer: Radiator

Radiators transfer hot steam to our cool rooms, heating them up. Recall that pumps are used in place of compressors to increase the pressure on the working fluid.


Question #10: Hydronic heaters are more efficient than furnaces.

  1. True

  2. False

Scroll down for the answer...












Answer: True

Hydronic heaters are more efficient than furnaces because moving hot water with pumps requires less energy than moving hot air with fans.


Question #11: Which of these components is used in boilers but not in furnaces?

  1. Heat exchanger

  2. Circulator pumps

  3. Burner system

  4. Exhaust pipes

Scroll down for the answer...







Answer: Circulator pumps

Recall that in furnaces, convection naturally moves hot air up through the system. Liquids do not naturally move up through convection, so we use circulator pumps to move the liquid in boilers.


Question #12: Which component acts as the brains of the boiler?

  1. Controls

  2. Heat exchanger

  3. Burner

  4. Pumps

Scroll down for the answer...












Answer: Controls

The controls are the brains of the boiler.They allow us to regulate the water temperature and pressure, as well as the amount of fuel we burn.


Question #13: Steam boilers heat water to temperatures below its boiling point.

  1. True

  2. False

Scroll down for the answer...












Answer: False

Steam water boilers heat water to temperatures above its boiling point. Hot water boilers heat water to temperatures below its boiling point.The question statement is false.


Question #14: Natural gas and ________ heating systems are the most common in the United States.

  1. Oil

  2. Wood

  3. Propane

  4. Electric

Scroll down for the answer...












Answer: Electric

Natural gas heaters are used in 49% of the homes in the U.S. Electric heaters are the second most popular, at 34%.


Question #15: Gas heaters can produce very high temperatures, so they are suitable for very cold regions.

  1. True

  2. False

Scroll down for the answer...












Answer: True

This statement is true. Gas heaters produce high temperatures. This allows them to heat even the coldest regions.


Question #16: Where do we commonly use electric heaters?

  1. Large homes

  2. Small spaces

  3. Businesses

  4. Cold regions

Scroll down for the answer...












Answer: Small spaces

Because electric furnaces are so compact, they are common in smaller spaces like apartments.


Question #17: Oil boilers are commonly used where there are gas pipelines.

  1. True

  2. False

Scroll down for the answer...








Answer: False

The convenience of gas heating allowed it to dominate oil heating in the U.S. Oil heating is most popular in areas where there are still no gas pipelines. The question statement is false.


Question #18: Heat pumps are better for our health and the health of the planet than furnaces and boilers, but they are more expensive.

  1. True

  2. False

Scroll down for the answer...













Answer: False

Heat pumps are better for people and the planet than furnaces or burners. They are also cheaper! The question statement is partly true, but they are not more expensive, so it is false.


Question #19: Heat pumps cannot heat air to as high of temperatures as gas heaters.

  1. True

  2. False

Scroll down for the answer...