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All about Heating Pumps

Heating Theory: Chapter 3


Heating Components


In this module, we will define the major components of heating systems and how to best place these components. Skip to quiz!


Overview of Major Components

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. Furnaces are the most popular way to heat homes in the United States!


Furnaces that heat water instead of air are called boilers. You can use this hot water in your home for showers, cooking, and other day-to-day tasks!


Furnaces and boilers have three main components in common:

  • Fuel,

  • Burner, and

  • Heat Exchanger

The only difference is that furnaces have ducts, and boilers have pipes.


Components


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. Furnaces and boilers both burn fuels inside of a heat exchanger.


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


Think of a heat exchanger as a shell. We combust fuel inside tubes in this shell, and it heats up. This hot air leaves the shell through these tubes. Outside air passes over this shell and is heated up by hot air from the shell. This heated air moves through our homes to keep us warm!


Ductwork provides a path for hot air generated by the furnace to flow into each room inside your home. A duct is a tube made up of thin, sealed metal sheets that make sure no heat escapes. Each duct has an outlet to allow the heat to leave the duct and enter the room we want heated.


In boilers, we use pipes in the place of ductwork. This is because water is heavier than air, so we need stronger tubes to transfer this heat. Pipes are much sturdier than the ductwork we use to move hot air from furnaces, so they are better suited for moving the hot water from boilers.


Think of pipes and ducts as straws. Just as we use straws to transfer a drink from a cup to our mouths, we use pipes and ducts to transfer heat from our boilers and furnaces to our rooms!


A burner is the section of the furnace or boiler where a fuel mixes with air. Burners mix fuel and air, then apply a spark of heat using a type of lighter called a pilot light. This starts the combustion reaction that provides our furnaces and boilers with the heat they use to warm our homes.


The burners used in furnaces and boilers are very similar to the burners on the gas stove in your home! Both types of burners use a small flame or spark to start the chemical reaction between fuel and oxygen. This reaction produces heat we can use in our stoves to cook food or in our furnaces and boilers to heat our homes!


We use furnaces to heat our air, and boilers to heat our water. Both use burners to mix a spark, fuel, and air to start combustion. This heats up the heat exchanger, which then passes this hot air or water to ducts and pipes. It is then distributed to our rooms!


Component Placement


Recall that hot air rises. Think of the smoke from a fire, flowing higher and higher in the air. If we place components such that we can capture rising hot air, our furnace will be more efficient. Recall that efficiency measures how much useful heat we get from a fuel.


To capture heat efficiently, 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. The heat exchanger then provides a path for this heat to enter ducts and get distributed to our rooms.


If we place the heat exchanger below the burner, there will be no path for the heat to enter our homes. Think about the heat exchanger as a pot of water you are trying to boil. You place the pot on top of a burner so that it receives heat from the flame. You don’t put the pot next to or underneath the burner!

It is best to keep the burner and the heating system as a whole in a room with lots of windows so that air can circulate freely. This ensures that the burner has access to enough air for the fuel to react with continuously. If there is not enough air circulation, the fuel will run out of air to react with and be wasted.


Recall that when there is not enough air for fuel to react with, incomplete combustion occurs. Incomplete combustion wastes fuel, so it is inefficient. The carbon and carbon monoxide it creates can also have severe effects on our health. If we do not keep the system in a room with good air circulation, we waste fuel and risk our health.


The components should be placed as shown in the graphic on this page. The fuel and burner are at the bottom, the heat exchanger is above the burner, and the ducts or pipes are above the heat exchanger. This way, the burner can start combustion in the heat exchanger, and this heat rises through the ducts or pipes to heat our homes.


We use furnaces to heat our air, and boilers to heat our water. Both use burners to mix fuel and air, starting combustion. This heats the heat exchanger. We then pass this heat to our rooms through ducts and pipes. Where we place these components impacts the efficiency of the system.



Introduction to Heat Pumps


In this topic, we will discuss the history of heat pumps and their benefits. We will also explain how heat pumps work and where and why we use them. Skip to quiz!


History of Heat Pumps


A heat pump is a device that moves heat from cold places to hot places. Recall heat flows naturally from hot to cold. Heat pumps move heat the opposite way! Heat pumps can be used to heat or cool our homes.


Heat pumps have been around for over 150 years! In 1856, Peter von Rittinger created the first practical heat pump we could use in heating systems. One advantage heat pumps have over other heating systems like furnaces is that they do not burn fossil fuels!

Heat pumps gained some popularity as the cost of fossil fuels began to increase in the 1900s. The first heat pumps were installed in Scotland in 1927. The cost of fossil fuels at this time was still relatively low, so heat pumps were not used very frequently.


Today, however, we know fossil fuels are both expensive and bad for the environment. Heat pumps do not burn fossil fuels, so they are cheaper and better for the planet than heating systems furnaces. Because of this, heat pumps are gaining popularity once again!


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!


Heat pumps run on electricity and do not require burning fuels. Heat pumps use a substance called the refrigerant to provide heat instead of a fuel. Unlike furnaces, heat pumps can be used to provide hot or cold air!


The cost of heating a home with a heat pump is about $500 per year. The cost of heating a home with a gas furnace is about $850 per year. The cost of heating your home with a furnace is almost double the cost of using a heat pump!


How Does a Heat Pump Work?


Heat pumps use the refrigeration cycle to move heat from cold places to hot places. Recall the refrigeration cycle uses a substance called the refrigerant to transfer heat. We will now outline the steps of the refrigeration cycle.


There are four components of a refrigeration cycle:

  1. Evaporator,

  2. Compressor,

  3. Condenser, and

  4. Metering Device — also known as an Expansion Valve.

The refrigerant begins its journey as a gas. We increase the pressure on this refrigerant gas in a compressor. Recall that as the pressure on a substance increases, the temperature increases as well. This increased pressure causes the refrigerant vapor to heat up.


This heated refrigerant gas then enters the coils of the condenser. We also call the condenser the outdoor coils because it is placed outside of your home. Cool outside air passes over the outdoor coils, causing the refrigerant to cool down and change phases into a liquid.


This cold refrigerant liquid then enters the metering device, where the pressure on it is reduced. The metering device we typically rely on is called an expansion valve.


This cold liquid refrigerant then enters the coils of the evaporator. We also call the evaporator the indoor coils because it is placed inside of your home. Warm air from inside your home passes over the cool indoor coils of the evaporator and transfers its heat to the indoor coils.


This leaves us with cool air inside our homes and warm refrigerant inside the indoor coils. Air conditioners use the refrigeration cycle to cool our homes in this way!


The refrigerant liquid in the evaporator boils into a gas as a result of the heat transfer from the warm air in your home. This gas then enters the compressor, and the cycle continues!


When we want heat pumps to cool our homes in the warm summer months, they work exactly like the refrigeration cycle we described. However, when we want heat pumps to heat our homes in the cold winter months, we must reverse the refrigeration cycle.


When we use heat pumps to cool our homes, they follow the refrigeration cycle exactly. A refrigerant flows through all of the same components in the same direction and provides cool air to our homes.


Think of an air conditioner. AC systems work by taking heat inside your home and moving it outside. If you stand by the outdoor coils of an AC unit, you feel the heat it is moving outside. If you stand by the indoor coils, you feel the nice cool air that AC systems are known for supplying. This is how heat pumps work when we want our homes cool in summer!


To heat our homes with a heat pump in the cold winter months, we must reverse the flow of the refrigerant. Instead of the hot refrigerant gas leaving the compressor and entering the outdoor coils of the condenser, it enters the indoor coils of the evaporator.


Because of this, the functions of the evaporator and condenser must also be switched! Instead of providing cold air for AC, the evaporator provides hot air to our homes for heating. The condenser switches from ejecting heat to the outside air, to absorbing heat from the outside air.


Let’s return to the air conditioner analogy. Imagine if you switched the outdoor and indoor coils of the AC system. You would feel the hot air inside your home and the cold air outside. This is how heat pumps work when we want our homes hot in winter!


Heat pumps are typically used where only moderate heating and cooling are required. They should not be used in regions with extreme temperatures. This is because the efficiency of heat pumps lessens in super cold or super hot weather.


Heat pumps have increased in popularity because they are cheaper and less polluting than furnaces. When we use heat pumps to cool our homes, they use the refrigeration cycle. When we use heat pumps to heat our homes, they reverse the refrigeration cycle.



Heat Pumps Cycle


In this module, we will describe the major components of the heat pump cycle. We will also discuss the purpose of these components and how they work. Skip to quiz!


Overview


Recall that a heat pump is a device that can heat or cool our homes using the refrigeration cycle.


Recall that the flow of refrigerant in the refrigeration cycle is as follows:

  1. Compressor, to the

  2. Condenser, to the

  3. Expansion Valve, to the

  4. Evaporator, back to the

  5. Compressor.

When we use a heat pump to provide cool air to our homes, the refrigerant flows in the same order.

However, when we use heat pumps to heat our homes, the refrigerant flow is switched to this order:

  1. Compressor, to the

  2. Evaporator, to the

  3. Expansion Valve, to the

  4. Condenser, back to the

  5. Compressor

Notice that the places of the evaporator and condenser have switched!


To reverse the flow of the refrigerant, we make use of a reversing valve. The reversing valve allows us to use heat pumps to heat or cool our homes. The reversing valve is the only difference between the heat pump and refrigeration cycles!


Heating Cycle


We will now outline the heating cycle of a heat pump when it is used to heat a home. Recall that this cycle is very similar to the refrigeration cycle, but it uses a reversing valve to switch the flow of the refrigerant.


In the compressor, we increase the pressure on the refrigerant, which starts as a gas. Recall an increase in pressure on a gas also increases the temperature. So as the refrigerant is compressed, its temperature increases. The refrigerant leaving the compressor is a high-temperature, high-pressure gas.

Recall the reversing valve controls the direction of the flow of refrigerant. Refrigerant enters the reversing valve after leaving the compressor. After this, if the refrigerant enters the condenser, the heat pump acts as an air conditioner. If the refrigerant instead flows into the evaporator, the heat pump acts as a heater.


Hot, compressed refrigerant leaves the compressor and enters the indoor coils of

the evaporator when we use heat pumps as heaters. The air from your home passes over these coils and absorbs their heat, thereby heating your home! After the refrigerant gas gives up its heat to the air in your home, it cools down and becomes liquid.


Next, the liquid refrigerant passes through the metering device. This reduces the pressure of the refrigerant, cooling it down further. The refrigerant leaves the expansion valve as a low-temperature, low-pressure liquid.


The cool refrigerant then enters the outdoor coils of the condenser. Here, the liquid refrigerant absorbs heat from the warmer air outside and once again becomes a gas. This low-pressure, high-temperature gas is sent to the compressor, and the cycle repeats!


Reversing Valve


The reversing valve has three main components:

  • The pilot valve,

  • The main valve, and

  • The solenoid coil

You can think of the reversing valve as a light switch. When it is off, the heat pump is a regular cooler. But when we turn it on, the heat pump becomes a heater!


The main valve is the hollow cylinder that the refrigerant flows through. It provides the path for the refrigerant to flow. There is also a solid cylinder within the hollow cylinder.


This solid cylinder slides back and forth, controlling the flow of the refrigerant. The pilot valve controls the movement of the solid cylinder within the main valve.


The solenoid coil controls the electric signals sent to the pilot valve. These signals control the pilot valve, which determines the direction of the flow of refrigerant. This is how the reversing valve switches the heat pump between heating and cooling mode.


Think of the reversing valve and its components as a TV. The main valve is the TV screen. The pilot valve is the power supply and cable connection. The solenoid coil is the remote control, deciding whether the TV is on or off.


A reversing valve is de-energized if the solenoid is in the “off” position. When the solenoid is off, it opens a hole in the top of the pilot valve. This pushes the solid cylinder in the main valve to supply refrigerant to the indoor coils. This reverses the refrigeration cycle and heats our homes!


A reversing valve is energized if the solenoid is in the “on” position. When the solenoid is on, it opens a hole in the bottom of the pilot valve. This pushes the solid cylinder in the main valve to supply refrigerant to the outdoor coils. This follows the normal refrigeration cycle and cools our homes.


Reversing valves have thin copper or brass outer bodies. Sometimes, the reversing valve slide gets stuck. If this happens, try tapping the reversing valve with a plastic object. Do not hit the valve with a hard, heavy object like a hammer — this could damage the reversing valve!


Heat pumps can cool our homes using the refrigeration cycle. They can also heat our homes by reversing the refrigeration cycle. Reversing valves allow us to switch between heating or cooling. Energized reversing valves cool our homes, and de-energized reversing valves heat them.



Question #1: Furnaces use pipes, while boilers use ductwork.

  1. True

  2. False

Scroll down for the answer...












Answer: False

Furnaces use ductwork, while boilers use pipes.

The question statement says the opposite, so it is false.


Question #2: Furnaces transfer hot air, while boilers transfer hot water.

  1. True

  2. False

Scroll down for the answer...












Answer: True

Furnaces transfer hot air, while boilers transfer hot water.

The question statement is true.


Question #3: We can think of a heat exchanger as a _____.

  1. Shell

  2. Tube

  3. Circle

  4. Candle

Scroll down for the answer...











Answer: Shell

Think of a heat exchanger as a shell.

We combust fuel inside tubes in this shell, and it heats up. This hot air leaves the shell through these tubes.

Outside air passes over this shell and is heated up by hot air from the shell.


Question #4: Which component do we use in a furnace to transfer heat to each individual room?

  1. Pipes

  2. Ductwork

  3. Heat Exchanger

  4. Burner

Scroll down for the answer...












Answer: Ductwork

Ductwork provides a path for hot air generated by the furnace to flow into each room inside your home.

We use pipes in the place of ductwork for boilers because they are better for transferring hot water.


Question #5: Burners are the section of the furnace where a ______ mixes with air and is burned.

  1. Fuel

  2. Water

  3. Both of the above

  4. Oxygen

Scroll down for the answer...










Answer: Fuel

Burners mix fuel and air, then apply a small spark or flame to start the combustion reaction and create heat.


Question #6: To capture heat efficiently, it is best to place the heat exchanger ______ the burner.

  1. Below

  2. Above

  3. Next to

  4. Inside of

Scroll down for the answer...











Answer: Above

To capture heat efficiently, it is best to place the heat exchanger above the burner.

This way, the heat from the burner rises up into the heat exchanger.

If we place the heat exchanger below the burner, there will be no path for the heatto enter our homes.


Question #7: It is best to keep the entire heating system in a room with lots of windows so that the fuel does not run out of air to react with.

  1. True

  2. False

Scroll down for the answer...












Answer: True

It is best to keep heating systems in rooms with lots of windows so that air can circulate freely.

If there is not enough air, the fuel will run out of air to react with, and incomplete combustion will occur.

Incomplete combustion is wasteful and dangerous.


Question #8: Heat pumps are safer, cheaper, and better for the planet than heating systems like furnaces that rely on fossil fuels.

  1. True

  2. False

Scroll down for the answer...











Answer: True

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

This is because they do not burn fuel, so they produce no smoke or toxic gases.


Question #9: Warm air from inside your home passes over the cool indoor coils of the __________ and transfers its heat to the indoor coils.

  1. Condenser

  2. Evaporator

  3. Compressor

  4. Expansion valve

Scroll down for the answer...












Answer: Evaporator

Warm air from inside your home passes over the cool indoor coils of the evaporator and transfers its heat to the indoor coils.


Question #10: When we use heat pumps to provide cold air, we must reverse the refrigeration cycle.

  1. True

  2. False

Scroll down for the answer...











Answer: False

When we use heat pumps to cool our homes in the warm summer months, they work exactly like the refrigeration cycle we described.

However, when we want heat pumps to heat our homes in the cold winter months, we must reverse the refrigeration cycle.


Question #11: To reverse the refrigeration cycle, we must reverse the flow of the refrigerant and switch the functions of the ___________ and condenser.

  1. Compressor

  2. Evaporator

  3. Expansion valve

  4. None of the above

Scroll down for the answer...













Answer: Evaporator

To heat our homes with a heat pump in the cold winter months, we must reverse the flow of the refrigerant.

The functions of the evaporator and condenser must also be switched!


Question #12: When we use heat pumps to heat our homes instead of cooling them, we must switch the evaporator and the ___________.

  1. Reversing Valve

  2. Compressor

  3. Condenser

  4. Expansion Valve

Scroll down for the answer...











Answer: Condenser

To heat our homes with a heat pump in the cold winter months, we must reverse the flow of the refrigerant.

Instead of the hot refrigerant gas leaving the compressor and entering the outdoor coils of the condenser, it enters the indoor coils of the evaporator.


Question #13: The only component used in the heat pump cycle but not the refrigeration cycle is the:

  1. Reversing Valve

  2. Compressor

  3. Condenser

  4. Expansion Valve

Scroll down for the answer...











Answer: Reversing Valve

The reversing valve is the only difference between the heat pump and refrigeration cycles!

It allows us to use heat pumps to heat or cool our homes.


Question #14: Cool air from inside your home passes over the warm indoor coils of the __________ and absorbs heat from the indoor coils.

  1. Evaporator

  2. Compressor

  3. Condenser

  4. Expansion Valve

Scroll down for the answer...












Answer: Evaporator

Hot, compressed refrigerant leaves the compressor and enters the indoor coils of the evaporator.

The air from your home passes over these coils and absorbs their heat, thereby heating your home!


Question #15: Which of the following is NOT one of the main components of the reversing valve:

  1. Pilot valve

  2. Main valve

  3. Solenoid coil

  4. Expansion valve

Scroll down for the answer...












Answer: Expansion valve

The reversing valve has three main components:

  • The pilot valve,

  • The main valve, and

  • The solenoid coil

Question #16: An energized reversing valve allows the heat pump to act as a heater.

  1. True

  2. False

Scroll down for the answer...











Answer: False

An energized reversing valve makes the heat pump follow the refrigeration cycle, acting as an air conditioner.

A de-energized reversing valve reverses the refrigerant flow and allows the heat pump to act as a heater.


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