In this video, we're going to talk about heat transfer in a building, specifically heat transfer in and out of a residential home. These are known as heat losses when heat is leaving the structure and heat gains when heat is entering the structure. Here we show a basic gas furnace setup in a garage as well as the condensing unit outside. When the gas furnace is operating in heating mode, it adds BTUs of heat to the home in order to balance out the BTUs that are leaving when it's in air conditioning mode.

The opposite happens. It removes BTUs of heat from the home to balance out the number of BTUs that are added. This begs the question though. What is a BTU BTU stands for British thermal unit, which is a simple measure of heat and it is equal to the amount of heat transfer required to change the temperature of one pound of water by one degree.

Fahrenheit This applies to both Heating and Cooling and we use it as an everyday measure to discuss the movement of heat by quantity. Now let's talk about some of the specifics of how heat is transferred into and out of a home. The first and simplest form of heat transfer is called conduction. This occurs when molecules come into direct contact with one another, transferring their heat from the higher temperature matter to the lower temperature matter.

Heat will continue to transfer until the temperature of the materials reaches equilibrium, meaning that the same temperature we use insulation to oppose conduction. Reducing the rate of heat transfer, Insulation that opposes conduction is rated by R Value R represents the resistance to heat movement. The higher the R value, the lower the heat transfer rate. In this example, we show a warm house and a cold attic.

Heat is going to transfer through the ceiling and insulation from the hotter to the colder or the higher temperature to the lower temperature. The opposite direction of heat flow also occurs. Here We show a hotter attic and a 75 degree Fahrenheit indoor temperature. It's very common here in Florida where we have hot addicts with cool indoor temperatures.

Insulation helps reduce this rate of transfer. The heat transfer will increase or decrease proportionally based on the temperature differential. A higher differential means there will be a higher heat transfer rate and a lower differential means there will be a lower heat transfer rate. Additional insulation will decrease the heat transfer rate and less insulation will increase the heat transfer rate.

All of these transfers again work towards equilibrium. The only time heat transfer stops is when the temperatures are equal. Here we show some very cold outdoor temperatures to show how heat could be transferred via conduction through the wall into a snow bank. Again, moving from higher temperature to lower temperature convection is another type of heat transfer.

Convection occurs when molecules themselves move and bring their heat with them. This is true in fluids in buildings. this would primarily be heat carried through the air, although convection can also occur in liquids. In the case of ponds and streams.

or think of when you're filling a bathtub with hot water and you kind of mix the hot water around in order to balance out the temperature. That mixing is convection. Inside a home, we often see undesirable convection in simple cases, like when doors and windows are open. If the temperature outside is lower than the temperature inside and we were to open a door, we would lose heat from the home as those molecules move outside.

or if the wind were to blow through the door with a cool breeze that would result in convection carrying lower temperature molecules into the home, reducing the home's average temperature. Anytime air is leaving or entering a home, we're either gaining or losing heat via convection. We call this infiltration when air comes in or exfiltration when air leaves. Some other common areas in which we can lose or gain heat via convection are around things like lights or attic vents.

The third type of heat transfer is radiation, which can be more difficult for us to understand because radiation happens via electromagnetic waves. Some of the most common ways that we observe radiation are by feeling the warmth from the Sun or being in front of a fireplace. Whenever we have Windows in a home, the inside of the home is heated by electromagnetic radiation from the sun. when sunlight enters and beats on a Surface As shown here, radiant heat can transfer both directions, though that's why when you stand in front of a cold wall, you'll feel yourself being cooled, even if the air temperature in the space is comfortable.

That's because radiant heat transfer occurs between bodies of higher temperatures and lower temperatures depending on the distance. In this case, it happens when our warmer bodies transfer heat to a cold wall, but most commonly it happens the other direction when there's a hot surface or a flame or the sun transferring to our bodies via radiation. Radiation always happens. line of sight.

Now let's talk about sensible and latent heat. Sensible heat is heat that we can measure with a thermometer, and latent heat is heat that we cannot measure with a thermometer. Here we show a common example of latent heat transfer. We know that when we're boiling a pot of water, much more energy is required to change the water from liquid water to steam than is required to increase the temperature of the water tooth boiling point.

It takes one BTU to raise the temperature of a pound of water by one degree Fahrenheit, but it takes about 970 BTUs to change that same pound of water from liquid to a vapor. The same is true when melting or making ice. There's much more energy contained in the actual conversion of the water to ice or steam than by merely raising the temperature. The amount of heat it takes to change a solid to liquid and vice versa is called the latent heat of fusion.

For water that's about 144 BTUs per pound. It's latent heat because its hidden heat. We're adding or removing BTUs but we're not seeing a change in temperature. The amount of heat it takes to change from a liquid to a vapor and vice versa is called the latent heat of Vaporization.

Here we show two pots of boiling water, one with a larger flame and one with a smaller Flame. The larger flame adds more heat to the water than the smaller flame and will apply more BTUs of heat over a shorter period of time. So the phase change will happen more quickly in the pot with a larger flame than the smaller flame. Notice how their temperature stays the same despite the difference in the added heat content.

This illustrates latent heat. That energy is going to the change of phase, not to a change in temperature. So when we cool a home in a typical climate in the U.S we're also removing moisture from the home. Air that enters the home contains water vapor and latent heat, which affects the relative humidity of the space and can negatively affect human comfort.

Here we show air which contains water vapor. Entering a home via convection, you will notice that the cooler air sinks and the warmer air rises. Many people will say that heat rises, but they're actually noticing that hotter air is less dense than colder air, so it floats naturally in colder air. Colder air is heavier and sinks in hotter air.

Heat itself doesn't rise or fall, but matter that is heated or cooled can float or sink in the same matter. You can see the air moving into the return of the HVAC system, which will take it into the unit. Here, when air passes over the evaporator coil, during the cooling process, water condenses on the coil. Water vapor becomes a liquid.

This type of transfer is called latent heat transfer because we can't measure the energy utilized with the thermometer. Like we mentioned before, much of that energy goes towards the phase change while the coil remains at the same temperature, similar to how the pot of water stays at the same temperature during the boiling process. In order to remove humidity from a home, it's necessary to have the evaporator coil drop to a temperature below the dew point of the air passing over it. In very simple terms, the colder the evaporator coil is, the more moisture it will remove from the home.

For an air conditioner to be ideal at removing latent heat or moisture from the home, the evaporator coil needs to be at a low enough temperature and the air conditioner needs to run. Humidity is only removed from a home when the air conditioning system is running. To calculate how much heat enters or leaves a home. Air conditioning contractors use a manual, which is now commonly integrated in the software programs called Manual J published by the Air Conditioning Contractors of: America.

This manual helps us use many local conditions and many other design factors in the home to design for how many. BTUs the system must add or remove from the space in order for humans to remain comfortable. just as BTUs can enter and leave the home via conduction, convection and radiation between indoors and Outdoors Human occupants also add heat to the structure. Human occupants radiate heat to colder surfaces, conduct heat into objects and air molecules they touch, generate convection when they move around, and add latent heat when they exhale.

These are internal gains that need to be considered when doing load calculations and sizing equipment. This video is a very simple overview of how heat moves in and out of a home and some of the basic principles in future videos in this series we'll be covering the residential HVAC design process and much more. Thanks for watching our video If You enjoyed it and got something out of it. If you wouldn't mind hitting the thumbs up button to like the video, subscribe to the channel and click the notifications Bell to be notified when new videos come out.

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