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Heat Transfer thru Walls and Windows

HVAC Engineering

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March 8, 2024

How does the “R” value of a material affect the cooling or heating load of a building? What is the basic equation for the conduction of heat through building construction such as walls and windows? What happens to the total heat when the temperature difference between the outdoors and indoors increases, or the “R” value is doubled?

If you prefer to watch the Video of this presentation, then scroll to the bottom.

We’re often asked to add air conditioning to a space that has never had air conditioning before. One of the factors that needs to be considered is the components of the building shell that will contain the conditioned air. This would be the walls, windows, roof and floors over unconditioned spaces,.

Here is the equation for the transmission of heat through building components by conductance. This doesn’t consider other heat gain or loss methods, such as infiltration, ventilation, lights, people, plug, or solar loads. This also just looks at single components and not the full wall assembly including interior and exterior air films.

Q = A x U x Delta-T

Where:

Q = Total Btu/Hr.

A = Area (Ft2) of wall or window

U = Building assemblies overall heat transfer coefficient (Btu/hr.ft2.°F)

Delta-T = Difference in outside and inside design temperatures. (°F)

Heat Transfer Examples

Here are a couple of examples of how different construction materials stand up against preventing heat transfer by conductance. This could either be heat coming into the building, or heat going out of the building.

We’ll start with a wall that has 1,000 square feet, and a design indoor temperature of 75 F and an outside design temperature of 90 F. This equates to a 15-degree F delta-T.

So far, we have the area of the wall and the delta T, the missing factor is the “U” value which is determined by adding up all the resistance of the components of the wall and taking the inverse value.

Typical Construction Materials and their "R" values (Resistance), and "U" Coefficients — Typical Construction Materials and their “R” values (Resistance), and “U” Coefficients

Here are four different building construction components and their “R” values.

1/8” Clear Glass R=0.76

1/2” Plywood (Douglas Fir) R=0.79

8” Light Weight Concrete Block “R” value = 2.33

3-1/2 R-11 Insulation “R” value =11

We performed the heat transfer equation on each of these building components. Starting with the 1/8” clear glass and an “U” value of 1.32, we get the following.

1,000 Ft2 of Glass x 1.32 “U” Factor x 15 F Delta-T = 19,800 Btu/Hr, or 1.65 Tons of cooling required for the heat gain through conductance. Obviously there glass windows with much better “U” values, but our purpose is to show you how the “R” value and inversely the “U” value effects heat transfer.

"R" Values and the Inverse "U" coefficient. — “R” Values and the Inverse “U” coefficient.

As, you can see if we built the wall with just plywood, we would do a little better with a heat gain of 18,975 Btu/Hr, or 1.58 tons. Using 8” Light Weight Concrete blocks helps considerably by reducing our total heat load in thirds. The greatest effect comes when we add R-11 Insulation to the wall, which in this case looks like this.

1,000 Ft2 of R-11 Insulated wall x 0.09 “U” Factor x 15 F Delta-T = 1,350 Btu/Hr, or 0.11 Tons of cooling. This is a huge reduction in the amount of heat that can transfer from outdoors to indoors. This shows the importance of properly insulating the shell of a building.

This chart shows that the greater the “R” value or resistance of the construction component, the smaller the “U” value will be. The smaller the “U” value, the smaller the heat transfer rate. This is why energy codes mandate a certain “R” value for insulation in walls and roofs.

If the outdoor temperature increased to 105 F, this would double the Delta-T in our equation, which would double the heat gain through our building components. This is one of the reasons why the temperature outdoors makes a difference in the energy consumption of the building for comfort cooling or heating.

A doubling of the Delta-T, causes a doubling of the Heat Load

Most energy codes have standard requirements for the total allowable transmission factor that can be used to construct a wall or roof for a space that is air conditioned. It wouldn’t make sense to allow the heating or cooling of a space built with paper walls, as heat would easily enter or escape the space, requiring enormous amounts of energy to condition.

A high quality wall, floor, window or roof assembly is essential to prevent heat gain or loss in buildings, ensuring energy efficiency and maintaining a comfortable indoor environment by effectively insulating against external temperature fluctuations.

1/8” Clear Glass R=0.76

Solve for U Value. U = 1/R

U = 1/0.76 = 1.32
Q = A x U x Delta-T
Q = 1,000 ft2 x 1.32 x 25 Delta-T
Q = 33,000 Btu/hr.

1/2” Plywood (Douglas Fir) R=0.79

Solve for U Value. U = 1/R

U = 1/0.79 = 1.265
Q = A x U x Delta-T
Q = 1,000 ft2 x 1.265 x 25 Delta-T
Q = 31,625 Btu/hr.

8” Light Weight Concrete Block R=2.33

Solve for U Value. U = 1/R

U = 1/2.33 = 0.429
Q = A x U x Delta-T
Q = 1,000 ft2 x 0.429 x 25 Delta-T
Q = 10,725 Btu/hr.

3-1/2 R-11 Insulation R=11

Solve for U Value. U = 1/R

U = 1/11 = 0.09
Q = A x U x Delta-T
Q = 1,000 ft2 x 0.09 x 25 Delta-T
Q = 2,250 Btu/hr.

How conductance heat gain works through walls and windows

Closed and Open Loop Controls

Controls

MEP Academy Instructor

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February 28, 2024

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In this article we’ll give you an example of a closed and open loop DDC control system. The use of open and closed loops is found throughout sequences of operation for most building DDC control strategies.

If you prefer to watch the Video of this presentation, then scroll to the bottom.

Closed Loop Control of AHU Discharge Air Temperature

In HVAC systems, a closed-loop control system using a discharge air temperature sensor and a control valve controlling the flow of hot or chilled water is employed to maintain a consistent supply air temperature from a coil. Here’s how it generally works:

A discharge air temperature sensor is placed in the air stream after the heating or cooling coil. Its purpose is to measure the temperature of the air leaving the coil, which is known as the discharge air temperature.

Supply Discharge Air Temperature Sensor in a DDC Closed Loop

The discharge air temperature is sent to a controller in the form of an analog input. The controller compares the actual discharge air temperature to the desired set point temperature.

The controller sends an output signal to the coil control valve, which is responsible for regulating the flow of heating hot water or chilled water through the coil. The control valve is positioned by the controller based on the comparison between the actual and desired discharge air temperatures.

If the actual discharge air temperature is higher than the setpoint, it indicates that more cooling is needed. The controller signals the control valve to open, allowing more chilled water to flow through the coil.

If the actual discharge air temperature is lower than the setpoint, it suggests that the space is adequately cooled. The controller instructs the chilled water control valve to close or reduce the flow, reducing the cooling effect. The heating hot water response in a similar manner.

This entire process forms a closed-loop system. The controller continuously receives feedback from the discharge air temperature sensor and adjusts the coil control valve to maintain the desired discharge air temperature.

Benefits

Energy Efficiency: By precisely controlling the coil operation, the system optimizes energy consumption.

Comfort: Consistent discharge air temperature ensures a comfortable environment for occupants.

In summary, a closed-loop system using a discharge air temperature sensor and a coil control valve allows for real-time adjustments to maintain a stable and desired supply air temperature from the cooling coil or heating coil in an HVAC system.

Open Loop System for Heating Hot Water Temperature Reset

In an open-loop control system using an outside air temperature sensor and a 3-way heating hot water control valve, the goal is to reset the hot water setpoint of a boiler based on the outside air temperature. Here’s a breakdown of how this system typically operates:

Boiler Controller - Heating Hot Water Temperature Reset Schedule — Boiler Controller – Heating Hot Water Temperature Reset Schedule

An outside air temperature sensor is located outdoors to measures the ambient temperature. The information gathered by the outside air temperature sensor is crucial for determining the heating requirements based on the actual outdoor temperature conditions.

The outside air temperature from this sensor is sent as an analog input to the boiler’s controller. The controller analyzes the outside air temperature and compares this to the Heating Hot Water Temperature Reset Schedule programmed into the controller before deciding on whether to adjust the set point temperature.

Heating Hot Water Temperature Reset Schedule Diagram

The controller lowers the hot water supply temperatures set point as the outside air temperature increases. When the outside air temperature gets colder, the boiler controller increases the heating hot water supply temperature set point.

Closed Loop using a Temp Sensor

Using a closed loop control sequence, the hot water temperature sensor sends an analog input signal to the boiler’s controller indicating the current temperature of the heating hot water. The boilers controller will compare the reset schedule with the current hot water temperature and send an analog output signal to adjust the 3-way valve to either increase or decrease the flow based on the current water temperature.

The 3-way control valve is responsible for controlling the flow of hot water in the heating system based on the analog output signal from the controller. It has three ports and can direct the flow in different ways based on the control signals it receives.

As the outside air temperature changes, the controller adjusts the set point temperature of the boiler and if required will send a signal to the 3-way hot water control valve to meet the current demand. This adjustment is based on the heating hot water reset schedule.

When the outside temperature is lower, indicating colder weather, the controller will adjust the boiler supply water temperature upwards. This is based on the HHW reset schedule. An analog output to the 3-way valve will more hot water to flow into the system. This will increase the temperature of the heating medium.

Conversely, in milder weather, the controller may adjust the 3-way valve to allow less hot water into the system. Thereby reducing the temperature of the heating medium.

Benefits

Energy Efficiency: By resetting the hot water setpoint based on outside conditions, the system optimizes energy usage by providing only the necessary amount of heat.

Weather Adaptation: The system can automatically respond to changes in external temperature, ensuring efficient heating in various weather conditions.

In summary, an open-loop system using an outside air temperature sensor and a 3-way hot water control valve allows for real-time adjustments of the hot water setpoint of a boiler based on external temperature conditions. This approach helps optimize energy consumption and provides effective heating in response to changing weather patterns.

DDC Control Loops Explained

Static Velocity and Total Pressure Explained

HVAC Engineering

MEP Academy Instructor

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February 19, 2024

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Static Velocity and Total Pressure Explained

In this article we explain the fundamental concepts of static pressure (Sp), velocity pressure (Vp), and total pressure (Tp) in the realm of HVAC systems. Understanding these pressure components is essential for designing, maintaining, and optimizing air conditioning ducts. In this article, we will explore the definitions of static, velocity, and total pressure, their significance in HVAC engineering, and practical insights on the instrument commonly used to measure them.

If you prefer to watch the video of this presentation, then scroll to the bottom.

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air above a specific point on the Earth’s surface. It is the pressure exerted by the Earth’s atmosphere on objects within it. At sea level atmospheric pressure is approximately 14.7 psia (pounds per square inch absolute) or 1,013 millibars.

Atmospheric pressure decreases with altitude. HVAC systems in high-altitude locations may require adjustments to account for lower atmospheric pressure, affecting factors such as air density and combustion processes.

Using a U-tube manometer with both ends open to the atmosphere, the liquid level on both sides of the tube rest at the same height. This is because both ends of the tube are open and exposed to the same atmospheric pressure.

Static Pressure

SP is the pressure exerted by the air in all directions within the ductwork when it is not in motion. It is a measure of the potential energy of the air as it remains static or stationary within the system.

Static pressure is the force exerted by the air on the walls of the ducts and other components when the air is not flowing. It is typically measured in units such as inches of water column (inWC) or pascals (Pa) using an inclined manometer as shown in image.

Air encounters resistance as it moves through ducts, filters, coils, and other components. This resistance generates static pressure. When choosing a fan for an HVAC system, consideration must be given to the static pressure the fan must overcome to deliver the required air to the space.

The layout and design of the ductwork, including bends, transitions, and fittings, can contribute to an increase in pressure.

High static pressure can lead to increased energy consumption, as the system must work harder to overcome resistance.

(Sp) affects the airflow within the ducts. Excessive pressure may result in reduced airflow, impacting the system’s ability to deliver conditioned air to the desired spaces.

Velocity Pressure

The Velocity pressure is the pressure component associated with the movement of air within a duct.

Consider a scenario where a fan is propelling air through a duct system; during this process, two distinct pressures come into play – velocity pressure and static pressure. The combination of these pressures is termed total pressure. Velocity pressure, arising from the movement of air, can be quantified by determining the variance between total pressure and static pressure, with pressure units denoted in inches of water (W.G.) or pascals (Pa).

Direct measurement of velocity pressure is not done; instead, the most straightforward method to ascertain the airflow velocity is by assessing the velocity pressure within the duct using a pitot tube assembly linked to a differential-P sensor, comprising both a static and total pressure probe.

The pitot tube can be inserted into the airflow to measure both static and dynamic pressures. The dynamic pressure measured by the pitot tube represents the velocity pressure.

Total Pressure

A total pressure probe, positioned in line with the airflow, detects both the duct’s velocity pressure and static pressure, resulting in the total pressure. Meanwhile, a static pressure probe, oriented perpendicular to the airflow, exclusively measures static pressure. The disparity between the total pressure reading and the static pressure reading represents the velocity pressure.

Devices used to Measure Pressure

In our example we used an inclined manometer, but there are many other ways to measure pressure, velocity and air flow. Manometers or pressure gauges are commonly used to measure static pressure within the ductwork.

Inclined Manometer

A manometer is a straightforward and widely used instrument for measuring pressure. There are different types of manometers, including inclined, vertical, and digital versions.

Inclined Manometer

To measure static pressure using a manometer, connect the instrument to pressure taps on the duct—one at the high-pressure point and one at the low-pressure point. The difference in fluid levels in the manometer provides the static pressure reading.

Manometer

Digital Pressure Gauge

Digital pressure gauges are modern instruments that provide accurate and quick static pressure measurements. These gauges typically have a display screen that shows the pressure readings directly.

Connect the gauge to the pressure taps on the duct, and it will display the static pressure.

Pitot Tube and Differential Pressure Sensor

A pitot tube can be used in conjunction with a differential pressure sensor to measure pressure. The pitot tube is inserted into the duct, and the differential pressure sensor measures the pressure differences.

Pitot Tube

The static pressure is then determined by subtracting the velocity pressure from the total pressure measured by the pitot tube.

Capture Hoods

Capture hoods are devices designed to measure both air volume and static pressure at diffuser and grille locations. They consist of a hood that covers the diffuser or grille, and a connected manometer or pressure gauge provides the static pressure reading.

Magnehelic Gauge

A Magnehelic gauge is a type of pressure gauge commonly used to measure air and gas pressures. It is easy to use and can be installed directly on the duct. The gauge displays the pressure difference between the duct and the ambient air.

Magnehelic Gauge

Balometer

A balometer is a specialized instrument used for measuring air volume and velocity, but it often includes a pressure measurement component. It is particularly useful for assessing airflow at diffusers and grilles.

Airflow Hood

When measuring static pressure, it’s important to follow these general steps:

– Identify suitable pressure tap locations on the duct.

– Ensure proper sealing of the instrument to prevent air leaks.

– Take measurements at various points to assess uniformity and identify potential issues.

– Compare readings with design specifications to evaluate system performance.

Always follow manufacturer guidelines and safety precautions when using any measuring instrument in an HVAC system. Regular static pressure measurements help HVAC professionals maintain and optimize system performance.

Air Velocity

Air velocity is the speed at which air moves through a duct or HVAC system, typically measured in feet per minute (fpm) or meters per second (m/s). Controlling air velocity is essential for maintaining comfort and efficiency. Low velocity may lead to inadequate air distribution, while excessively high velocity can result in noise, pressure drops, and increased energy consumption. Engineers need to optimize air velocity to achieve the desired airflow without causing discomfort or system inefficiencies.

In summary, understanding and carefully managing air pressure and velocity are critical for designing HVAC systems that deliver optimal performance, comfort, and energy efficiency in the construction projects you work on, such as commercial buildings, hospitals, hotels, and other facilities.

Static, Velocity, and Total Pressure Explained

BTU Meter

MEP Academy Instructor

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February 10, 2024

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In this article we’ll discuss BTU Meters, also known as Energy Meters, which is a device used to measure the amount of thermal energy (heat) consumed or transferred in a system. These meters are commonly employed in various applications where accurate measurement and billing of heating or cooling energy are essential. Here’s how a BTU meter works and some applications for their use.

If you prefer to watch the video of this presentation, then scroll to the bottom.

How a BTU Meter Works

Temperature Sensors

BTU meters typically consist of two temperature sensors, one placed at the inlet and the other at the outlet of a heat exchange system, which could be a chiller, boiler, air handler or the heating hot water and chilled water service lines feeding one of many campus or district buildings. These sensors measure the temperature of the fluid before and after it undergoes the heating or cooling process.

BTU Meter measuring energy consumption in a Heating Hot Water System

Flow Sensor

A flow sensor measures the rate of flow of the fluid within the system. This is essential for determining the volume of fluid passing through the heat exchange system.

Heat Calculation

Using the temperature difference (ΔT) between the inlet and outlet, along with the flow rate, the BTU meter calculates the amount of thermal energy transferred or consumed. The specific heat capacity of the fluid is often considered in this calculation.

The formula for calculating the energy consumed is Q = GPM x 500 x Delta-T

Q represents the total amount of energy in BTU’s, while GPM is the volume of water flowing through the pipes, 500 is a constant, and the Delta-T measures the difference between the supply and return temperatures.

Display and Data Transmission

The calculated energy value is then displayed on the meter’s interface, and/or is transmitted to a building automation system.

Applications of BTU Meters

Heating and Cooling Systems in Buildings

BTU meters commonly used in residential, commercial, and industrial buildings measure the energy consumption of heating and cooling systems. This includes applications such as central heating, air conditioning, and district heating or cooling networks.

District Heating and Cooling Networks

In large-scale district heating or cooling systems, BTU meters are essential for accurately measuring and billing customers based on their actual energy consumption. This is common in urban areas where a centralized plant supplies heating or cooling to multiple buildings.

BTU Meters measuring energy consumption at each building on a large Campus

Industrial Processes

BTU meters find applications in various industrial processes where precise measurement of thermal energy is critical. Industries such as manufacturing, chemical processing, and food production use these meters to monitor and manage energy consumption.

Renewable Energy Systems

BTU meters used in solar thermal systems, geothermal systems, and other renewable energy applications to quantify the thermal energy harvested or produced. This information is crucial for assessing the efficiency and performance of these systems.

HVAC Systems

Heating, ventilation, and air conditioning (HVAC) systems in commercial buildings often utilize BTU meters to monitor the energy used for space heating and cooling. This helps in optimizing system efficiency and identifying opportunities for energy savings.

Utilities and Billing

Utilities use BTU meters for accurate billing based on actual energy consumption. This is particularly important in situations where tenants or customers are individually responsible for their energy costs.

Energy Efficiency Assessments

BTU meters used in energy audits and assessments to evaluate the efficiency of heating and cooling systems. Measuring actual energy consumptions generates recommendations for energy efficiency improvements.

In summary, BTU meters play a crucial role in measuring and managing thermal energy in various applications, providing accurate data for billing, optimizing system performance, and promoting energy efficiency.

BTU Meters, also known as Energy Meters

Heat Transfer Examples

Closed Loop Control of AHU Discharge Air Temperature

Benefits

Open Loop System for Heating Hot Water Temperature Reset

Closed Loop using a Temp Sensor

Benefits

Atmospheric Pressure

Static Pressure

Velocity Pressure

Total Pressure

Devices used to Measure Pressure

Inclined Manometer

Digital Pressure Gauge

Pitot Tube and Differential Pressure Sensor

Capture Hoods

Magnehelic Gauge

Balometer

Air Velocity

How a BTU Meter Works

Temperature Sensors

Flow Sensor

Heat Calculation

Display and Data Transmission

Applications of BTU Meters

Heating and Cooling Systems in Buildings

District Heating and Cooling Networks

Industrial Processes

Renewable Energy Systems

HVAC Systems

Utilities and Billing

Energy Efficiency Assessments

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