57 Heating Systems and Human Comfort

Introduction

Heating modern buildings is one of the most important functions that HVAC systems can provide. The energy required to heat a space, or a house comes from either the burning of natural gas, or it comes from electric heating loads. The method of distributing heat throughout a building can also vary, depending on the medium used to transport the heat. Usually forced air or water is used, though some heating loads, such as baseboard heaters, rely on passive heat dissipation.

Heat Gain

Heat gain in commercial HVAC refers to the increase in thermal energy within a building’s interior spaces due to various factors such as solar radiation, heat transfer through building envelopes (walls, roofs, windows), internal heat sources (occupants, lighting, equipment), and outdoor air infiltration. It is a critical consideration in HVAC system design and operation, as excessive heat gain can lead to discomfort, reduced indoor air quality, and increased energy consumption for cooling. Effective management of heat gain involves proper insulation, shading strategies, efficient building envelope design, and the selection of appropriate HVAC equipment to maintain comfortable and energy-efficient indoor environments.

Sources

Conduction

Conduction is the process by which heat energy is transferred through solid materials from areas of higher temperature to areas of lower temperature. In commercial HVAC systems, conduction contributes to heat gain when heat flows from warmer external environments into the interior spaces of buildings. This occurs through various pathways, including windows, doors, walls, roofs, and building materials.

For example, during hot weather, sunlight can heat up windows and doors, causing them to conduct heat into the building’s interior. Similarly, external walls and roofs can absorb heat from the sun and transfer it into the building through conduction, especially if they lack adequate insulation. Additionally, materials commonly used in construction, such as concrete, metal, and glass, can conduct heat from the outside environment into the building’s interior.

Furthermore, HVAC equipment located outside or in unconditioned spaces can absorb heat from the surrounding air and transfer it into the building through conduction. Overall, conduction plays a significant role in heat gain in commercial HVAC systems by allowing heat to flow through solid materials, thereby increasing the demand for cooling to maintain comfortable indoor temperatures.

Infiltration

Infiltration occurs when outdoor air seeps into a building through cracks, gaps, or other openings in the structure. This uncontrolled airflow can introduce heat into the building, contributing to heat gain in commercial HVAC systems. When warm outdoor air enters the building, it raises the indoor temperature, making it more challenging for the HVAC system to maintain comfortable conditions. Additionally, sunlight can also penetrate through these openings, adding solar radiation and further increasing heat gain. Infiltration may also bring humidity indoors, which can exacerbate the discomfort and strain on the HVAC system. To mitigate heat gain from infiltration, it’s crucial to seal and insulate the building envelope effectively to minimize the uncontrolled flow of outdoor air into the conditioned spaces.

Infiltration or ventilation load refers to the amount of heat or cooling required to maintain indoor air quality and comfort levels in a building due to the exchange of air with the outdoor environment. This load encompasses the energy needed to compensate for the infiltration of outdoor air into the building and the ventilation requirements necessary to remove contaminants and maintain acceptable air quality. Managing infiltration and ventilation loads is crucial for ensuring efficient HVAC system operation and indoor comfort while minimizing energy consumption and operating costs.

Ventilation

Ventilation in commercial HVAC systems introduces outdoor air into the building to maintain indoor air quality and provide fresh air for occupants. However, this process can lead to heat gain through two main mechanisms:

1. Outdoor Air Temperature: If the outdoor air temperature is higher than the desired indoor temperature, bringing it into the building during ventilation can raise the overall temperature inside. The HVAC system then needs to expend energy to cool down the warmer air, resulting in heat gain.
2. Humidity: Outdoor air often contains moisture, especially in humid climates. When this humid air enters the building through ventilation, it increases the indoor humidity levels. Higher humidity can make indoor spaces feel warmer, prompting occupants to demand more cooling from the HVAC system. As a result, the system works harder to remove moisture and maintain comfort, leading to additional heat gain.

It is preferable to use this source rather than infiltration, as ventilation provides a constant, steady, and controlled input of air.

Radiation

Radiation causes heat gain in commercial HVAC systems through the transfer of thermal energy from warm surfaces to cooler surfaces without the need for direct contact or an intervening medium. This phenomenon occurs in three primary ways:

1. Solar Radiation: Sunlight enters the building through windows, skylights, or other transparent surfaces, heating up interior surfaces such as walls, floors, and furniture. These heated surfaces then radiate thermal energy into the surrounding air, increasing the overall temperature inside the building and leading to heat gain.
2. Internal Heat Sources: Equipment, lighting fixtures, electronic devices, and even occupants within the building generate heat. This heat is emitted in the form of infrared radiation, which can be absorbed by nearby surfaces, causing them to warm up and contribute to the overall heat gain in the space.
3. Thermal Radiation Between Surfaces: Surfaces within the building, such as walls, ceilings, and floors, exchange radiant heat energy with each other based on their temperatures and emissivity properties. Warmer surfaces radiate heat to cooler surfaces, resulting in a net transfer of thermal energy and potential heat gain in areas where it is not desired.

Overall, radiation contributes to heat gain in commercial HVAC systems by transferring thermal energy from warm surfaces to cooler ones through electromagnetic waves, thereby increasing the temperature of the indoor environment. Effective strategies for mitigating radiation-induced heat gain include using shading devices, reflective surfaces, and insulation to minimize solar heat gain and optimizing the layout and operation of internal heat sources to reduce unnecessary heat generation.

Heat Loss

Heat loss in commercial HVAC systems is the process by which thermal energy escapes from the building’s interior to the outside environment, leading to a decrease in indoor temperature. This phenomenon can occur through several mechanisms, including conduction, where heat transfers through solid materials like walls and windows, convection, where heat is carried away by air currents, and radiation, where heat is emitted as infrared energy. Effective insulation and sealing of building envelopes are crucial in minimizing heat loss and maintaining indoor comfort levels while maximizing energy efficiency.

Sources

Conduction, Convection, and Radiation

Heat moves through building walls, ceilings, and floors from warmer internal surfaces to cooler external ones, facilitated by conduction. As warm room air near outer walls and ceilings loses heat, it becomes denser, descends, and is replaced by warmer air, creating convection currents that contribute to heat loss. Additionally, warmer objects and surfaces emit heat through radiation towards colder outside walls or windows. A practical example is feeling cold when standing close to an exterior wall or window in winter, as heat radiates from your body to the cooler surface. These heat losses can be mitigated by enhancing insulation, such as adding layers of materials like fiberglass or Styrofoam to walls, and by installing multiple panes of glass in windows. Double-pane windows, particularly those filled with argon gas, significantly reduce heat loss compared to single-pane windows. For instance, a single-pane window with an outside temperature of -18°C (0°F) and an indoor temperature of 21°C (70°F) would have a glass temperature of -9°C (16°F). In contrast, a double-pane window with argon gas would maintain an inside glass temperature of 13°C (55°F), approximately 22°C (40°F) warmer.

Infiltration

Buildings also experience heat loss due to air leakage, referred to as infiltration. This occurs primarily around windows and doors and, in poorly constructed buildings, to some extent through walls. When cold air infiltrates a building, it needs to be heated by the heating system, leading to increased fuel consumption. Conversely, warm air may escape from the building as well.
To mitigate infiltration, it’s important to thoroughly seal around window and door frames using caulking and employ weatherstripping techniques.

Calculation Example:

Steam Heating Equipment and Systems

Steam heating involves the production of steam in a centralized boiler. This steam is then distributed throughout the building via pipes connected to radiators, convectors, and unit heaters. As the steam moves through the system, it releases its latent heat within these heating devices, causing it to condense back into liquid form. The resulting liquid, known as condensate, returns to the boiler where it is reheated and converted back into steam. In contrast, hydronic (hot water) and warm air heating systems are more commonly used in residential and smaller commercial buildings. Steam heating, however, is often preferred for larger structures like hospitals and factories due to its efficiency. Over time, various types of steam heating systems have been developed, each consisting of numerous components referred to as auxiliary equipment. It is important for power engineers to be familiar with these components and understand their respective functions within the steam heating system.

Equipment

Unit Heater:

Systems

The arrangement of piping and heating units in a steam heating system must meet several criteria:

• Steam must flow freely and in sufficient quantities to all heating units.
• Condensate should be easily removed from the units and returned to the boiler.
• Air must be able to be expelled from the heating units and piping.

In steam heating systems, the main horizontal steam supply pipes that run between the boiler and the heating units are known as mains. The vertical pipes that extend to each floor are called risers.

In an upfeed system, the supply mains are positioned below the heating units, and steam is supplied upward through the risers to the heating units. Conversely, in a downfeed system, the mains are located above the heating units.

The pipes responsible for carrying condensate back to the condensate tank or boiler are termed return risers and return mains.

In a gravity return system, if the return main is positioned below the water line in the boiler, it remains filled with water at all times, known as a wet return. Conversely, a return main located above the water line is typically only partly filled with condensate and is termed a dry return. Dry return mains will drain completely when condensate flow stops.

Various types of steam heating systems have been developed, each with its own set of advantages and drawbacks compared to others. These systems are categorized based on piping arrangement, operating pressure, and condensate return method:

• Piping arrangement classification distinguishes between one-pipe and two-pipe designs.
• Operating pressure classification includes high-pressure, low-pressure, vapor, or vacuum systems.
• Condensate return method classification encompasses gravity return, return trap, condensate return pump, or vacuum pump systems.

Maintenance

Hot Water Heating Systems Equipment and Operation

A hot water heating system, also known as hydronic heating, shares many similarities with steam heating systems. It utilizes a boiler to heat water, generated from fuel combustion in a furnace. Similar heat transfer devices, such as radiators, convectors, and unit heaters, are employed in both systems, along with a piping network connecting the boiler to these devices.

However, unlike steam systems, hot water heating does not produce steam; instead, water is heated below its boiling point. The entire system, including the boiler, piping, and heating units, is filled with water or a water/glycol mixture, with minimal air space in an expansion tank.

Water heated in the boiler circulates through the supply piping to heating units, releasing heat before returning to the boiler through the return piping for reheating. Typically, the temperature difference in a hot water heating system from the boiler outlet through the piping and back to the boiler return connection is around 11°C (20°F).

Equipment

Systems

Radiant Panel Heating

Radiant panel heating involves warming rooms through sections of ceilings, walls, or floor panels primarily via radiation, unlike steam and hot water heating systems that rely on convection for heat distribution.

Several methods can be employed to warm these panels, including the passage of warm air through hollow floor tiles or above hung ceilings, electrical heating cables embedded in the panels, or piping through which hot water circulates.

The most common method involves piping or tubing embedded in the panels, through which hot water circulates. To avoid discomfort and prevent damage to plaster, temperature limits are enforced: ceilings should not exceed 46°C (115°F), walls 38°C (100°F), and floors 30°C (86°F).

Usually, continuous serpentine coils made of plastic or thin copper tubing heat the panels, although in the past, steel piping was used for strength, particularly in concrete floor slabs. Nowadays, special plastic tubing like PEX is preferred for most residential, commercial, and industrial in-floor heating installations.

Water temperatures in the coils are generally lower than those in conventional hot water heating systems, typically not exceeding 80°C (176°F) for ceilings and 60°C (140°F) for floors.

Snow Melting Systems

Snow melting systems are increasingly popular for clearing driveways, sidewalks, and parkade ramps, enhancing safety and eliminating the need for manual snow removal. These systems typically rely on a heat transfer solution heated by either steam or hot water from the building’s boiler system, which is then circulated through coils embedded in the sidewalk via a pump.

Alternative snow melting equipment includes electric resistance heating cables buried in concrete or infrared radiant lamps mounted above surfaces.

Operators should be mindful of the energy consumption of snow melting systems and strive to use them only when necessary.

In the Sidewalk Snow Melter design, the heater connects to the boiler, and the system is filled with an antifreeze solution to prevent freezing when not in use in below-freezing temperatures.

For the Snow Removal Panel, an open expansion tank placed above the coil level is standard, but if the tank must be positioned below the coils, a closed expansion tank is required.

Maintenance

Equipment and Maintenance

Systems

Warm air heating systems utilize various types of heat sources, which can be categorized as follows:

1. Directly fired: Furnaces in this class are self-contained units that use a motor-driven blower to circulate air through a heat exchanger. Heat is generated by the combustion of oil or gas within the heat exchanger, but the circulating air does not directly contact the combustion byproducts.
2. Electric resistance coil: Circulating air is heated by passing through electric resistance coils. However, due to the high cost of electricity, this type of furnace is primarily used in smaller warm air heating systems, such as residences, where electricity is readily available.
3. Fin-coil heat exchanger: In systems where steam or hot water serves as the primary heat source, part of the generated steam or hot water is directed to fin-coils installed within the air ducts. These coils heat the air in the secondary warm air heating system.

Air Filters

Fresh air admission into a room should meet the following criteria: a. Ensure even distribution of air across the entire area to prevent stagnant air pockets. b. Avoid direct airflow onto occupants.

Ventilation methods can be categorized into two basic types:

1. Natural ventilation: Air movement is induced by temperature variations or wind effects.
2. Mechanical ventilation: Air movement is facilitated by motor-driven fans or blowers.

Natural Ventilation

In smaller buildings like older homes, offices, and shops, ventilation is often managed by opening windows, although this method can be inefficient. It may lead to cold drafts near the floor and might not effectively remove stale air from the upper part of the room, especially if the window is positioned low. Modern building codes mandate the installation of a primary exhaust fan to ensure adequate ventilation. Some systems integrate this fan with the heating furnace, featuring a fresh air duct connected to the furnace’s return side.

When the exhaust fan activates, it triggers the furnace blower to start, facilitating the intake of ventilation air at a rate equivalent to the air leaving. Typically, the main bathroom fan serves this purpose and is sized up to meet ventilation requirements. If the influx of fresh air causes the home’s temperature to decrease, the furnace may activate in response to a thermostat signal.

Mechanical Ventilation

Mechanical ventilation systems can be categorized into three groups based on how the fan or blower is employed to circulate air:

To ensure a clean atmosphere within conditioned spaces, the air supplied by an air handling system must undergo thorough filtration. Both the recirculated indoor air and the outside air brought in for ventilation purposes typically carry various contaminants. Therefore, it’s essential to purify all air passing through the air conditioning system.

Airborne contaminants can be categorized into several general classes:

1. Solid particles of visible size: This includes dust, dirt, lint, pollen, insects, and other larger particulate matter.
2. Solid particles of microscopic size: Fine dust, fumes, and smoke fall into this category, consisting of smaller particles that are not visible to the naked eye.
3. Liquid impurities: Mist and fogs composed of extremely tiny droplets of chemicals are considered liquid impurities.
4. Vapors and gases: This category encompasses cooking odors, human and animal occupancy odors, as well as exhaust gases from vehicles and other internal combustion engines.
5. Living organisms: Bacteria and spores are examples of living organisms that can be suspended in the air.

Airborne contaminants exhibit a wide range of sizes, measured in microns, with 1 micron equal to one-thousandth of a millimeter. Here are some examples of particle sizes:

• Tobacco smoke particles: 0.2 microns in diameter
• Bacteria: Vary from 0.2 to 5 microns
• Pollen: Range from 5 to 150 microns
• Dust particles: Vary from 1 micron to well over 1000 microns

The diverse range of particle sizes makes it challenging to design a universal air cleaner capable of removing all contaminants. Consequently, various types of air cleaners have been developed to address different applications. The selection of an air cleaner for an air conditioning system hinges on factors such as the type and concentration of contaminants present and the desired level of cleanliness.

Types

Electric Controls

There are several main styles of fixed electric heating loads, including baseboard heaters, unit heaters, heating cable sets (in-floor heating) and central electric heating, which is similar to central gas heat, in that there is one source that distributes heat, usually by blowing warmed air through ductwork.

Electric heating loads are very reliable methods of heating a building. By passing current through a resistive element, our electric heaters produce heat proportional to the square of the applied voltage.

$\text{P}={\displaystyle \frac{{\text{E}}^2}{\text{R}}}$

This means that if you connect your heating load at half its rated voltage, you will produce one quarter of the power, or consequently if you connected at twice the loads rated voltage you would develop four times the rated power. It is for this reason that fixed electric heating loads are connected at 240V, which is the line-to-line voltage inside most houses.

Central Electric Heat

A central electric furnace is comprised of a large bank of resistive heating coils, and a blower motor that pushes air across them. Most furnaces allow for the installation of air filters to help purify the air as it circulates through the building.

Control of the furnace comes from a low-voltage thermostat installed in a central location of the house. This room thermostat energizes a 24V solenoid contactor, which in turn energizes the 240V heating elements. As the temperature in the plenum chamber rises, eventually the blower motor thermostat engages and begins to push heated air throughout the building.

A central electric furnace control circuit

The furnace houses the relays which control the heating elements. These are normally switched on and off in increments of 5 kW or less. This helps reduce the strain on the line due the heavy currents drawn when large heating loads are all switched on simultaneously. This helps reduce the flickering of lights and other under voltage concerns in parallel circuits.

The furnace will also contain a high temperature safety cut-out switch that will disconnect all power to the heating elements if the temperature rises above a pre-set safety threshold. This can happen if the blower fan fails to engage and drive the heated air through the house, and thus draw cool air into the plenum chamber.

Electric furnaces are sized according to their power consumption and can generally range anywhere from 5 kW to 40 kW and be supplied with either single-phase voltage for household use, or three-phase voltage for industrial and commercial heaters, depending upon the availability at the point of installation.

Baseboard Heating

The simplest form of fixed electric heat is the baseboard heater. These small, modular units with their long horizontal profile are installed underneath windows and other areas of high ambient heat loss.

Energized at 240V in households to maximize heat production, baseboard heaters are commonly controlled by line-voltage thermostats installed in each room or heated area though some can also be controlled by built-in thermostats installed directly into the unit housing.

Because line-voltage thermostats directly control the flow of current to the heater element they must be rated for whatever value of current flows through them.

Baseboard heaters are rated for their power output, in watts, and are generally available in 250W increments, thus allowing installers to select the appropriate size of heater for the area being heated. To find the current requirements of a device, divide the rated power by rated voltage.

For example, a thermostat rated 3kW at 240V would be able to handle 12.5 amps of current.

$12.5\text{A}={\displaystyle \frac{3000\text{W}}{240\text{V}}}$

When installing any kind of baseboard heaters, care must be taken not to install any other electrical equipment above them. No receptacles may be installed above a baseboard heater because of the risk of a cord resting on the heater and getting damaged by the heat. If a receptacle is required in a section of wall that also has baseboard heat, some manufactures will provide spacing inside the heater for installation of a receptacle.