Table of Contents
Introduction
Heating, cooling, and ventilation systems are central to how comfortable and healthy buildings feel, and they also account for a large share of energy use in homes, offices, and cities. In a course on renewable energy and sustainability, these systems matter because improving how we heat, cool, and ventilate spaces can dramatically cut energy demand, reduce greenhouse gas emissions, and make better use of renewable sources.
This chapter focuses on the functions and main types of heating, cooling, and ventilation systems in buildings, and how their design and operation affect energy use and sustainability. Broader design ideas, such as passive strategies and net‑zero buildings, are treated in other chapters, so here we concentrate on the active systems that move heat and air.
Thermal Comfort And Indoor Air Quality
Heating, cooling, and ventilation are not goals in themselves. They are tools to achieve thermal comfort and good indoor air quality. Thermal comfort is the condition where most people feel neither too hot nor too cold. It depends on air temperature, mean radiant temperature of surrounding surfaces, air speed, humidity, clothing, and activity level.
Indoor air quality refers to the cleanliness and freshness of the air inside a building. Without adequate ventilation, pollutants such as carbon dioxide from people, volatile organic compounds from materials and cleaners, moisture from cooking and showers, and sometimes combustion products from poorly vented heaters can build up to unhealthy levels.
Heating and cooling systems change air and surface temperatures and sometimes humidity, while ventilation systems bring in outdoor air and remove stale air. In sustainable buildings, the aim is to achieve acceptable comfort and air quality with the least possible energy use and environmental impact.
Basic Principles Of Heating And Cooling
All heating and cooling systems work by moving heat from one place to another or converting another form of energy into heat. Three physical processes are involved: conduction through materials, convection by moving air or water, and radiation between surfaces.
In heating mode, energy is supplied to raise indoor temperatures or to heat a fluid that then transfers heat to indoor spaces. In cooling mode, heat is removed from indoors and released outdoors. In both cases, the building envelope and passive design strongly influence how much heating or cooling is required, but the active systems determine how that demand is met.
Energy performance of heating and cooling systems is usually described by:
- Efficiency for combustion or direct electric systems, often given as a percentage, comparing useful heat output to energy input.
- Coefficient of performance (COP) for heat pumps and refrigeration systems, defined as useful heating or cooling provided divided by electrical energy consumed.
For heating or cooling systems, a key performance metric is the coefficient of performance:
$$\text{COP} = \frac{\text{Useful heating or cooling output}}{\text{Electrical energy input}}$$
A higher COP means more heating or cooling provided per unit of electricity, which is essential for energy efficient and low carbon buildings.
This ratio can be greater than 1 for heat pumps because they move heat rather than create it from electricity alone.
Common Heating Systems
Heating systems vary by energy source, distribution method, and the way they transfer heat to indoor spaces. While their exact configuration differs between climates and building types, several families of systems are common.
Central hydronic systems use water as the main heat transfer fluid. A boiler or other heat source heats water, which circulates through pipes to radiators, convectors, or underfloor heating loops. Radiators mainly provide radiant and convective heat to rooms. Underfloor systems spread low temperature heat over large floor areas, which can improve comfort and allow lower water temperatures that work well with condensing boilers and heat pumps. Hydronic systems can connect to multiple heat sources, including biomass boilers, solar thermal collectors, or district heating networks.
Central air based systems use air as both the heating medium and the distribution medium. A furnace, heat pump, or electric heater warms the air, which is then distributed by fans through ducts to different rooms. Return ducts carry cooler air back to the unit. Central forced air systems are common in some regions, especially in combination with cooling, because the same duct network can be used for both.
Localized or room heaters operate in individual spaces without central distribution. Examples include wall mounted gas heaters, electric resistance heaters, and small stoves. These systems can make sense in small or intermittently used spaces but are usually less efficient at whole building scale, especially when they rely on direct electric resistance or unvented combustion.
Heat pumps used for heating are particularly important for sustainable buildings. They can extract heat from outdoor air, ground, or water, and deliver it indoors. Because they move heat instead of generating it only from electricity, they can reach COP values significantly above 1. This allows substantial reductions in delivered energy, especially when electricity comes from renewable sources. Details of specific renewable systems such as geothermal heat pumps are covered in other chapters, but here it is important to recognize that heat pumps can often replace fossil fuel boilers and furnaces.
Common Cooling Systems
Cooling systems also take several forms, from centralized chilled water systems to simple room air conditioners. At their core, most modern cooling systems rely on the vapor compression refrigeration cycle, which uses a refrigerant that circulates between evaporator and condenser components.
Central chilled water plants produce cold water in a chiller unit and pump it through pipes to air handling units or fan coil units located in different zones. In the air handling unit, air passes over cold coils, is cooled and often dehumidified, and then distributed through ducts. Chilled water systems are common in large commercial or institutional buildings, where they can be more efficient and easier to manage at scale.
Direct expansion systems, where refrigerant flows directly to indoor units, are widespread in smaller buildings. Room air conditioners and split systems are examples. In split systems, an outdoor unit contains a compressor and condenser coil, while an indoor unit contains an evaporator coil and fan. Multi split and variable refrigerant flow systems can connect multiple indoor units to a few outdoor units and modulate refrigerant flow to match demand, which can improve part load efficiency.
Evaporative cooling systems use the principle that water evaporation absorbs heat from the air. In dry climates, simple or indirect evaporative coolers can provide cooling with much lower electricity use than vapor compression systems, because they mainly power fans and water pumps. However, they are limited by outdoor humidity and do not dehumidify air.
In sustainable design, sometimes non mechanical or hybrid cooling methods are integrated with active systems. For example, night time ventilation can cool the building structure, and mechanical cooling is then used only when passive strategies are insufficient. Central focus here remains on the active equipment that provides controlled cooling.
Distribution And Emission Systems
The way heat and cooling are transported and delivered inside a building has a strong influence on energy use and comfort. Three main distribution media are used: air, water, and sometimes refrigerant directly, each with its own characteristics.
Air distribution uses ductwork and fans. Supply ducts carry conditioned air from a central unit to rooms, while return ducts bring air back. Proper duct design, insulation, and sealing are crucial. Leaky or poorly insulated ducts can waste large amounts of energy, especially when they pass through unconditioned spaces such as attics or service shafts. Air terminals such as diffusers and grilles must be positioned to provide even temperatures without drafts.
Water distribution uses pipes and pumps. Because water has a higher heat capacity than air, it can transport the same amount of heat with smaller volume flow and pipe sizes. Hydronic distribution can therefore be more compact and often more efficient. Terminal units like radiators, convectors, fan coils, and radiant panels or floors then transfer heat or cooling to rooms. Low temperature heating and high temperature cooling systems increase overall efficiency.
Direct refrigerant distribution is used in variable refrigerant flow and similar systems. Refrigerant runs in relatively small diameter pipes between outdoor and indoor units, and each indoor unit controls local conditions. These systems can offer zoning flexibility, but refrigerant leakage must be minimized because some refrigerants have high global warming potential.
Emission systems are the final devices that transfer heat to or from the occupied space. Radiators, convectors, floor and wall heating, chilled beams, and air diffusers all shape how occupants experience comfort. For example, radiant floor heating can allow lower air temperatures with the same comfort level because warm surfaces reduce radiant heat loss from occupants.
Ventilation: Natural, Mechanical, And Hybrid
Ventilation systems control the exchange of indoor and outdoor air. They remove indoor pollutants and excess moisture and supply oxygen for occupants. How this exchange is achieved has a direct impact on energy use, because outdoor air usually must be heated or cooled before it reaches comfort conditions.
Natural ventilation relies on wind and temperature differences between indoors and outdoors to move air through openings such as windows, vents, and shafts. Stack effect, driven by warm air rising and cooler air entering lower openings, can create vertical airflow through buildings. When climate, building form, and outdoor air quality are suitable, natural ventilation can provide fresh air with very little energy, relying mainly on controllable openings. However, its performance is variable and difficult to guarantee in all weather conditions, and it can lead to uncontrolled heat loss or gain if not carefully designed.
Mechanical ventilation uses fans to provide controlled airflows. There are three basic forms. Exhaust only systems use fans to remove indoor air, relying on infiltration to bring outdoor air in. Supply only systems blow outdoor air into the building, pushing indoor air out through leaks and designated exhaust points. Balanced ventilation systems use both supply and exhaust fans to control airflows in and out. Balanced systems allow filtration of incoming air and better distribution of fresh air throughout the building.
Hybrid, or mixed mode, ventilation combines natural and mechanical methods. For example, windows may provide ventilation in mild conditions, while mechanical systems operate during extreme weather or when outdoor air quality is poor. In some buildings, window opening is coordinated with automation systems to avoid energy waste.
For sustainable buildings, mechanical ventilation with heat recovery is especially important in cold and temperate climates or in airtight, energy efficient buildings. In such systems, a heat recovery ventilator or energy recovery ventilator uses a heat exchanger to transfer heat, and sometimes moisture, between outgoing and incoming air streams.
In heat recovery ventilation, the thermal efficiency is often expressed as:
$$\eta = \frac{T_{\text{out,supply}} - T_{\text{outdoor}}}{T_{\text{indoor}} - T_{\text{outdoor}}}$$
where $T_{\text{out,supply}}$ is the temperature of the supply air after the heat exchanger, $T_{\text{outdoor}}$ is outdoor air temperature, and $T_{\text{indoor}}$ is exhaust air temperature. Higher $\eta$ values indicate more effective heat recovery and lower ventilation heat losses.
By recovering energy from exhaust air, these systems can significantly reduce heating or cooling demand while still maintaining high indoor air quality.
Integrated HVAC Systems
In most modern buildings, heating, cooling, and ventilation functions are combined into integrated HVAC systems. The acronym HVAC stands for heating, ventilation, and air conditioning, and it highlights that these functions are closely related. An integrated system decides how much outdoor air to bring in, how much heating or cooling to apply, and how to distribute conditioned air or water to zones.
Central air handling units are a common example. They take in outdoor air, possibly mix it with return air, filter and condition it through heating and cooling coils, and then supply it through ducts. Local terminal units or variable air volume devices then adjust conditions for each zone. Alternative configurations use dedicated outdoor air systems that handle ventilation, while separate systems such as radiant panels or hydronic loops handle most of the heating and cooling load.
In integrated systems, controls play a crucial role. Sensors measure temperatures, humidity, occupancy, and sometimes air quality indicators. Controllers decide when to operate equipment, at what setpoints, and at what capacities. Well designed control strategies can reduce energy use by matching system output to actual demand instead of running equipment continuously or at full load.
Variable speed drives for fans and pumps, modulating valves and dampers, and advanced thermostats all help match output to demand. For example, in variable air volume systems, supply air temperature may remain relatively constant, but air volume to each zone is adjusted by dampers in response to local needs. In hydronic systems, variable speed pumps adjust water flow based on temperature differences and valve positions.
A key concept in integrated HVAC design is zoning. Different areas within the same building can have very different loads and occupancy patterns. Separating them into zones with independent control reduces energy waste. For instance, unoccupied rooms can be set to setback temperatures that require less heating or cooling.
Energy Efficiency Strategies For HVAC
Because HVAC can be one of the largest energy uses in a building, even simple improvements in system design and operation can lead to major savings and emission reductions. While passive design reduces the underlying heating and cooling demand, efficiency measures ensure that the remaining demand is met in the most effective way.
At the equipment level, selecting high efficiency boilers, chillers, furnaces, and especially heat pumps with high COP or efficiency ratings is fundamental. The same applies to fans, pumps, and motors that move air and water. In many cases, the lifetime energy savings of more efficient equipment far exceed the initial cost difference.
At the system level, appropriate sizing is essential. Oversized systems can cycle on and off frequently, operate at low part load efficiency, and cost more to install. Right sizing based on realistic load calculations supports both comfort and efficiency. Distribution systems must also be designed to minimize losses by insulating ducts and pipes, avoiding unnecessary bends and restrictions, and maintaining correct pressures.
Ventilation rates should match health and comfort requirements but not exceed them unnecessarily, because every extra unit of outdoor air in extreme weather adds to heating or cooling demand. Demand controlled ventilation is one strategy that adjusts ventilation based on occupancy or measured indoor pollutant levels, such as carbon dioxide. In this way, spaces are well ventilated when in use, but not over ventilated when empty.
Effective control of setpoints and schedules is another powerful tool. Allowing indoor temperatures to float within a reasonable comfort range can significantly cut energy use. For example, cooling setpoints can be a few degrees higher, especially if air movement from fans improves perceived comfort. Heating setpoints can be lower in winter, especially when occupants can adapt with clothing. Night setback and scheduling ensure that systems do not run at full comfort conditions when spaces are unoccupied.
Regular maintenance is vital for actual performance to match design intent. Dirty filters, fouled coils, stuck dampers, and poorly calibrated sensors all increase energy consumption and reduce comfort. Basic maintenance measures such as filter replacement, leakage repairs, and proper refrigerant charge management are essential for sustainable operation.
Thermal Comfort, Controls, And User Interaction
Even the most efficient HVAC system cannot deliver sustainability benefits if occupants feel uncomfortable or cannot use the system properly. Human behavior and control strategies must therefore be part of the design.
Occupants vary in their preferences. Some like cooler or warmer conditions than others. Fixed single setpoints are often a compromise. Providing some local control, such as individual thermostats for zones, operable windows, or personal fans, can increase comfort without extensive energy waste, as long as the controls are well integrated and limits are set to avoid conflicting actions.
Adaptive comfort approaches recognize that people can adapt to a wider range of temperatures if changes are gradual, if occupants can adjust clothing or open windows, and if expectations match the building type. For naturally ventilated buildings in mild climates, acceptable indoor temperatures can be higher in summer than in fully air conditioned buildings, which reduces or eliminates the need for mechanical cooling.
Automation can support energy efficient operation, but it must be transparent and easy to understand. Overly complex interfaces can lead to misuse, such as occupants overriding schedules or setting extreme temperatures when they feel a momentary discomfort. Clear feedback, such as visible indicators of system status and recommended setpoints, can help align occupant actions with energy goals.
Urban Perspectives: District Systems And Waste Heat
At the scale of sustainable cities, heating, cooling, and ventilation systems are not only a building level issue. Groups of buildings can share infrastructure and energy sources, which opens up new efficiency and renewable integration opportunities.
District heating and district cooling systems distribute hot or chilled water from a central plant to multiple buildings through underground pipe networks. Instead of each building owning separate boilers or chillers, a centralized plant can use large scale heat pumps, biomass, geothermal sources, or waste heat from industry or data centers. It can also integrate thermal energy storage to shift heat production to times when renewable electricity is plentiful.
Waste heat recovery is particularly important in urban areas. Data centers, industrial facilities, or even sewage systems can provide low temperature heat that, when upgraded by heat pumps, becomes a valuable source for space heating and domestic hot water. By connecting buildings through district networks, cities can reduce reliance on individual fossil fuel heating systems and improve the overall efficiency of energy use.
At the same time, urban form, such as building heights and street layouts, influences wind patterns, shading, and the potential for natural ventilation. Urban planners and building engineers must coordinate to make sure that future neighborhoods are not only connected to low carbon heating and cooling sources but are also designed to reduce cooling loads and enhance safe ventilation.
Sustainability Considerations In HVAC Choices
Every decision about heating, cooling, and ventilation affects energy consumption, emissions, and often also material use and refrigerant impacts. Sustainable systems aim to minimize life cycle environmental burdens while maintaining or improving comfort and health.
From a climate perspective, systems that rely on fossil fuel combustion, particularly for space heating, contribute directly to greenhouse gas emissions. Moving toward electric heat pumps, especially when powered by renewable electricity, significantly reduces direct building emissions. Efficient cooling technologies with low global warming potential refrigerants reduce indirect emissions from electricity use and direct emissions from refrigerant leakage.
Another important dimension is resilience. Heating and cooling systems must maintain safe indoor conditions during heat waves, cold spells, or grid disturbances. Buildings with low energy demand, good envelope performance, and passive design are inherently more resilient. At the system level, buildings that integrate with microgrids, on site renewables, and storage can maintain critical HVAC functions even when external systems are stressed.
Finally, sustainable HVAC choices consider the broader social and economic context. Affordable, efficient systems can reduce energy bills and improve living conditions, especially for vulnerable households that might otherwise face energy poverty or live in unhealthy environments. Urban scale solutions like district heating and cooling must also address equity, ensuring that benefits are accessible across neighborhoods rather than concentrated in a few developments.
By aligning heating, cooling, and ventilation systems with efficiency, renewable integration, and good indoor environmental quality, buildings and cities can move closer to genuinely sustainable operation, making active systems a key partner to the passive strategies and design approaches developed elsewhere in this course.