The death of HVAC
The death of HVAC can lead to the rise of thermal environmental engineering. Here are some thoughts on the effectiveness, efficiency, and installation and operating costs of modern air conditioning systems
Today we stand at the 100th anniversary of Willis Carrier’s introduction of the science of psychrometrics to ASME and of the introduction of air conditioning systems to the United States. It is a good time to analyze and synthesize recent building mechanical systems from a building science and engineering principles standpoint. We will examine systems from the fundamental concepts of human thermal comfort and health requirements, the current and future cost, and the availability of energy in the 21st century, along with the lifecycle costs of installation, operation, and remodeling.
The history of modern air conditioning systems over the past 40 years is not a story to be proud of. The energy crises of the 1970s should have spawned a flurry of innovation in energy conservation and the introduction of systems prudent both in installation and operating costs. This has not been the case. The 1990 study on air conditioning systems in government offices authored by Amory Lovins found that newer mechanical systems cost more to install, cost more to maintain, used more energy, and were less comfortable than older systems.
The study had two major flaws. One was the simple fact that there was no research into why recent systems, which were post-energy crises, were less efficient and effective but more costly than older systems. The second was Lovins’ accepting suggestions from the industry on a remedial path; that was asking the fox to guard the henhouse. The result was that the HVAC industry was allowed to plow onward and upward, developing ever more complicated systems that cost even more to install and maintain and were no more energy-efficient or comfortable.
The ultimate result is that we now have what may be considered the worst HVAC system ever introduced to the market and one that is currently the most popular system selected for major buildings in the United States: the fan-assisted variable air volume (VAV) system. To make a complete mockery of energy conservation, the industry is promoting ground source heat pumps to supply hot and chilled water to these systems, claiming, in some cases, that buildings using this system are fossil fuel free. All-electric buildings are probably the electric utility companies’ dream for the future. Unfortunately, this system is the opposite of a sustainable, high-performance system.
I came to the United States in 1978 to perform energy conservation studies in 100 hospitals in Philadelphia, Chicago, and Cleveland. My instrumented tests on air conditioning systems corroborated Lovins’ work. My introduction to energy conservation in the United States was greeted by roughly half of the hospital chief engineers stating “I’ve tried energy conservation, and it doesn’t work!” I witnessed the failures firsthand: black boxes hanging off boiler and chiller room walls, new VAV systems being operated almost manually to keep the occupant complaints down to a minimum, centralized computer control systems being used to monitor half of the connected equipment, and the controls being operated by local maintenance staff. The legacy of the oil crises was not a victory for owners, but a victory for snake oil salesmen selling silver bullets to owners who acted like deer caught in the headlights every time the price of gasoline jumped at the pump.
Because systems have not changed for the better, I suggest we develop HVAC systems that fulfill the owners’ detailed lifecycle performance requirements, details that are rarely documented and even less often fulfilled; systems that provide excellent thermal comfort; are least expensive to install, operate, and maintain over a very long lifecycle; and are most able to become carbon neutral in the near future, if not immediately, for economic reasons.
Comfort HVAC systems are installed in buildings for one purpose and one purpose only: the comfort and productivity of the occupants. Sadly, this primary performance is generally relegated to minimum current standards and accepted criteria for mediocre comfort and performance of the occupants. A nominal 75 F air temperature combined with 50% relative humidity (RH) is the summer indoor design and 75 F air temperature for the winter with no humidity control are the usual conditions. As for thermal comfort, these design temperatures will often produce poor comfort conditions for over half of the year and draw futile complaints from occupants. In fact, ASHRAE-specified comfort conditions assume 20% discomfort by the occupants, down from 25% in previous attempts.
Although it is almost impossible to make 100% of the occupants comfortable, we should be able to control the indoor environment to expect much better comfort while improving productivity by up to 10% against many current HVAC systems. Improving productivity by 5% in office buildings can be worth over $10/sq ft/year, more than the energy and maintenance costs combined. This increase in productivity is very achievable if we develop a mechanical system that provides consistent thermal environmental conditions suitable for human comfort. There are three primary criteria to consider for human thermal comfort:
Mean radiant temperature (MRT)
Ambient air temperature combined with air movement (T)
Relative humidity (RH)
The first thing we should recognize is that the MRT can have a greater effect on thermal comfort than the combined effects of air temperature and air movement. Engineers and designers have neglected this most important fact in human thermal comfort for far too long. This also indicates that if we expect a higher MRT in the summer than the winter, controlling air temperature to one setpoint for an air system will not provide a good solution for controlling thermal comfort in both winter and summer. This is why we often reset the temperature from 75 to 72 F in the summer and 78 F in the winter to feel more comfortable.
If we were to install a surface temperature detector on the floor together with a radiant floor system, we could control the comfort feeling of a human more closely. Many people have experienced radiant floor warming systems and appreciate the superior comfort conditions that this system is capable of providing. Radiant floors provide superior comfort not only by providing radiant energy, but by also controlling the air temperature close to the ideal temperature in the space without air movement. Therefore, while the air temperature may be less than provided by an air system for warming, it feels more comfortable.
Likewise, if we were to install a surface temperature detector on the ceiling together with a radiant ceiling, we could control the comfort feeling more closely. (Author’s note: radiant ceiling, not radiant panels.) Installing a radiant ceiling that covers more than 2/3 of the ceiling will provide quite an even distribution of temperature control throughout a space. Radiant ceilings work the same way as radiant floors except that they can provide more cooling than radiant floors, which most residential, commercial, and institutional buildings require in many parts of the United States. Radiant systems may be controlled from surface temperature detectors or a combination of surface and air temperature detectors. The air temperature is offset from the MRT opposite to an air system. The air temperature is moved from 75 to 78 F in the summer and 72 F in the winter to feel most comfortable. Using this knowledge, we can devise energy reduction strategies.
The second thing we should realize is that RH can have almost as great an effect on comfort as the air temperature. Writing from my home office outside Philadelphia where we have recently had 90 F days with the wet bulb in the mid-70s and the heat index in the mid-100s, I bear witness to the fact that humidity in the summer plays a major part in comfort in the United States, at least east of the Mississippi.
Controlling the RH in a conditioned space down to 40% rather than 50% during the summer season will provide a greater sense of comfort for the occupants. Likewise, controlling the RH up to 30% during the winter warming season will also provide a greater sense of comfort for occupants. Cooling coils in air conditioning systems dehumidify as a secondary function of cooling. Controlling the indoor humidity down to an acceptable level during periods of high humidity and low sensible gains can be achieved by either resetting the thermostat down to 70 F or having a wet bulb override in the control system and using reheat to control the air temperature. This situation occurs most noticeably with a summer thunderstorm over Philadelphia providing an outdoor temperature of 83 F with 90% humidity. Air conditioning systems with cooling coils use a great deal of energy either way this is controlled.
Examining the ASHRAE comfort chart (Figure 4) suggests we can have humidity levels between 80% and almost 0% for the summer and winter and expect a reasonable level of comfort. I suggest that the humidity should be between a 30% low in the winter and a 40% high in the summer for the best comfort conditions, with an operative temperature between 72 and 78 F, as indicated by the blank area almost exactly in the middle of the shaded comfort zone. The operative temperature is a combination of radiant and air temperatures.
Controlling the humidity below 60% is essential to eliminate mold growth, a problem that seems to be on the increase in U.S. buildings. Controlling the humidity above 15% in the winter prevents drying the occupant’s mucous membranes. Controlling the humidity level at low levels in the summer, below 50%, allows us to raise the operative temperature and remain very comfortable. Similarly, raising the humidity levels in the winter, above 25%, allows us to lower the operative temperature while remaining very comfortable.
Maintaining excellent comfort conditions for the whole lifecycle of the mechanical system should be the primary goal of every mechanical system. What we have established by a cursory evaluation of human environmental comfort is that we can radically improve comfort provided by environmental systems that are not based on moving large amounts of air in the space but require a substantial amount of radiant thermal control and closer humidity control that is not readily available from standard air conditioning systems.
The ASHRAE comfort temperature is unhelpful for practical system design and operation:
Effective temperature ET = to + wim LR(pa – 0.5pET,S)
I won’t even begin to demystify this equation. Instead, we can use the surface temperature of a radiant floor or ceiling to directly control the temperature or control the room air temperature to indirectly control the temperature.
Examining green building standards in the United States and the European Union (EU), the greatest difference is that some EU standards have much more stringent building envelope performance requirements. Most of the EU has a temperate climate compared to the Mid-Atlantic states, yet Passive Haus and Minergie standards have superior envelope performance requirements both in insulation and air tightness to any U.S. standard. The most comfortable buildings in the EU have been consistently reported as those with a radiant ceiling where there is a substantial cooling load and those with a radiant floor where the load is predominantly a warming load.
One more criterion must be addressed to provide excellent thermal comfort for building occupants: individual control. Studies have shown that individuals who have direct control of their temperature report higher comfort levels. Because individuals vary considerably in their thermal comfort requirements, providing individual control allows each occupant to have his or her own setting. Providing small zones within each space will allow individuals to have their own temperature control.
A further health and life support requirement is outside air ventilation. Ventilation is required to provide air for breathing and also dilute and exhaust contaminants generated within the indoor environment. The amount of ventilation required has varied from 30 to 5 cfm per person over the past 40 years. With the elimination of smoking within buildings and the elimination of off-gassing materials, fixtures, and fittings, the amount of air required for dilution and exhaust has been dramatically reduced. The amount of air required for occupant life support varies from 1 to 7 cfm, depending on activity. Most sedentary work or occupations within buildings will be satisfied with 2 to 3 cfm per person.
Providing the minimum of ventilation air to a space requires an efficient ventilation strategy. Displacement ventilation can be more than 90% efficient in providing ventilation to the occupants and is the most efficient form of ventilation available. Generally, the efficiency varies between 75% and 85% in most applications. This means we need a minimum of approximately 5 cfm of outside air ventilation per occupant, consistent with the latest requirements from ASHRAE. If we have approximately 100 sq ft of space per occupant, we need 0.05 cfm/sq ft for ventilation air. Note that most air conditioning systems circulate 0.75 to 1.5 cfm/sq ft in the conditioned space, 15 to 30 times the required ventilation air circulation required with displacement ventilation.
Recirculating air that has been exhausted from a space can provide a path for unwanted toxins and diseases. It is preferable to provide 100% outside air in order to prevent spread of indoor pollutants and disease. Minimizing and optimizing the method of introducing outside air takes considerable diligence.
Building envelope performance
The shape and orientation of a building affects the thermal performance of the building considerably and should be analyzed for new buildings and major retrofit projects. Optimizing the shape and orientation can improve the energy performance by more than 30%, as established by using building energy simulation programs during the initial design and planning.
The indoor environment is cushioned from the outside environment by the building envelope. The performance of the building envelope is critical to the comfort and performance of the occupants and the thermal environmental control system. The better the insulation and higher the mass in the walls and roof, the less heat and cold will get through to the indoors and the later it will happen. Having high-performing walls and roofs requires complementary performance from the windows and the building entrances in order to complete the integrity of the building envelope performance.
Building air-tightness is another huge factor in building thermal performance; the tighter the building envelope, the less energy is required for conditioning the interior. There have been instances of super-airtight buildings causing an IAQ problem, but with the introduction of new materials, fittings, and fixtures that emit no contaminants or pollutants, this problem has been overcome.
The overall thermal performance of a building is therefore highly dependent on the building envelope performance. If we believe in climate change and in planning for the next 50 to 100 years for a building, we should allow for both higher and lower design temperatures for cooling and warming, and most importantly allow for much higher winds that are predicted to occur with much greater frequency. We should substantially strengthen these two standards for higher temperature and wind occurrences. Currently, even the best buildings in the United States would fail to meet the EU standards; a huge improvement in building envelope performance is required to both improve occupant comfort and substantially reduce energy consumption.
For the most prudent lifecycle performance, improvements in the building envelope performance can be one of the best investments. Building walls and roofs can have a lifecycle of 50 to 200 years. Windows and doors usually have a 20- to 50-year lifecycle. Investing in excellent airtight envelopes can have a short payback when planned for and completed by a competent construction team.
An airtight envelope that provides a vapor-proof barrier is absolutely necessary for providing a great difference in the vapor pressures between the indoor and outdoor pressures. A strong vapor barrier in the building envelope allows for low humidity within the building to be easily maintained when the outside humidity is high, and vice versa.
As discovered in the 1990 Lovins’ report, HVAC systems were becoming less maintainable than older systems. Current systems are even less maintainable than those of 1990. Fan-assisted VAV systems require almost 50% more maintenance than the VAV systems of the 1990s. Fifty years ago, 80% of mechanical systems had preventive maintenance. Today, as much as 80% of buildings suffer from deferred maintenance in mechanical systems, or worse. This reversal of maintainability means current systems are not being fully maintained and thus cannot provide optimum comfort or energy performance. It is therefore imperative to have a total preventive maintenance program if we want optimum performance from our mechanical systems.
There are three simple rules of thumb to determine if a total preventive maintenance program can be conducted throughout the system whole lifecycle. While most engineers and contractors will deny such rules, maintenance directors and personnel understand the importance of them.
1. Ninety percent of maintenance is to inspect, clean, and lube all moving parts.
2. All moving parts must be easily accessible in plant rooms or equipment rooms.
3. If it’s not easy to maintain, it won’t be.
The extra cost of reconstruction due to lack of preventive maintenance is often hidden within the construction budget, thus disguising the true cost of lifecycle maintenance. Today we see remarkable, heretofore unheard-of situations: major institutions with directors of deferred maintenance, major hospitals that require triage on their mechanical systems to keep them operating, and so it goes on.
It is easy to understand why designers don’t want to acknowledge the three rules; 75% of the moving parts in a fan-assisted VAV system are hidden in false ceilings. This makes them impossible to maintain throughout their lifecycle, and in practice impossible to even balance and commission 100% during initial construction. This is the reason many current systems are reconstructed every 15 to 20 years at great expense to the owner, usually with the excuse of being “upgraded” to avoid embarrassment to the owner’s pride.
Now we need to move on to more difficult concepts: planning systems to be expandable and adaptable to building internal churn and change of occupancy. The 1970s mantra of long-life, loose fit is a great performance requirement that should be applied to current mechanical systems so they will require the minimum of remodeling during a long lifecycle of, say, 100 years.
The longest-lived components of mechanical systems are the piping and ducting systems. These systems are tentacled and embedded throughout every building to deliver the required conditions. These components can represent more than 50% of both the total mechanical system cost and energy use and yet are usually arbitrarily selected.
Most textbooks credit reverse return systems with being self-balancing; there are further advantages that should make this system the default choice. Reverse return systems have no index run so all zones receive equal pressure, which, in turn, means that if one zone requires an increase or decrease in flow of up to 50% at some later time, it is very doable with simple local piping changes. This allows the system to be very adaptable to major changes without replacement, unlike the flow and return system.
Murphy’s Law states that the first increase in flow required is near the index run, requiring major changes for a minor increase in flow. We will discuss the comparative energy requirements between the two systems later on. From a maintenance standpoint, suffice it to say that when the flow and return system will usually have a 25-hp pump with a VFD, the reverse return can use a 3-hp pump, which requires less maintenance.
Energy performance of mechanical systems
Most mechanical systems for new buildings or major remodeling projects are expected to last for only 20 to 30 years. It would be more cost effective for the major parts to last for nearer to 100 years. These systems are the piping and ducting systems, as explained above. The generating plant usually lasts for 20 to 30 years and the major plant, pumps, fans, etc., have the same length of service. However, when designs are produced, the energy data used are nearly always the current year energy costs with little or no regard to future costs and availability of energy sources.
We would be wise to consider the future of energy costs and supply. Several energy cost scenarios seem quite certain: energy costs will rise much greater than the general cost of living; and demand charges for electricity and gas/oil will continue to rise, punishing electrical use between June and October and gas/oil use between December and March.
If we want to design a mechanical system that can become less dependent on electricity and gas/oil for energy sources, we need to develop a system that is easily and readily adaptable to clean renewable energy sources such as earth, sun, and wind. If we want to design a system that does not use more electricity when the heat increases in the summer and uses no more gas/oil when the temperature drops in the winter, we need to develop a system that has no demand for peak cooling or heating, or can be easily adapted to use clean renewable sources for the peaks.
In a typical fan assisted VAV system (Figures 2 and 3), we have a filter, a damper, and a complete fan coil unit hidden in the false ceilings. Some buildings will have several hundred of these units, each supposed to control the temperature by blowing huge amounts of air into the space below. It is impossible for any maintenance staff to maintain all these units with all their individual moving parts. In most cases the access is limited at best and there are simply too many units. Even with computerized controls, dampers get stuck, filters need to be cleaned, etc.
However, because VAV systems are the ubiquitous system in the United States, we need to adapt/modify these systems to become easily maintainable while reducing energy use and improving occupant comfort. By removing the fan coil units from the false ceilings and replacing them with reheat/recool coils with cool and warm water, and by adding desiccant humidity control and heat recovery, we have lessened the overall maintenance requirements of the systems and allowed for easy adoption of renewable energy sources at some future date. These modified systems have lowered the maintenance costs and energy use, and improved occupant health and productivity by offering a higher quality indoor environment and eliminating air recirculation.
Thermal Environmental Engineered Systems
In the 1960s, a professor of architecture in England developed a concept that buildings should use “ambient energy systems” and sources. That is to say, the mechanical system was designed to maintain 75 F indoors by using 85 F warming and 65 F cooling sources. This is an excellent concept to use if we want to develop mechanical systems with the specifications derived above. With a good building envelope, particularly with a good air and vapor tightness, we can use 65 F fluid for cooling with a radiant ceiling and 85 F fluid for warming with the same radiant ceiling. The dehumidification and humidification can be derived from a liquid desiccant air system that supplies 0.1 to 0.3 cfm/sq ft of 100% outside air to the space via displacement ventilation for both ventilation and humidity control. The desiccant system can use 65 F and 85 F to supply 67 F air year-round to the space.
We can supply the 85 F and 65 F fluids extremely efficiently from boiler and chiller plants or from renewable energy sources in the future. Chillers that supply 65 F cooling water work at twice the efficiency of chillers working to provide 45 F chilled water. Boilers supplying 85 F warming water are working at their most efficient condensing temperature, almost twice as efficient as when they are developing 200 F hot water. The 85 F warming fluid can also be easily supplied from solar thermal, particularly as solar energy is abundant when the temperature is at its lowest during the short winter daylight hours. The 65 F cooling fluid is available from a ground heat exchange for supplying all the cooling and can be used for pre-warming during the winter. Where the ground source temperature is above 65 F, using the 70 to 75 F available as condensing water will allow absorption refrigeration machines to work at their peak efficiency, using solar thermal as the heat source and thereby developing systems that can use clean renewable energy sources for anywhere and everywhere in the United States.
The mechanical system developed (at right) does not fit the terminology of an air conditioning system well because it is subtler in both concept and operation. The term “thermal environmental system” fits well as it is controlling the thermal environment more fully than an air conditioning system ever could.
This system has a small air system for distributing ventilation and providing humidity control. It also separates and transfers the distribution of warming and cooling from the air system to an effective and efficient piping system. Distribution systems are sized on the maximum energy requirements of every zone. This causes the system to be oversized by over 50% for actual operating design. This is good design, but we can take advantage of it for energy reduction strategies. If we have a distribution system that is 50% oversized, we can install a pump that pumps 40% of the maximum design flow. If we have a reverse return piping system, we can install a pump with 7% of the pump power and it will provide an even distribution to all zones. With an existing flow and return piping system, we have to install a pump that will provide sufficient power to pump to the index circuit, probably a 20% size motor.
To address concerns about climate change, climate chaos, and global warming, we need to transition from accepting buildings as energy users to viewing them as potential energy generators. To accomplish this, we need to reduce the carbon footprint of building mechanical systems by over 90% and, to achieve this goal, we need to throw out our 100-year-old Carrier ideas of air conditioning and begin to think in terms of thermal environmental engineering. We must rely on architectural and building science together with engineering principles and learn to understand and appreciate the many sides of thermal environmental engineering. Understanding the role of MRT and humidity in providing both comfort and strategies to minimize energy use will be a major challenge for most engineers and one that should addressed by professional societies as soon as possible.
The 21st century requires mechanical systems that are able to double occupant comfort; extend the lifecycle up to 100 years with minimum, easy, preventive maintenance; and offer carbon neutral energy strategies. Where we have warm, humid conditions, we must separate humidity and ventilation control from temperature control so we may improve comfort while greatly reducing energy use. Lastly, a much higher standard for the building envelope is required, not only for insulation but particularly for air and vapor barriers. To do all this requires a paradigm shift in thinking not only from engineers and designers, but also from building owners and their representatives, who must demand superior performance and drive the changes needed.
Maisey is chief engineer at Building Services Consultants Inc. He has worked for more than 40 years to save U.S. buildings energy while reducing maintenance and improving occupant comfort. His upcoming book, “Buildings for a Sustainable Climate,” will cover all this and more.