Advances in HVAC technology

BUSRide Maintenance gathered experts from the HVAC industry to discuss HVAC’s effect on bus efficiency, the relative advantages of different system types and how zero-emission technology is affecting air conditioning.

The panelists for this discussion are:

Chad Sleimanchief executive officer SanUVAire

Gary Eubank product manager DCM Manufacturing

Matt Lishbusiness development managerBitzer

Steve D. Johnsonproduct marketing manager, bus HVACThermo King

What impact does the weight of an HVAC system have on bus efficiency?

Gary Eubank: Increasing overall vehicle weight has a negative effect on fuel consumption, while reductions in weight have a positive effect. The significance of this will depend upon the operational cycle of the vehicle and the weight of HVAC equipment as a percentage of overall vehicle weight.

Chad Sleiman: The HVAC system is the second or third largest bus component, after the engine and transmission, and it takes up considerable real estate on the bus. The weight of such a system has a direct impact on the vehicle’s mechanical components’ performance. This can affect life cycle and fuel consumption, regardless of whether it is CNG, diesel, or electric; as well as CO2 emission and heat dissipation, which contributes to global warming.

Matt Lish: There are many variables involved in the concept of “bus efficiency,” including drag coefficients, drive-train design, driver behavior, and operating terrain, therefore it’s hard to make definitive statements regarding this issue. There is a large body of testing evidence from Society of Automotive Engineers (SAE), the National Renewable Energy Laboratory (NERL) and the Environmental Protection Agency (EPA) showing the reduction of vehicle weight (also referred to as “lightweighting”) in conventional-drive automobiles has a positive impact on vehicle efficiency, and reduces total fuel consumption over the life of the vehicle. Reductions in overall vehicle weight and specifically in the HVAC system contribute to fuel efficiency. Simply changing compressors can achieve reductions of 70 pounds or more  This is why Bitzer designs our bus air conditioning compressors out of cast aluminum in lieu of older, heavier, cast iron models.

Steve D. Johnson: Start with the fact that an empty bus gets better fuel mileage than a full bus and you know weight matters. While the impact may not be huge, it is also not insignificant. Every one-tenth of MPG improvement counts.

What are the advantages of a top-mounted system? A rear-mounted system?

Eubank: Traditionally, rear-mounted HVAC has been approximately three to four times more popular than top-mounted systems. This has been due to compactness, which in turn is viewed to have a positive effect on system robustness and reliability. Recently top-mounted systems have been gaining in popularity, because of improvements in technology, and a need to provide more space in rear-engine compartments for emissions control equipment. Most buses have a maximum weight limit per axle, and top-mounted systems also allow transferring some of the system weight from the rear axle to the front axle.

Sleiman: There are advantages and disadvantages to both. With a rear-mounted system, you have ease of install, access and maintenance since the system is not too compact. It also helps balance weight distribution on the bus. However, this creates higher exposure to grime, dirt and dust, as it is closer to the roads.

Top-mounted systems tend to be more linear and compact internally, which doesn’t leave much room for ease of access, maintenance and replacement parts after the initial install. It is directly exposed to rain, and heat and ultraviolet rays from the sun, so it needs to be well-constructed and sealed to withstand the elements and provide for a long life-cycle. It also needs to be lightweight, because the roof of the bus is not typically designed to support heavy components. It is less exposed to dirt and grime, but we still must keep an eye on condensation and drip-pan leakage issues.

Lish: Regarding the heart of any air-conditioning system, the compressor, it doesn’t matter where the rest of the system is located. What matters is that the system is designed to properly protect the compressor to ensure its long-term operation. Each location has challenges and may benefit from different protection strategies in order to properly protect the compressor from damage-causing events like liquid slugging or oil migration. Issues like belt alignment, belt tensioning, and shaft seal operation are all related to traditional diesel engine-driven piston compressors.  All electric, rooftop mounted compressors and systems come with different sets of challenges such as how repairable is the compressor, and challenges related to the heat from the sun.

Johnson: Rear-mounted systems generally weigh less, cost less and are easier to maintain, but can create an issue with axle weight. Space for the system is limited and so are the configuration options. Top-mounted systems provide more flexibility for installation locations, and more options on size, capacity and air distribution. They can be strategically placed to optimize axle weight. Having to cut holes in the roof and providing a good seal to prevent water ingress adds complexity and risks at installation. Being on the roof also adds safety risks during service functions.

What effect have zero-emission buses had on
HVAC technology?

Eubank: Increased emissions controls have been a major driving force in migration from rear mounted HVAC systems to roof mounted systems. They have also been a driving force in reducing energy consumption of all the HVAC components, from the compressors to the fan motors.

Technological advancements can be a double-edged sword.

Sleiman: It puts more pressure on the HVAC manufacturer to produce HVAC units that can meet such requirements. This caused manufacturers to go back to the design boards and come up with a new design, better components and compliance. This triggered a change in size, dimensions and weight of HVAC units as well as some of their components to function with newer more eco-friendly refrigerants.

Lish: Zero-emissions and anti-idling legislation particularly are driving innovation in transport A/C. The adoption of electric buses particularly has driven Bitzer to develop a range of fully electric, variable speed compressors. We have seen enormous interest in electric a/c systems globally, with real traction here in the USA within the last year and a half. The ELV series is highly efficient and multiple system companies are adopting them as key components in their new electric system designs.

Johnson: While it is still early in the game, the push for more efficient systems has started. That means delivering the required capacity using the least amount of power possible from the battery. To achieve this, more system integration between the HVAC and the bus power management system must be developed. With the absence of the engine, noise reduction technology will be applied to the HVAC system.

How do you see HVAC technology evolving 5 years from now? 10 years from now?

Eubank: Life-cycle cost will gain importance in the coming years. Heat pump technology will also grow dramatically with the importance of electric vehicles.

Sleiman: In the next five years, we see the mandates and legislations for more efficient HVAC systems, zero emissions and eco-friendly refrigerants continuing to shape and drive the evolution of HVAC technology.

In the next 10 years, we believe that, like everything else that needs to be connected and accessible over a network, HVAC technology will follow suit and be smart, connected and accessible. However, we also see that such developments will play a role in making HVAC units bigger and heavier.

Lish: As market and legislative pressures continue to drive reductions in greenhouse gas emissions, new refrigerants will be adopted, and compressor innovations to work most efficiently with those new refrigerants will undoubtedly be developed. Additionally, innovative electric A/Cs, which are being widely adopted today in Asia, will become more prevalent in North America. A range of variable speed electric compressors, such as the ELV series, will allow component selections to possess variable speed capability allowing for very precise cooling and relying on only the power needed.

Johnson: In the near term, systems won’t change that much because of the low number of zero-emission buses and the slow adoption rate. There is a lot of in-service testing to be done to establish base lines. Suppliers will be developing the next generation HVAC platforms. In 10 years expect systems to be smaller and quieter, more modular, and lighter in weight. Also, expect them to be using new environmentally-friendly gases, and have more efficient strategies for heating.

As HVAC technology advances, what affect does it have on preventative maintenance?

Sleiman: We have seen advancements that were a step forward in the right direction, but we have also seen backward steps. It’s a double-edged sword. On one hand, you equip the HVAC system with sensors and the ability to integrate to other monitoring systems to provide alerts and notifications, but that comes at a cost, which is more components that are too delicate and prone to frequent failure due to the harsh nature of the system functionality.

There are always great benefits to implementing technology advancements to allow for self-diagnostics, monitoring and alerts, but it means there are more specialized components that tend to be costly and require the maintenance staff to be trained to troubleshoot and replace them, or incur extra services cost. The question would be: what is the ROI like, overall, comparing systems with and without such advancements?

Advancements in the type of gas and refrigerants used is another serious issue.

Lish: With regard to compressors, those companies willing to invest in thoughtful design can integrate improvements more quickly thanks to advanced modeling and shortened development timelines. As an example, recent integration of the Bitzer shaft seal oil reservoir and drain tube allows for the quick check of proper oil weeping from the shaft seal, allowing service staff to perform this routine compressor check more quickly and intelligently than in the past.

Another example would be the use of fully hermetic, or sealed, scroll compressors. There have been studies performed by transit authorities that have demonstrated fully hermetic compressors and systems with minimal access points experience fewer leaks and thus require less maintenance.

Johnson: Overall, there are fewer mechanical parts to fail and maintain. The incidence of refrigerant leaks will drop dramatically. Maintenance costs can conservatively drop as much as 50 percent. Most of this can be attributed to the cost associated with checking, fixing and replacing the mechanical components.

Are there advances in HVAC systems and component manufacturing that ensure durability, as well as eco-friendliness? If so, what are they?

Eubank: There is continual advancement in both average service life and consistent performance in all electrical components. Hazardous metals and materials have been removed both from the final HVAC components and from the processes used to manufacture them.

Sleiman: Some of the advancements that we are aware of or have knowledge of are:

• Better compressors for better performance and durability

• Type of coolants used per legislations and various evaluations

• Layout of certain components within HVAC unit to provide for better performance, reliability and durability

• Implementation of sensors and smart components for better diagnostics, analytics and self- management.

Johnson: Durability is a given and systems will need to observe today’s standards as they change. Eco-friendliness will be driven by the new gases. Legislation will drive systems to lower leak rates which means designs with fewer fittings and no hoses. There may be laws dictating what the recyclable content must be or, leading suppliers could set those standards independently.

Tell us about the legislation that affects refrigerants in transportation applications.

Eubank: R134a is scheduled to be phased out by 2025. HFC1234YF is the probable successor. This will require
new refrigerant recovery and recycling equipment, and possible redesign of heat exchangers and sealing methods.

Lish: The U.S. EPA greenhouse gas regulation set standards based on a presumed across-the-board conversion to HFO-1234yf between 2017 and 2022 for domestic automobiles. There have been no definitive timelines set for bus adoption. The good news, however, is when adoption does occur, provided nothing changes, those shops servicing HVAC systems which have passed the required one time certification with their EPA Regional Office proving they have already acquired and are properly using approved refrigerant handling equipment for CFC-12 or HFC-134a equipment will not be required to re-submit certification to EPA when they purchase new equipment for a different refrigerant, such as HFO-1234yf.

Johnson: Bus and Rail are no longer just lumped into the transportation category and left to try and interpret the rules.  The EPA has recognized that the lower GWP refrigerants do not meet the flammability standards required to transport people safely.  Currently the two refrigerants used for buses and trains, R407C and R134a remain acceptable for those applications. A list of refrigerants currently acceptable under EPA’s Significant New Alternatives Policy (SNAP) Program for air conditioning in passenger trains and buses is available at

Sleiman: We already know that as of January 1, 2010, virgin HCFC-22, HCFC-142b and blends containing HCFC-22 or HCFC-142b may only be used to service existing appliances. Consequently, virgin HCFC-22, HCFC-142b and blends containing HCFC-22 or HCFC-142b may not be used to manufacture new pre-charged appliances or appliance components or to charge new appliances assembled onsite.

The legislation focuses on the reduction of the F-GAS by 79% by 2030 and its impact on the Global Warming Potential (GWP) limit of 2500 of it by 2020. If you have R404A, all systems with more than 10KG will fall under the new F-Gas legislation.

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