In 2010, new engines must comply with lower emissions standards. Engine manufacturers have added diesel particulate filters (DPF) to buses and R-22 refrigerant will be banned for use in new equipment.
“In general, the trend has been toward roof-mounted units, and the 2010 emissions regulations, which require more equipment in the rear of the bus, are increasing the need for these systems,” says Fil MacNeil, Systems Group Leader for New Flyer Industries.
“Roof-mounted units provide many benefits: better weight balance for improved ride quality, lower noise levels for passenger comfort and the additional real estate on the roof provides more flexibility when deciding on the size and capacity of units.”
Steve Johnson, bus HVAC product manager with Thermo King, says that while many are aware of these changes, they may not be aware that the configuration and location of the HVAC unit could be affected.
“In the United States, the most commonly used HVAC in the transit industry is mounted at the rear of the bus above the engine,” he says. “As bus OEMs are forced to find space to accommodate the DPF, they are re-arranging the components in the rear of the bus. The available space for the traditional rear-mount HVAC unit is being reduced or eliminated. The robust, one-piece, compact design is the most reliable configuration available today and is easy to install and service. Will it go away permanently? As of now we don’t know, but we do know that some type of change is inevitable.”
Roof-mounted units are what the industry is seeing more of. “This is not a novel concept,” says Johnson. “Some U.S. transit properties already have experience with roof-mount HVAC, particularly on articulated buses where two cabins have to be cooled. Also, in all markets outside of the United States, roof-mount HVAC is the standard configuration.” He adds, “For the most part, the rear-mount HVAC is preferred in the transit industry, and in fact, more than 80 percent of what Thermo King supplies has been rear-mount over the years.”
Johnson explains how the roof-mount systems are installed. Holes are cut in the roof for the air openings and sealing of the HVAC unit to the roof becomes critical. The refrigerant, water lines and some harnesses have to be routed within the body of the bus to connect the roof-top unit to the compressor in the engine compartment.
To realize the same capacity, the roof-mount unit will be heavier, mainly due to the configuration of the coils and the addition of covers, which are not needed on the rear-mount system. Some roof-mount configurations have separate condenser and evaporator modules that have to be connected after they are on the roof, an additional step in the installation process not required for rear-mounted units.
In addition, some maintenance and repair work has to be done from the roof of the bus instead of from the ground or on a short ladder.
“The roof-mount configuration allows the operator to have a rear window if they choose, and there are more opportunities to manage air distribution, weight distribution and noise because the location of the unit can be anywhere from the back to the front of the bus,” explains Johnson.
Without an air-conditioning unit in the rear, the bus is able to have a rear window, which allows for more natural light so the passengers don’t feel enclosed or uncomfortable when sitting in the upper deck. Also, the driver has a rear window to look out and he or she can keep a closer eye on the passengers. Another convenience for the passengers with the roof-mounted system is that it is quieter and there is less heat as the noise and heat are already going up in the air.
A roof-mounted system accounts for less porpoising. The weight is leveled out more on the bus having the air conditioning unit moved forward. A lot of weight in the rear with the engine and air conditioning system in the back makes the bus porpoise.