There are four key challenges in designing an air conditioning system for a commercial building – meeting the comfort requirements for the occupants, ease of installation, energy efficiency and cost. Today’s ductless split systems score highly in the first three requirements, but the upfront cost has tended to be high in North America.
But that’s changing, notes Skip Hayden, senior research scientist at Natural Resources Canada’s CanmetENERGY lab in Ottawa.
The cost in Europe has traditionally been about one-third of that in North America. “It has dropped in half in the past year-and-a-half, but there’s still a big difference,” he says.
There are a number of reasons for the difference. These systems are based on heat pump technology and where the system is used for heating and cooling, the heating load is typically much higher in North America. As well, the voltages, safety and certification requirements are different for North America, which requires a special production run for a relatively small market.
Technician Cash Curtis (Curtis Heating & Cooling) adjusts the refrigerant charge in one of the Fujitsu Halycon mini-split systems at the historic 56-room Hotel Seville in Harrison, Arkansas.
Ductless systems have been widely used in Japan and Europe for over 20 years and make up the majority of air conditioning systems sold. This compares to about three percent in North America, although that is rapidly changing. They offer a number of advantages in commercial installations – easy zoning, energy efficiency, outstanding comfort levels and relatively unobtrusive installation where they must be retrofitted into existing buildings. Most HVAC contractors are already familiar with “mini-split” systems used in residential applications. With up to four indoor evaporator units per outdoor condensing unit, these can also used in smaller commercial applications. However, larger commercial applications require “multiple split systems” or “variable refrigerant flow” (VRF) systems, which can have up to 50 indoor units operating with one outdoor unit.
Zoning and comfort
Two advantages of mini-split and VRF systems go hand in hand – easy zoning and high comfort levels. Each indoor unit has individual on/off and climate controls and the system, which is inverter driven and provides variable output, provides only as much cooling (or heating) as required by the indoor load. “You are automatically forcing the zoning and, in effect, lowering the overall energy requirements of the application,” says Hayden. “(The user) can control the amount of flow and the temperature that you are getting, so from a comfort point of view it can be particularly attractive.” VRF systems control indoor air temperature by varying the amount of refrigerant that flows across the indoor coil while maintaining constant air circulation. Because VRF systems offer intelligent zoning, there are numerous ways to make the indoor environment more comfortable for the occupants while at the same time saving energy, says Hayden. “If I’m an engineer and I’m going to do a layout, I start thinking about that. The possibilities are very high to reduce energy use and yet increase comfort.”
Mitsubishi’s City Multi VRF system provides heating and cooling for Enermodal Engineering’s new 22,000 sq. ft. headquarters in Kitchener, Ont.
Location, location, location
Installing a ductless mini-split system requires the designer to shift the approach somewhat if their previous experience is with ducted central systems. Rather than having at least one outlet in every room, a VRF has one indoor unit per living space. Strategically locating the indoor units where people congregate saves energy and minimizes ductwork. The key is to locate the indoor unit in a relatively open area that allows easy convective flow through the building. “You can run them at a continuous low speed to aid the convective flow,” adds Hayden. While some commercial VRF systems are truly ductless, like residential systems, most use short ductwork to deliver the air. Sizing, while important, is not as critical with a ductless system because they modulate. “You could probably oversize a certain amount, but it’s very important that you locate them properly,” says Hayden. “Technicians should perform a proper load calculation to select the best system,” says Fujitsu General America’s Erin Mezle. “While variable speed systems are more forgiving to under-sizing or over-sizing errors, a properly sized system provides the greatest comfort and efficiency.” As far as controls go, there are a number of options. Typically, each indoor unit has its own localized remote control. However, units can also be tied together into a central control or tied into a building management system. One advantage of a central control is that it can monitor electrical usage in each individual unit, in an apartment building, for example. Owners/managers can also program limits into the room controllers, such as setting a temperature range or preventing a change of operational mode.
Short ducts keep Enermodal’s office comfortable year round.
The efficiency or residential mini-split systems is well established. While it is possible to buy a 14 SEER system, they can be as high as 24 or 25 SEER, notes Hayden. Inverter technology, efficient compressors and motors all play a role. However, there is no efficiency rating system for commercial VRF units. One of the problems is that it is very difficult to establish an accurate efficiency test and standard for equipment that modulates. VRF systems derive their efficiencies from the removal of duct losses (10-20 percent), and high part-load efficiencies as a result of two or three compressors in each outdoor unit, one of which is variable speed, “enabling wide capacity modulation” reported the <it>ASHRAE Journal<endit> (April 2007), Manufacturers, however, have put together considerable information on their equipment through research and case studies. The manufacturer should be able to provide an accurate estimate of energy use for a given building, which can be compared to previous energy use records and presented to the owner. Much information is also available online. Fujitsu, for example, offers an online Dealer Toolbox that includes a BTU load calculator that takes into account many variables, such as room size, windows, other appliances, etc.
Heating and cooling
While this article has primarily focused on the cooling aspect of VRF systems, one advantage is that they can simultaneously offer heating in some zones and cooling in others, says Maggie Yuen, B.Eng., of Mitsubishi Electric Sales Canada. “VRF systems can provide both heating and cooling at the same time, while recovering the heat energy harvested from cooling zones and applying that to heating zones, which can save up to 50 percent in energy consumption.” VRF systems use heat pump technology, which can provide considerably higher efficiencies than traditional chiller/boiler central systems, she added. The ability to install air conditioning easily and with minimal impact in historic buildings and other difficult installations has caught the eye of a number of contractors. However, wherever space is at a premium, comfort requirements are high and low energy use adds LEED points, a mini-split or VRF system can offer an excellent option.
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