Ask the Engineer: What Are the Concerns When Installing Ductless Air-Conditioners

THE ENGINEERS ANSWER: Ductless air-conditioners consist of an air-cooled condenser unit, typically installed outside the building, and an inside evaporator or air handler unit that cools the interior space. Instead of using a packaged system of ducted air, the ductless air-conditioning system uses refrigerant-filled tubing that connects the exterior and interior units.

Because the condenser and evaporator are separate units, ductless air-conditioners are commonly called split systems. In a traditional split air-conditioning system, one condenser unit provides cooling for just one interior unit. In a multi-split system, one external unit powers several interior units that can be individually controlled for a particular room or space. Some systems can be configured to provide both cooling and heating.

Split air-conditioning systems have become increasingly popular in residential buildings because of their low-profile design and ease of installation. However, the placement of outdoor units and the refrigerant tubing that connects the exterior and interior units can pose potential problems for boards because the tubing can intrude on the building’s common spaces.

Roof Installation

Roofs are a common location for installing the exterior condenser units because the units don’t take up much space and it (usually) places them out of the way and out of sight. Lighter units are typically placed on pads so they don’t damage the roofing membrane, while larger units may require steel dunnage. Another option when roof space is limited is to mount the unit on a bulkhead wall.

The main concern for boards is the path of the refrigerant tubing as it runs from the rooftop unit to the air handler/evaporator unit in the apartment. For aesthetic reasons, most boards will not allow the tubing to run along the outside of street-facing façades. Similarly, buildings located in designated New York City historic districts are prohibited by the Landmarks Preservation Commission from running tubing that is visible from the street. In addition, boards will commonly declare stairwells, hallways, lobbies, and other common spaces off limits for the tubing.

Many boards will, however, give residents the option of running the tubing on a side or rear façade, but they may charge the resident for use of roof space for placement of the condenser. Another option is running the tubing through a chase or conduit if one is available.

For apartments directly below the roof, it may be possible to run the refrigerant tubing through the roof itself. In such cases, the penetrations will have to be waterproofed and firestopped. But some boards may object to allowing penetrations through the roof out of fear that leakage problems could result.

Another factor to consider is that for optimal performance, the outdoor unit and indoor unit must be positioned within a certain distance of each other, depending on the type of split system. For apartments far below the roof, that distance needs to be taken into consideration.

Given your apartment’s location two stories below the roof, running the tubing along a rear or side façade may be the best option for you. But again, it will require your board’s approval and may also require paying a rental fee to your condominium for use of the roof. Keep in mind that split air-conditioning systems run on electrical power, so you would also need to run wiring from your apartment to the outside unit.

Inside Units

There are a number of options for your indoor evaporator/air-handler units. The inside units, which are typically wall- or ceiling-mounted, come in various designs and styles suitable for the particular room being cooled. Units can be recessed to reduce their profile, and some residents conceal the units with customized millwork. Aside from mounting, the units require a plumbing connection for condensate drainage. Some units can be piped into an existing indirect drain, such as one used by a washing machine, for example. Other setups require a dedicated funnel drain fitted with a device known as a trap primer to redirect the condensate.

Maintenance

Maintenance of split air-conditioners is similar to most cooling systems. Typically, maintenance involves cleaning the filter and condenser coils, reviewing the system charge, checking fan operation, repairing or replacing tubing and insulation, and checking electrical connections.

Before you install a split air-conditioning system, your board will require a plan review to make sure the installation does not adversely affect the building as a whole, and it will decide if it will require you to pay a monthly rental charge for placement of the outdoor condensers. The installation will also require that a licensed engineer file plans with the New York City Department of Buildings.

Split air-conditioning systems are a convenient option for cooling your apartment. Just be sure to work with your board to address any installation concerns and avoid potential problems.

Stephen A. Varone, AIA, & Peter E. Varsalona, PE, are principals in RAND Engineering & Architecture.

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