Rooftop Unit Install and Changeout Considerations
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Rooftop units, or RTUs, are commercial HVAC units and may sometimes be called “package units” when the application is on the ground. When installing or changing out an RTU, you need to think about the unit’s size (including weight), the area around the unit, adapters or curbs, orientation, climate conditions, duct configuration, and if the unit requires reheat.
In many cases, you’ll need a crane to get RTUs on top of buildings. To operate a crane, you’ll need to know the basics of rigging and signaling. Rigging refers to the connection of a load to a crane, and signaling refers to the process of controlling a crane with visual gestures or audio cues. When rigging, you want to try to minimize damage to the unit, especially by doing close visual inspections throughout the process. Certification is often required for rigging and signaling.
When thinking about crane picks, location is perhaps the top concern; you don’t want cranes to stand over grease traps, drainage, soft ground, uneven terrain, or other potentially catastrophic areas. Some areas may require permits or further consideration, including near power lines or trees, too close to airports or runways (you may need a permit), or inside a structure. You’ll also want to consider the weight of your load (plus the rigging) and the crane’s distance from the load. Weather (especially high winds) and the need for a spreader bar are other things to keep in mind.
Commercial units may be subject to ventilation standards, so we need to size units for the appropriate fresh air intake to hit the required air changes per hour. In our market, a common assumption is that there should be one building occupant per 200 square feet, and 17 CFM of outdoor air is required per occupant. In some cases, you may have to consider the effects of economizers, exhaust fans, vestibules, and makeup air units on the fresh air intake, air mixing, and air pressure. There must also be sufficient distance between fresh air intakes and exhaust structures. Cooling towers also introduce humidity and should be placed away from fresh air intakes to prevent the fresh air from picking up extra moisture. Intake openings should also not be positioned near lot lines or other buildings on the same lot.
The electrical side of RTU installation also requires a lot of thought. For example, on changeouts, you’ll have to determine if existing wires and breakers meet the correct sizing requirements per the MCA and MOCP. If you change the unit’s orientation during the changeout, you also have to make sure your wires can reach their new location. You may also have to weatherproof electrical penetrations and extensions to prevent moisture from posing a hazard to the electrical equipment.
When thinking about condensate drainage, you must consider the unit’s tonnage, roof pitch, putting in an appropriately sized and placed trap and air gap, and the drain pipe material (especially. The diameter should be at least 3/4″ and no smaller than the drain pan outlet diameter, and the slope should be at least 1/8″ of fall for every foot of horizontal run (1%). Drains also need to discharge to different types of areas depending on what they may contain; for example, drains that may contain oils may need to drain into a grassy area instead of a gutter.
Tie-down specifications also matter, especially in windy or hurricane-prone climates. These specs may vary wildly based on location, but the ultimate goal is to keep the RTUs from coming off the buildings in extreme weather.
Curbs allow you to affix an RTU to a building. In changeout scenarios, you may either need to introduce a new flat curb or install an adapter to the existing curb. Flat curbs typically connect the RTU to the building envelope, not supported by the roofing material. Curbs must also be leveled properly so that the RTUs can drain appropriately, so you may have to shim the curbs. If you decide to use an adapter with an existing curb, you’ll have to keep in mind that the specs will usually be customized, though the manufacturer can often help you out. The curb, however, is usually NOT custom-made, and that’s what we need to pay attention to. You also can’t install adapters to other adapters or shim the adapters.
Minimum steel requirements are usually the points where the peak load at first concrete cracking matches the ultimate load after steel yielding. In other words, the steel must support the load but not overload the concrete. Steel reinforcement must be done thoughtfully with careful attention to curbs, adapters, etc.
The supply and return dimensions refer to the outlet and inlet sizes of an RTU. You must ensure that curb submittals take these dimensions and duct sizing into account.