A packaged rooftop unit, commonly referred to as RTU, is a type of HVAC system that contains all the system components required to provide conditioned air into a building. It is essentially a self-contained air-handling unit with heating and cooling components. RTUs are commonly used in low-rise commercial building applications and are especially popular with retail spaces and industrial warehouses. Packaged rooftop units are typically located on the roof, hence the name, and connect directly to a ductwork system that distributes conditioned air from the RTU to various rooms and spaces within a building. Ensuring that packaged rooftop units are well maintained with adequate heating, cooling and airflow capacity is very important to ensure the operation of a building and comfort of its occupants is maintained. In this article we discuss the basics of packaged rooftop units (RTUs) in HVAC systems along with good preventative maintenance practices, and common pitfalls to look out for during an inspection.
Packaged Rooftop Units (RTUs): The Basics
In its basic form a packaged rooftop unit (RTU) is a large box containing a motorized supply fan, direct expansion (DX) cooling/heating coil(s) and compressor(s), air filters and condenser fans. Sometimes packaged units are provided with gas or electric heating components, such as furnaces or electric coils, and economizer heat exchangers for optimum efficiency. Figure 1 below details the components of a basic packed rooftop unit (RTU) with cooling only.
Outside air, controlled by the open-air damper, is bought into the packaged RTU to provide fresh air. The outside air first mixes with the return air from the building. Mixing the air allows a portion of the conditioned return air to flow back into the building resulting in energy savings.
The mixed air then flows over the filters which clean the air of dust and debris. The air then flows through heating and/ or cooling coils to become “conditioned”. The conditioned air is then drawn into the supply fan before being distributed into the building via a ductwork system.
The return air, previously ran through the RTU and building, then returns into the packaged unit via the ductwork system. This mixes with the outside air to start the cycle all over again.
Some RTUs have electric or gas heating elements near the blowers to further adjust the humidity of the air. In packaged rooftop units, there is typically a clear and insulated divide in the unit between the evaporator coils that cool the return air and the condenser coils and compressor rejecting heat to the atmosphere.
Packaged RTUs can be single-zone controlled by a single thermostat or part of a more complex system with multiple-zones.
What to Look Out for During an Inspection?
During an inspection of packaged rooftop units (RTUs) the following conditions should be observed:
The condition of the RTU housing panels should be inspected for missing sections, screws and corrosion. Typically, the older the unit, the worse the condition the housing will be, as it exposed to weathering and other outdoor environmental conditions.
Data plates, usually located on the on the outside of the RTU, can provide manufacturing information, performance and a serial nomenclature which will provide the capacity, age and heating/cooling specifications. If the data plates are faded and worn, then it’s an indication that the unit is likely to be old unless it has word prematurely. RTUs typically have an estimated useful life of 15 to 20 years before replacement is required. This can however vary significantly based upon the operating environment and levels of proactive maintenance.
Most RTUs are located on the roof of a building, hence the name, however some do get installed at grade level or at a low-level roof adjacent to walls and openings. It is important that the air intake is not blocked or near odors, such as kitchen exhausts or sanitary risers, as exhausts from these sources will affect the quality of the conditioned air being supplied into a building. As a matter of note, owners maybe in violation of Operational Safety and Health Administration (OSHA) regulations if the RTU is close to a roof edge or roof opening and adequate fall protection/ safe working procedures is not implemented during preventative maintenance works.
A RTU should be adequately sized and programed with just enough airflow, measured in cubic-feet-per-minute (CFM), so that conditioned air can circulate all intended areas but not create a problem such as positive or negative building air pressure. It can be difficult to calculate the required CFM and therefore you should consultant with a qualified mechanical engineer if you are having issues with air flow.
Condensate Lines and Pans.
As the air is heated or cooling in the RTU, humidity is released. This humidity will turn into condensation; therefore, condensate lines and pans are installed to manage and funnel the condensate water outside of the RTU. The condensate lines are components that are most susceptible to water leaks. The leaks often in turn corroded metal components contained within the RTU. If the RTU is opened the condensate pan is heavily corroded, then it’s an indication that there are excessive humidity levels within the unit or that leaks have occurred. If the condensate pans are corroded then other metal components, such as the heat exchange or motors, may have corroded also.
If left, dirt and debris will accumulate and build up on the filters, which in turn has a negative impact of the efficiency of the RTU and indoor air quality. The filters are easy and cheap to clean or replace and therefore there is no excuse for having dirty filters. If you find dirty filters within an RTU, or any other HVAC system for that matter, then this is an indication that the systems has been neglected and not subject to adequate preventative maintenance. Also, it should be noted that dirty filters and coils put additional strain on the components of the RTU as fans and motors need to work harder to draw air through. This will result in more frequent break downs and a reduced service life.
Heating and Cooling Coils.
The heating and cooling coils should be inspected for any signs of corrosion or damage to the fins. A variety of things, including debris, impact and corrosion, can damage a coil. Because this is such an integral part of the overall component, a damaged coil instantly compromises the entire RTU unit. RTUs near a costal marine environment are particularly susceptible to corrosion due to salt spray from the sea. If the coils are not protected with a marine type coating then they may prematurely corrode, sometimes as soon as a year from install. It is important to ensure that RTUs in a marine environment has a coil coating product that has passed ASTM B117 test standards for salt spray. It also should have multiple marine and industrial ASTM test standard listings for resisting ultraviolet (UV) rays, acid rain, high concentrations of urban vehicle emission air pollutants and other outdoor contaminants.
Regular servicing of RTUs help prevent potential breakdowns, increase the unit’s efficiency, extend equipment lifespan and save money on utility costs. Servicing should be undertaken every 3 months or so, and include, but not limited to, the following; general cleaning of unit, replacing the filters, lubricating motor bearings, cleaning of coils, fans, motor etc., checking oil/refrigerant levels, and inspecting components such as the heat exchange, motors, electrical wiring, and connections. The maintenance record should be requested prior to any inspection.
If the packaged RTU uses ozone-depleting R-22 refrigerant, then this is a concern. We will not go into detail into the legislation on this page but basically R-22 will no longer be readily available after January 1, 2020 and is being phased out due to environmental concerns. Once the existing compressor within the RTU runs out of R-22 refrigerant then replacement of the entire RTU maybe required as conversions can be problematic. Sometimes RTUs can be converted to a sustainable refrigerant, like R-410A, but often this is not economically viable as coils, line sets etc. cannot be cross-contaminated with different refrigerant types and usually must be replaced when changing refrigerant types.
https://technologyportal.ashrae.org/journal/articledetail/564 – ASHREA Journal Article: Smart Maintenance Tips for Rooftop Units
https://www.epa.gov/ods-phaseout – US Environmental Protection Agency (EPA) – Phaseout of Ozone – Depleting Substances (ODs)
https://www.americanstandardair.com/owner-support/maintenance/packaged-system.html – American Standard Packaged System Preventative Maintenance Checklist
https://www.trane.com/residential/en/for-owners/maintenance-tips/packaged-systems/ – Trane Packaged System Preventative Maintenance Checklist