A rooftop unit (RTU) is a self-contained air conditioning system. They are typically mounted on roofs but can be installed in any outdoor location. These units provide conditioned air for a facility and distribute it through an established ductwork system. When installed correctly, RTU’s are designed to be tough and impermeable: built to prevent snow, debris, rain, and even small animals from getting inside while still permitting air to go through.
Still, these complex systems can encounter problems, especially with improper maintenance and over time. This guide can help you understand your system and common problems to look out for.
RTU Components
Dampers: are set at specific positions to balance the air flow into the unit. These can be manual or modulated depending on the manufacturer.
Filters: From high grade HEPA to inexpensive box designs, filters clean the air before it is distributed to the occupied space. The size of the unit, frequency of use, and indoor/outdoor conditions will determine how frequently these filters should be changed.
Coils: A cooling or heating coil is used to change the temperature of the air. As the air enters the unit, it travels through these coils and the temperature changes based on the internal system needs.
Heat Exchangers: Help to improve the efficiency of the unit. This system allows for preheating or precooling the incoming outside air by running through two opposing coils. The two air streams never mix, however, the temperature of air changes as both streams of air travel through the unit. This offers a significant improvement in overall efficiency as the RTU works less to achieve its temperature goal.
Fan: Pumps the air through the ducting and into the facility. The most common are forward curved centrifugal fans. When a higher capacity of air flow is needed, an airfoil design is used.
Heat Pump: An improved efficiency RTU is called a Heat Pump. In differs from a traditional RTU in that it uses the outside air or ground temperature to collect or reject the buildings’ heat. These units typically cost more and have larger internals coils which allow them to be more efficient than standard units.
Common RTU Problems
1. Dirty Coils
When the coil of the RTU unit is very dirty, the particles pack in between the fins which reduces air flow, and the unit will have a tough time pulling heat out of the atmosphere. Dirty coils can lead to heat-rejection problems. In this case, the refrigerant will be unable to make the system cool as efficiently or effectively as it otherwise would.
2. Thermostat Problem
The thermostat is placed close to the evaporator coils inside your HVAC unit and adjusts the timing and amount of cold or hot air the RTU unit should make. If the thermostat has a malfunction, then the building can suffer from irregular temperature variations, which impacts occupant comfort.
3. Inappropriate Air Balance
The cooled or heated air that comes out of your HVAC unit vents move through a duct system before reaching each room. Dampers regulate the air by lowering airflow in one line and allowing extra conditioned air to run to other lines. Each room needs a defined amount of air flow to condition it. An inappropriate air balance means some lines are getting more or less than is needed.
4. Leaking Refrigerant
The liquid component that makes air conditioning possible is the refrigerant. Refrigerant, which is contained inside the coils of an RTU unit system, dehumidifies and cools the indoor air. If the RTU unit does not have sufficient refrigerant, then the air produced will not be very cold.
5. Uncommon Furnace Noises
A few furnace noises are part of the routine operation, but other noises may be an indication or warning of a technical problem. A squealing or whining motor indicates the failure of bearings in a blower motor or inducer motor. Other noises can be caused by dirty burners or airflow issues. It is always better not to ignore these noises as they could lead to early RTU failures.
6. Tripped Breakers or Blown Fuses
Overworking of blowers can lead to tripped breakers. If anything is blocking airflow in the RTU, then the blower will work harder to compensate. A common cause is a dirty air filter; other causes include blocked-off or closed air registers, duct leaks, or dirty coils. Regardless of the cause, once the blower has to work harder, it raises the energy consumption, which sometimes leads to tripping the breaker.
7. Continuous Running of the Blower
There are a few reasons why a blower might run continuously. One can be the switch of the fan on the thermostat. If the thermostat is arranged to the “fan on” position, the motor runs continuously. Second, the fan relay, which is available in the furnace, can be stuck. There are various limits and safeties on the furnace, but if the safety or limit doesn’t reset, then the blower will never shut off.
8. Mechanical Wearing and Tear
The RTU’s unit heating system relies on numerous mechanical components. As these components wear out, it can impact performance. Worn belts and bearings can result in poor heating, airflow issues, or overheating. Regular inspections and good preventive maintenance will avoid this problem.
9. Water Leakage
Every RTU unit produces condensate, which is why proper drainage is essential. If drainage lines are cracked or clogged, leaks will occur. Regular cleaning, including using bleach to limit mold, can solve this problem. Other likely sources of a leak could be the heat exchanger, evaporator drain tank, or collector box.
10. Ignition or Pilot Problems
Ignition or pilot problems can happen due to numerous causes. A flame sensor, burner, or dirty pilot can lead to an outage of the pilot, delayed ignition, short cycling, or the burners lockout. Other causes can be a gas supply problem or deterioration of an ignition constituent that needs to be changed, such as the hot surface thermocouple or ignite.
Adopting a precautionary maintenance program is crucial to ensuring your rooftop unit system operates efficiently and properly and to extending your rooftop unit’s life. Without system maintenance, RTU units lose their energy efficiency very fast. We recommend obtaining a regular maintenance plan from an experienced HVAC service company (like CM3) to keep your RTU operating at peak performance.