Small packaged rooftop units usually have the following components; a DX cooling section (DX stands for Direct EXpansion and refers to the process of evaporation, or expansion of refrigerant to absorb heat from the air being cooled), a natural gas-fired heating section, a constant-volume supply fan, motorized dampers that may control outside air and return air flow, and electronic controls to operate all components and interface with the building’s temperature control system. All of these components tend to be fairly standardized in nature and don’t have great capability to be customized or adapted to specific situations. This lack of ability to be customized can make these types of units tricky to select.
Let’s start with the heating and cooling loads for a hypothetical building which we might wish to match with an RTU. Many cooling loads will calculate a supply airflow in cubic feet per minute (or CFM) based on a particular cooling supply air temperature, typically 55 F. There is also a cooling load expressed in BTU/H or tons of cooling (one ton of cooling is 12,000 BTU/H) associated with the supply airflow and supply air temperature. An RTU usually has discrete cooling load and airflow values that it can provide. For instance, you may calculate a cooling airflow of 4,500 CFM and require 8.8 tons of cooling for your building. An RTU may be able to provide a nominal 7.5 tons or 10 tons of cooling for 4,500 CFM, not 8.8 tons of cooling. Each of these cooling capacities will result in a cooling supply air temperature which is different than the design temperature of 55 F for 4,500 CFM. The cooling airflow is based on the supply air temperature (among other things), so the calculated cooling airflow will need to be revised as the supply air temperature changes. As the supply air temperature changes, the airflow and cooling load will then change, so now we’re back to matching the RTU selection with the supply airflow and cooling load. The point here is that selection of packaged RTUs is an iterative process between the discrete airflow and cooling capacity values an RTU can provide and the more exact (although still approximate) values obtained from cooling loads. A similar analysis needs to be performed with the heating section for the RTU, although heating is typically over-sized compared to what the calculated heating loads are. Always check the unit discharge air temperature to make sure that it isn’t too high or too low.
Another potential issue with small packaged RTUs is that they may not be able to accommodate high flow rates of outside air. If you have a design that requires a high flow rate of outside air and it’s a cold day outside, the air temperature that hits the heat exchanger can be cold enough to make the flue gasses condense. This condensation is extremely acidic. It can corrode the heat exchanger and cause it to fail very quickly. Often, corrosion-resistant stainless steel heat exchangers are available, but RTU manufacturers may have a minimum recommended heat exchanger entering air temperature to prolong heat exchanger life.
It’s also important to understand the function of the “supply” fan in a packaged RTU. Despite the label of this fan as being for supply, it actually accommodates the supply and return airflow capacities. This is true even if a power exhaust fan is provided with the unit. It can become a troublesome mistake to neglect including the return ducting in the duct pressure drop calculations when selecting the supply fan motor size of a unit. Always include the return duct pressure drop with the supply duct pressure drop calculations. As with most other aspects of packaged RTUs, there is a limited amount of fan pressure available, and the available pressure of the fan will be reduced as the airflow increases.
Finally, RTUs just don’t have many options for systems that aren’t simple constant volume. Packaged RTUs can be fitted with variable speed controllers on the supply fans to accommodate VAV systems, but the DX cooling section may not be able to modulate its cooling capacity to accommodate varying supply airflow rates. Packaged RTUs also typically have options for power exhaust (which is recommended for superior building pressurization control), but power exhaust options may not have variable speed controller options for the fan, even if there is a variable speed option for the supply fan. If any of these options are included with an RTU, it may also need to be fitted with electronic controls components in addition to the factory-provided controls. All of these options add cost to the unit, and may make it a bit more difficult to integrate the unit with the building temperature control system, but it can be done. As always, consult a qualified engineer and manufacturer’s representative when selecting any packaged RTU. This can greatly lessen the probability of a problem after the unit is installed.