Can you use a standard rooftop for a makeup air application?
We get asked this question often. And for good reason, DOAS equipment is expensive.
If a building owner needs makeup air, in most cases a DOAS will be required. In a SMALL number of cases, however, it may be possible to use a unitary rooftop unit (or split system) for 100% makeup air. But beware, there are a LOT of drawbacks.
Let’s start by looking at what happens when we run a rooftop unit as a makeup air unit on the hottest days of the year.
Summer Design Day Analysis
First, check to see if the manufacturer’s selection program will let you run performance for the hottest days of the year. In York’s case, their program maxes out at 90db/77wb. So right off the bat, we can’t run performance for anything warmer than 90 degrees dry bulb. The good news is that York allows us to run a selection for 77 degF wet bulb though, which is very close to the max wet bulb in our territory (Kansas, Missouri, Nebraska, Iowa).
If we select a 25 ton unit at say 7,000 cfm, our discharge temperature is 68.8db/64.2wb. If we could theoretically run performance at 98/78 entering air, the leaving air temperature would end up in the low 70s.
For most applications, this is where you walk away. Typically a discharge of approximately 55db/55wb is required to maintain a comfortable space temperature and humidity level.
However, if you only need to reduce the entering air temperature a little bit, you can move on to the Low Load Analysis.
Low load Analysis
One of the biggest differences between a DOAS unit and a basic rooftop is the evaporator coil in a DOAS unit is much bigger: 6-8 rows vs 3-4 rows. This allows a DOAS unit to operate in low load conditions. I’ll explain:
When the ambient starts to fall, let’s say to 74 degrees and 50% rh, the amount of energy in the air starts to decrease. This energy is what boils the refrigerant in the evaporator, and if there isn’t enough energy in the air, then the refrigerant remains a liquid and then you will have frozen coils and slugged compressors.
Just as we did on the design day analysis, let’s look at our selection tool and see what the MINIMUM entering air temperature (EAT) we can handle. In York’s tool, that number is 65db/57wb.
BUT, keep in mind that this minimum EAT assumes we have NOT lowered airflow like we may have done earlier to achieve a colder discharge temperature in the summertime.
When we try to run a “low load selection”, the lowest entering air temperature that York allows us to run in this case is 74db/62wb (~51% rh). That means if the ambient conditions are any less humid or colder, our unit will begin to experience a frozen coil.
So, to be able to use a standard rooftop as a make up air unit in low load conditions, you must cut out cooling operation somewhere in the high 70s or low 80s.
If this is acceptable, you can move on to the Winter Analysis.
Most unitary rooftop units will not have enough heat to warm up 0 degree air to neutral temperature.
For example, the 25 ton unit we have been studying in this example has a max heating capacity of 320,000 btuh OUTPUT. So at 7,000 cfm, that would provide a 42 degF temperature rise. Clearly not acceptable in most applications.
If you can use the SMALLEST tonnage in cabinet footprint (for example using a 15 ton unit in the 15-25 ton cabinet), you will often find better temperature rises. In this case, the 15 ton unit has the same max heat capacity, but it would be running at a lower airflow (maybe 5,000 cfm), so the temperature rise would be higher (59 in this case). In most cases, you will be short of heating capacity, though, and you will need supplemental heat or will need to go for the DOAS option.
One other item to note: Most manufacturers have a minimum entering air of 25-40 degF. If you plan on bringing in 0 degree air, make sure to include a stainless steel heat exchanger due to condensation. Also note that since these heat exchangers are not designed to bring in air this cold, the heat exchange life could be shortened.
If this is acceptable, you can move on the Controls Analysis
Standard rooftop units are not set up to be controlled via discharge temperature. And even if you order the rooftop unit as a “VAV” unit to get this option, it wouldn’t work very well. The best way to control your make-shift makeup air unit, is probably to use ambient stats to bring on first cooling stage at 80-ish, and then stage 2 around 85 or 90 ish (assuming a 2 stage cooling unit). This means that the discharge temperature will not be constant, however, it would allow you to cool down the makeup air that your building needs.
If this is acceptable, you might be able to use a rooftop unit as a makeup air unit. But just in case you missed something, here is a review of the drawbacks:
- High discharge temperatures in the heat of the summer (high 60s and low 70s)
- Cooling not available on mild days (temperatures below ~80 deg ambient)
- Not sufficient heating capacity for neutral temperature delivery in the winter
- Limited control options which provides inconsistent discharge temperatures
If you do decide to go for it, make sure to add some safeties to keep your system safe:
- Coil Freezestat
- Low pressure switch
- Ambient stat(s) to control compressors on temperature rise
- Hot gas bypass
- Head pressure controllers to maintain head pressure when hot gas bypass is bypassing max capacity
Have you used a standard rooftop or split system as a makeup air unit before?