DOAS Dehumidification: DX Coil Bypass vs. Hot Gas Reheat
Ventilation air is a significant source of humidity within buildings. The role of DOAS equipment in delivering effective dehumidification is paramount in ensuring building comfort and indoor air quality (IAQ). So, let’s look at a couple options that we typically see specified throughout Ontario.
Before digging into the details, let’s set the stage with conditions.
Outside Air (OAT): 88oF db, 75oF wb
Space Setpoint (SP): 75oF db, 50% RH
DX Coil Bypass – This method of dehumidification has been a staple for many years due to it’s simplicity and relatively low first cost. The premise is that a percentage of total airflow is cooled below dewpoint, while the remaining airflow bypasses the coil. This is usually accomplished with a simple dampered opening above the DX coil. Dampers can either be manual or modulating depending on how fancy you want to get. Unfortunately, at the end of the day, the mixed air condition downstream still obeys the laws of thermodynamics. With two mixing airstreams, you can draw a line between the two conditions on the psychrometric chart and the mixed air conditions will always fall on that line. Referencing the psychrometric chart below, we draw a line between our OA and typical comfort cooling DX coil leaving air temperature (LAT), 55oF saturated . What you’ll notice is that at 75 oF your relative humidity is approximately 70%, well above recommended space RH condition of 40-60% for optimal indoor air quality.
So, what if we lower the DX coil LAT? Comfort refrigeration systems can only deliver LAT from coils around 40oF at best. Even at 40oF, you’re in danger of freezing coils because you’re running a saturated suction temperature (SST) very close to 32F. Nobody is doing this for comfort cooling as it adds unnecessary complexity. But, even if you had 40/40 LAT conditions from the DX coil, you’re not getting to 75/50%. Draw the line, check it out!
DX Coil bypass is not an effective solution to deliver proper space dehumidification. Let’s look at a solution that delivers real results!
Hot Gas Reheat (HGRH) – Let’s not get this confused with hot gas bypass! Hot gas bypass is an OG means of providing capacity control, which has been displaced in favour or inverter and digital scroll compressors. Variable speed compressors are no longer a special feature but have become a standard used by all commercial AC manufacturers. HGRH is the addition of condenser coil downstream of the DX cooling coil within the primary airstream. (see Image)
Both the DX and reheat coils condition the total airflow delivered by the air handling unit (i.e. no bypass). The reheat coil uses the heat absorbed in the refrigeration circuit from the DX coil to reheat the airstream. This is important because ASHRAE 90.1 does not allow a secondary energy source for reheat applications. In days long ago, we would use the gas heat exchanger downstream of the DX coil for reheat. That is a waste of energy and is now a “No-No”! Referencing the psychrometric chart below, you can see the thermodynamic process. The airflow is cooled from our OAT to typical DX coil LAT of 55oF, saturated. Following the DX coil, the airflow passes through the reheat coil where sensible heat drives the drybulb temperature up. In this example, we’re hitting our desired SP of 75oF, 50% RH.
HGRH used to be an expensive option when integrated into air handling units. Today, most major DOAS manufacturers offer HGRH for an extremely reasonable cost. Given the value it delivers, the cost is insignificant. When dehumidification is important, use HGRH to deliver true performance.