Humidity Question for make up air
Humidity Question for make up air
(OP)
What should the discharge air be on a make up air handler unit in the summer (cooling) mode of operation? The make up air unit has 2 stages of DX cooling and a hot water reheat coil with a three way modulating valve. I want the coldest discharge temp so not to load the existing mech cooling units. This is a general question and a made up problem.
Outside conditions are: 90* F D.B. and 100% humidity
Inside conditions are: 70* F D.B. and 45% humidity
Outside conditions are: 90* F D.B. and 100% humidity
Inside conditions are: 70* F D.B. and 45% humidity





RE: Humidity Question for make up air
Given your scenario, relative humidity will fluctuate wildly. To maintain the Inside condition of 45% humidity at 70F, you must cool to saturation at approximately 47.5F. That will give you the amount of water vapor in the air to result in 45% RH at 70F. This is true for any condition of higher moisture content in the outside air.
The problem is trying to get 2-stage DX cooling coils to always cool to saturation, regardless of outside conditions. You can size for the peak condition - 90F and 100% RH, or the condition that will completely saturate with only the 1st stage. Every other condition will result in over-humidifying or de-humidifying. This is because the DX coils will only have 3 settings of cooling - OFF, 1 stage, or 2 stages - period. It matters little at that point if the hot water reheat is able to maintain precise temperature for an infinite number of conditions.
A throttling chilled water coil can reduce or increase flow - at will - to perfectly saturate and dehumidify the air under any condition, allowing the hot water reheat to return the dehumidified air to the precise temperature desired.
RE: Humidity Question for make up air
Then use a preccoling coil to say mid point
Then a final cooling coil to say 44 deg F DB LAT
Use a DDC based LAT controller for each stage
Intertwined DX coils
Whats the air volume?
RE: Humidity Question for make up air