Humidity, the 8th wonder of the world, is a tough nut to crack. Unlike sensible heat transfer, it’s very difficult to measure its movement into buildings through cracks, doorways, and other openings.
But, there are ways to combat these higher than expected humidity levels.
Here are 11 ways to decrease the relative humidity in your space. The answer could be one of these 11, or in many cases it’s a combination of a few.
- Check and Reduce Airflow if Necessary
This is a great place to start because if your airflow is not in a good range, you will avoid future issues by correcting it now. We discussed this in our previous blog, but basically, the lower the discharge air temperature off the coil, the lower the space humidity will be. And when you reduce the airflow, the discharge air temperature will decrease.
So, step #1 would be to measure airflow, and if it is HIGHER than 400 cfm/ton, reduce it to the nominal rating. If that doesn’t help, lower it some more, but don’t go much lower than ~320 cfm/ton.
CAUTION #1: If you lower airflow below ~320 cfm/ton, you risk freezing the coil in standard A/C operation. Make sure you check filters consistently, because a plugged filter will freeze the coil faster at lower airflow speeds.
CAUTION #2: When you lower the discharge air temperature, you will lower the temperature of the ductwork and supply registers, which means they might sweat.
CAUTION #3: Lowering the airflow reduces your unit’s total capacity. If you can’t afford to be low on capacity during the heat of the summer, this might not be the right move.
- Check economizer minimum position
Most rooftops and split systems are now being installed with an economizer for free cooling, but if it’s not set up properly it can cause humidity issues. If the thermostat fan mode is set to “on” instead of “auto”, and if your economizer minimum position is open, you might be bringing in HUMID air constantly. And if that air doesn’t have a very high (dry bulb) temperature, then you might not get a call for cooling to remove the excess humidity.
If local code requires fresh air for people, consider installing a CO2 sensor, and setting the economizer to be 100% CLOSED unless the CO2 calls for fresh air.
- Add a VFD to stage airflow
On units that have multiple cooling stages, this will help. When first stage of cooling is running and the supply fan is at full speed, the evaporator coil doesn’t remove as much moisture. If you install a VFD (Variable Frequency Drive) on the supply fan motor, and then operate the fan at 50% air flow on the first stage cooling call, you will lower the coil temperature and remove more moisture. You also will lower the capacity of the unit (at the half speed setting) and thus will run longer to satisfy, which will remove even more moisture.
CAUTION: Check the evaporator coil circuiting. If it is row split or intertwined split, you are good to go. But if you try to do this with a face split coil, you will probably freeze the coil at the lower airflow.
- Auto-Fan control is your friend
We discussed this briefly in #2, but its worth further discussion. If the thermostat fan setting is set to “on”, then the unit will be bringing in outside air constantly, which could be humid air and might be causing the issue you are having.
The other problem with “on” fan, is after the cooling call goes away, all the moisture on the evaporator coil will evaporate back into the air stream and into the space.
Setting the thermostat or controller to “auto fan” control avoids these two issues.
- Different setpoints
If you have multiple units serving the same space, you can set one unit at a 1 or 2 deg higher setpoint. For example: unit #1 set temp at 74, unit #2 set temp at 75. When you do this, the first unit runs full cooling before the other one comes on. If you don’t do this, each unit will cycle on/off on first stage during mild weather, and neither will run at full cooling. Which means the evaporator coils never get cold enough to maximize moisture removal. This is the same concept as adding a VFD (#3) or lowering your airflow (#1).
- Raise the thermostat set temperature
It’s probably worth your time to review our other blog, How to avoid a humidity problem. When you lower the space temperature setpoint, you will actually RAISE the space relative humidity. Yes, it’s counterintuitive, so in the best interest of time, check out our other blog.
- Check building pressure
Are there exhaust fans removing a lot of air? If so, is there a makeup air unit making up that air and cooling it? (Like a DOAS unit). If the makeup air is coming in through louvers or through a standard rooftop, this could be the cause of the issues.
If there IS a make up air unit, make sure its running properly. If it’s running, but the building is still under a big negative, there is something still missing from the equation.
- Check the ductwork
Is all of the supply and return ductwork serving the same area? If the return ductwork is pulling air from another space (that is humid for some reason), that might be the issue.
- Vapor barrier
Is there one?
- Spot removal
Does the building have a piece of equipment generating steam or humidity in a concentrated spot? You might look into a hood and exhaust fan to remove it. Beware though, this might put you in a negative pressure situation (#7).
- Dehumidification units
If every option has been exhausted, this is where you might end up. The only way to TRULY control humidity is by using a unit with a dehumidification circuit. If you are prepared, this feature can be built into packaged rooftop units during the manufacturing process. If you don’t have that luxury, then you might need to look at adding electric, steam, gas, or hot water reheat of some kind. This allows the unit to run in full cooling mode on a call for dehumidification, but then reheat the air so that you don’t OVERCOOL the space.