One of the problems we come across during our inspections is Air Conditioning Dehumidification Problems: This is where the air conditioning system cools but does not dehumidify the room. The most common cause of inadequate dehumidification by an air conditioning system is the installation of a cooling unit, which has too much capacity, or is “over-sized” for the space it is being used to cool. What happens then is simple:

If an air conditioning compressor unit is oversized (too many BTUH of cooling capacity) what happens is it cools the room so quickly that the system does not move enough total volume of air across the cooling coil to remove much moisture before the room temperature has dropped to the air conditioning cut-off point.

In other words, an air conditioner needs to run longer, and move more total volume of air through itself to drop room humidity than it does to just cool the air. So “bigger” cooling capacity or higher BTU capacity for an air conditioning system is not necessarily better, it can actually be a problem.

The main symptom is if your cooling system is otherwise working normally, and it cools the room temperature quickly but the room humidity stays high, an over-sized unit is likely to be the problem.

System Checks

First, make sure that the equipment is operating properly and is cooling – check that temperatures are dropping normally when the system is running.

Check that there is not an abnormal moisture source such as leaks into the building or its wall or ceiling cavities from any other source. Check to see if the condensate drain from the air conditioner is producing water.

If you see lots of a/c condensate dripping out of the system condensate drain but room humidity is still high, there may be an abnormal humidity source.

Adjust the a/c unit controls to increase dehumidification and discuss with your HVAC service engineer whether there are technical changes that might be made to fan speed or other controls that might improve a/c dehumidification.

Try increasing the cooling load on the installed air conditioner by making it cool a larger area – for example, leave the doors open to other rooms. This may alleviate the problem but is not always a suitable solution.

Unfortunately, in the case of an oversized unit the fix is to replace the unit with one that is the right size for the job. Alternatively, a dehumidifier can be added to try to balance the humidity but this is an additional energy usage overhead.

Remember that condensation can create problems with dampness and may lead to black mould, which is a health hazard, so we always advise that this situation should be rectified especially in an enclosed area where people are working.


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