Question

What can I do to lower the humidity level in my home?

Asked by Lida Wilson
New Orleans, LA

I have a raised cottage in New Orleans with a slab addition of 2 rooms to the back of the house. After Hurricane Katrina, I was able to have the house wrapped, put on new siding, hurricane windows and shutters and spray foamed the raised portion under the house. The 4 ton A/C unit was replaced last year with I believe a 2.5 unit with a dehumidifier (the square footage is just over 1,300). There is insullation in the attic (not foamed). I still see high humidity readings in the house (over 50). Is there anything I can do to regulate this problem?

Answer

Stillman Jordan

Answered by Stillman Jordan

Stratford, CT

Encon

June 18, 2013

This humidification problem is a common one experienced by many building owners around the country. To solve it you have to understand a little bit more about how de-humidification works.

How a dehumidifer works

The most common way we de-humidify air is by cooling the room air down to its dew point temperature where the moisture condenses out of the air before the air is returned back to the room.

A dehumidifier typically cools the air to remove the moisture and then heats the air back up again before it is returned back to the space so you don’t super cool the room. 

An essential component is usually missing

You need two things in order for an HVAC system to correctly dehumidify a space:

  1. An HVAC system that can cool the air to the dew point temperature without super cooling the building. This often requires some form of reheat in the equipment.
  2. A humidistat and controller that senses the humidity and then activates the HVAC to dehumidify the building

Unfortunately it is very common for buildings to be missing one of these essential components.

If an HVAC system is oversized or if it does not have some form of reheat then the system will shut down because it is over cooling the space. When the system shuts down it is no longer dehumidifying the space. This can often cause the system to short cycle or “hunt” for the correct climate setting.

The solution to this problem is to have an HVAC system that either has several stages of cooling, multiple compressors, or modulating compressors that will allow your system to adjust the capacity to the load experienced in the space.

On commercial projects the reheat is often done with a technology called hot gas bypass reheat. Residential grade equipment usually does not have this many options on it so your only option will probably be a stand alone dehumidifier.

Got a humidistat?

A lot of buildings only have a thermostat that senses the room temperature and will then only activate the HVAC system whenever the temperature is out of the acceptable range. As Lida describes it is possible for many days that the temperature in the room is okay but the humidity higher than desired.

In this case, unless you have a humidistat reading the humidity in the space that controls the dehumidifier then, you will not be able to effectively activate the required equipment.

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