Humidity has a large impact on indoor comfort and construction integrity. A home that’s more humid in the winter months will feel warmer and cozier, but that added humidity is not what homeowners want during the warmer seasons. 

It’s important to advise customers on why controlling humidity is important so that the proper equipment is utilized correctly to prevent both minor and major issues. A too-humid home can promote mold growth and result in condensation stains on ceilings and walls, and can even affect a building’s frame and structural integrity. Excessive humidity can also damage artwork, fabrics, paint and wallpaper. On the other hand, a too-dry house will cause static electricity, aggravate sinus problems, lead to irritated, scratchy throats and can affect things like wood furniture or musical instruments. 

What is a good humidity level? 

A good humidity level to prevent issues is 30 to 50 percent. At this humidity level, there’s not enough moisture in the air to make occupants uncomfortable, but it’s also not so dry as to cause dry skin, itchiness and a scratchy throat. A humidity level of 70 percent is considered too high for comfort.

Adding humidity to a too-dry house is very easy. In a smaller space like an apartment, just leaving a big pot of water simmering on a stove can be enough to up humidity levels as well as providing a bit of radiant heat. How to reduce humidity in a house, on the other hand, is a lot more of a challenge. We’re going to talk a bit about how to handle and manage humidity control in home or business HVAC design so you can best advise your customers. 

Best Practices to Optimize Humidity Control

HVAC humidity control problem #1: Improperly sized AC system

This refers to the capacity of the system, not the size of the unit itself. It would seem that excess cooling capacity in an HVAC system would be a good thing by taking some of the stress off the unit, but that’s not really the case. 

An oversized HVAC system will cycle on and off more frequently, and won’t serve well as a humidity control system  because it never runs long enough to do an effective job of reducing humidity content in the air. There’s a huge percentage of homes and businesses that are set up with an over-capacity HVAC system, and designers have very specific formulas that take into account square footage, air flow and other factors for a right-size system. 

In some cases, a homeowner may just need to install a smaller-capacity HVAC system to address a whole house humidity control problem. 

HVAC humidity control problem #2: AC system is limited to only one speed

This is an issue that’s pretty common with older systems that can run at two settings only: either On, or Off. The problems are similar to what we mentioned above with an oversized system; the unit runs full-tilt until it reaches the target temperature set at the thermostat. It then shuts off until the temperature comes back up again, cycling the HVAC back on. 

Like with a too-big system, the unit can’t ever run long enough and consistently enough to do a good job of regulating humidity. The next generation of HVAC systems are designed as Variable Refrigerant Flow (VRF) units, which means more consistent comfort as the compressor runs only at the capacity needed. A VRF system also has several smaller air handlers rather than one big one that cools the whole house, meaning more efficient zone cooling and lower energy bills. 

HVAC humidity control problem #3: Negative air pressure 

Here we come to another problem that could have been prevented with the right design and installation. Negative air pressure means that the system is pulling too much air out of a space and causing a partial vacuum. With a situation like this, it feels like a gust of wind is blowing through every time someone opens an exterior door. 

In a negative air pressure situation, the system is struggling to pull in more outside air to balance things out. On a hot, muggy day, that extra humidity outside is going to be drawn inside and then you have a problem with humidity and temperature control. A switch of components might be necessary to remedy a negative air pressure problem. 

HVAC humidity control problem #4: Wrong thermostat setting

Often a homeowner will run the system with the thermostat just set to Fan, under the assumption that they can keep air moving and save a little on the utilities by not having the AC engaged. Sometimes this can be enough to keep a home comfortable when it’s not terribly hot and humid outside. Other times, though, this can just make a humidity problem worse; the AC system is designed to pull humidity from the air, and by just running the fan all the time you’re recirculating the same sticky air that the AC worked to dehumidify, before it has had a chance to be vented to the outside. 

HVAC humidity control problem #5: An older system that can’t handle the load

We all know that HVAC components have a limited service life. Parts wear and degrade, and a poorly-maintained system will age a lot faster. Humidity control is part of the system’s job along with regulating temperature, and that will be compromised as well. This will be a case-by-case situation and will come down to the property owner; does it make more sense to replace components as needed, or would it be best to just upgrade the whole system for something more modern? 

A few strategies for managing humidity control effectively

Not surprisingly, some of these best-practices tie right back into how to keep your HVAC properly maintained and the importance of that maintenance to overall HVAC system health. 

 

  • Clean the coils: So many problems with HVAC performance lead straight back to dirty coils or a dirty filter. Outdoor coils that “sweat” will quickly start collecting dust and dirt, compromising their ability to effectively work as a heat exchange. Consequently, the whole system is going to run at a lower efficiency and will have a harder time regulating indoor humidity.
  • Get a preventive maintenance program: As with just about any system, a well-maintained HVAC is a healthy HVAC. Along with coil cleaning and filters (encourage the homeowner to mark a date on the calendar for filter changes, every 30 to 90 days), the whole system should get an inspection and tuneup yearly. At tuneup time, a tech can also spot any potential problems before they develop. 
  • Adjusting airflow: A good HVAC tech should know how to make the calculations to adjust airflow through the system, which can force the evaporator  coil to run colder and can enhance moisture removal from the home. This isn’t foolproof, though; it can only work with certain system designs and can actually result in a frozen coil in some instances. 
  • Install a dehumidifier: If all else fails, it might just be necessary to incorporate a dehumidifier and dehumidistat into the system. This can remove moisture from the air as the system runs and can also prevent condensation and mold growth inside the ducts. This might also require an upgrade to a multi-speed blower, which is designed for enhanced humidity control. A variable speed blower will typically kick on at about 200 cfm, then will ramp up to about 80% airflow capacity for several minutes. If the target temperature on the thermostat isn’t satisfied, the unit might run at full capacity for awhile longer. To find out more about whole-house dehumidifiers, airflow calculations and multi-speed blowers, click here. 

 

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