How to Plan A Whole-Home Dehumidifier Installation
Has your home been extra humid recently? If you’re not ready to replace/renovate your HVAC system but just need to reduce humidity, you may consider adding a whole-home dehumidifier. Here are some things you’ll need to consider, and how different ducting arrangements affect its performance and the performance of your HVAC. Note: planning makes all the difference, so you’ll want to take some humidity measurements in different areas of your home, consider the capacity of dehumidifier you’ll need (see our article), check on the power, ducting and drain requirements of the model(s) you’re considering, and generally research the market. This is where a walk-through of your home and system with an HVAC tech who has installed lots of whole-home dehu’s is invaluable. They will have a better idea of the clearances needed for ductwork and the unit itself, where to place the hygrometer (a device that measures the relative humidity to send a signal to switch the dehumidifier on and off), how to route the drain, and what kind of performance you’ll get from different models and ducting configurations.
Whole-home dehu’s (dehumidifiers) are permanently installed, and assist your HVAC in keeping your home more comfortable, and being more efficient. The reasons for this is that excessive humidity (above 60%) increases human discomfort, and the more water vapor in the air, the more energy your HVAC requires to change the temperature, because it is not only adding or removing heat from the air, it is also adding or removing heat from the water vapor. In general, central AC systems remove some water vapor, but are not great at dehumidifying. If your HVAC system does not maintain your home below 60% humidity, you should consider adding a whole-home dehu.
According to GreenBuildingAdvisor.com, there are Four Ways to Duct a Dehumidifier (at least four they recommend). Their article lists “best” to “next best” but since every home is different, the best one for your home may not be the first listed here.
1. Independent return/supply method
The simplest configuration is to install the dehu independently of your HVAC–that is, give it separate ducts/grilles from your central air system: one for supply and one for return. Ducting is simple because it will be the same size as the inlet/outlets on the unit, and if possible, it can be placed near the area of highest humidity in your home (usually the upstairs in a 2 story home, because warm, moist air rises). For this matter, “in wall” dehu’s like this popular Santa Fe model further simplify installation because there is no ducting. They still need professional installation because they are hard-wired (not plug-in) and require a drain line to be installed or connected to an existing drain, but very convenient if you don’t have room for ductwork in the area where you would like to place the dehu.
The biggest drawback of installing a dehu independently of your central AC is that the dehu produces warm air, so this makes the area near the unit warmer. Also, isolating the dehu from the HVAC doesn’t utilize the ductwork you already have with central AC, and pulling moisture out of one area of the home will lower humidity in other areas because the HVAC “homogenize” your air’s humidity, but more slowly and to less extent than if the dehumidifier distributed dry air to all areas via central ducts.
2. Dump dry air into the supply ducts of your central AC (dedicated return method)
This configuration is recommended in a video by HVAC School (a trusted source for all things HVAC) for one of its own teachers’ homes. It uses an independent return for your dehumidifier, and the supply side is joined to the supply plenum of your central system. It is slightly more complex because you’ll need to find area near the supply side of the air handler to duct in the dehumidifier supply, but in actuality you can place the dehumidifier some distance from the air handler (the teacher decides to mount it in his encapsulated attic where the vibration and noise won’t bother his family). He plans to pull return air from the 2nd floor, close to the dehumidifier, where it is more humid. The controller for the dehumidifier can be a hygrometer placed anywhere in the home (even in the return of the air handler) and wired to turn the dehu on and off.
A downside of this option is that because both the air handler and the dehu are sometimes using your duct system at the same time, there will be more pressure on your air handler (you are trying to force more air through your ducts). This is where “check dampers” prevent air from going backward in the system. Additionally, if your dehumidifier is equipped with a second port to bring in fresh air, you can do this via a motorized damper.
According to Greenbuildingadvisor.com, sending the dehu air into the supply side of the air handler is a better choice than dumping dehumidified air into the return side, because dehumidification is shared by both the air handler and the dehu.
Source: INNOVATIVE DEHUMIDIFIER WH200 Whole House Dehumidifier Instruction Manual
3. Bypass Method
This is easy to visualize: the dehu takes air from the existing return and delivers it back into the existing supply plenum. It ranks third, however, in preference because the air that bypasses the air handler is not cooled, in fact, it’s heated…so this causes more work for the air handler. On a good point, however, the airflow/pressure on the air handler is not increased, unlike Method #2 above. It also utilizes the existing ductwork to reduce humidity throughout the entire home.
Source: INNOVATIVE DEHUMIDIFIER WH200 Whole House Dehumidifier Instruction Manual
4. Injection Method
“Injecting” dry air into the supply takes air from the air handler, dehumidifies it, and sends it back into the supply plenum. This method actually increases the efficiency of the dehu because air is cooler going into it (the cooler the air, the easier it is to remove water vapor from it). It also uses the ductwork of the central system to distribute dehumidified air.
One drawback of this method is that the air handler needs to run at the same time as the dehu in order to prevent short-cycling (stopping and starting) of the dehu. However, in a humid climate that has lots of air handler run time, this method could be used to “inject” dehu air into the return side.
Source: INNOVATIVE DEHUMIDIFIER WH200 Whole House Dehumidifier Instruction Manual
As with all projects, planning is key! And, don’t be afraid to call back the HVAC techs to make changes if the installation doesn’t perform as well as you expected. It’s possible that you need to move the hygrometer or program the unit for a different run time. Even checking with the dehumidifier manufacturer can yield information that will help “dial in” the humidity and comfort of your home.
Photo by Cindy Bartillon on Unsplash
