Installing a Whole-House Dehumidifier the Right Way
Muggy. Sultry. Moist.
Here in Atlanta, we experience humid conditions from March through October. Sometimes, we even have warm, sticky days in December or January! Assuming your air conditioner is sized properly, it probably does a good job removing excess humidity.
But boy, are there exceptions.
During the spring and fall, it isn’t always hot enough to run the AC. When you aren’t running it, indoor humidity can easily creep up to an uncomfortable level. And what if your AC is too large for your home? If that’s the case – and it’s a common problem – you’re probably not removing enough humidity to stay cool and comfortable.
A whole-house dehumidifier solves all of these problems.
We love whole-house dehumidifiers, and so do our clients who have them installed.
Any time the relative humidity (RH) rises above a preset threshold – 50%, say – your dehumidifier kicks in to remove the excess moisture. During the shoulder seasons, it removes humidity when your air conditioner doesn’t. And on hot summer days, it continues removing humidity while your air conditioner is between cycles.
We’ve even had clients who were able to increase their thermostat setting from 68 to 78 degrees after installing a whole-house dehumidifier. No joke.
But to reap all of the benefits, you’ve got to install a dehumidifier properly.
The best way to install a whole-house dehumidifier: Add a dedicated return duct and send dry air to your supply plenum.
If you encountered this article prior to September of 2018, you would not have read about this method of installing a whole-house dehumidifier. The reason? We used to install most dehumidifiers by only tying the unit into your existing return ductwork.
That method remains one of the most common ways to install dehumidifiers in homes. In our experience, many homeowners are perfectly happy with it.
However, after hundreds of dehumidifier installations and countless hours of testing and troubleshooting, we’ve modified our approach. Today, our preferred method is a “dedicated return to supply” installation. Here’s how we do it:
- Install a new return duct in your home. In a two-story home, we usually only need to add this duct to the bottom story. It will have its own grille, just like your main HVAC return.
- Discharge dry air into your supply plenum. The dehumidifier pulls air through the return, removes humidity, and dumps the dry air into the same supply plenum that’s connected to your main indoor HVAC unit. The dry air enters your home through normal HVAC supply pathways.
- Measure static pressure and make adjustments. If the dehumidifier and air conditioner are running at the same time, there’s a chance you’ll have excess static pressure to the system. Depending on the size of your ducts, some additional static pressure might be ok. In any case, we always measure the static pressure to ensure it doesn’t get too high. If it does, we simply set the dehumidifier to only operate independently of your primary HVAC system.
Benefits of the “dedicated return to supply” installation method
There are several benefits to installing a whole-house dehumidifier this way! Here are the main ones:
- Lowest possible static pressure. Compared to simply tying the dehumidifier into your existing return ductwork, this method does not increase static pressure on the dehumidifier. High static pressure can make it harder for the dehumidifier to remove as much moisture as it should.
- Independent operation. In a typical “return to return” installation, your air handler fan turns on to help move dry air into your home. If this happens shortly after your AC turns off, the dry air actually picks up more moisture from your AC’s evaporator coil. That moisture is then pushed back into your home! Our preferred method avoids this scenario.
- Dew point stability. Know what happens when relatively warm, dry air passes over your evaporator coil while the system is running? You prevent the coil from getting as cold as it should, which can prevent the AC from removing humidity. The dehumidifier will have to work even harder. However, when you push dry air directly into the supply plenum instead of the return, this problem never materializes. Why? Because you’re introducing dry air downstream from the indoor coil.
- Better efficiency. Since the dehumidifier doesn’t have to work as hard, it doesn’t run as often. That alone saves energy, but it’s not the only way this installation method reduces power consumption. For the dehumidifier to be effective in a “return to return” installation, the unit typically activates your air handler fan to help move air through the system. In contrast, the “dedicated return to supply” method does not require the use of the air handler fan when you operate the dehumidifier. The bottom line? You don’t have to pay to run the fan.
- Even lower humidity. It’s true! If your dehumidifier struggles to remove enough moisture to keep you comfortable (maybe it only gets you down to 57% RH instead of 50% RH, for instance), reinstalling it per this design will probably get you where you need to be.
There’s just one problem with this installation method…
And it’s really not that big of a deal.
Some clients report that the dry air blowing into their homes feels warm. That happens because the air isn’t blowing over your cool evaporator coil and because the dehumidified air isn’t mixing with the rest of your home’s conditioned air.
Most people won’t notice this or be bothered by it. What’s more, if the warm air actually increases the temperature of your home, your AC will turn on. Problem solved.
Another way to install a whole-house dehumidifier: Connect a dedicated return to your existing return.
Sometimes, it’s simply not feasible to connect the dehumidifier to your supply plenum. When this is the case, we add the dedicated return but discharge the dry air into the HVAC system return.
This method works, but it still requires you to use your air handler fan to move the dehumidified air into your home. The downsides of pushing air over the evaporator coil still apply.
The thing to remember is that there are almost always options. Many people have dehumidifiers that are installed this way, and they work really well! Are they as efficient as units connected via the “dedicated return to supply” method? Probably not. Do they still remove excess humidity and improve comfort. You bet they do.
Fresh air ventilation: the ultimate fringe benefit of whole-house dehumidifiers
If you care about indoor air quality, be sure to enable the fresh air controls on your whole-house dehumidifier! Your dehumidifier can function as a mechanical ventilation system, which is necessary in airtight homes but beneficial for all homes, airtight or not.
Using ASHRAE guidelines for ventilation, we calculate the amount of fresh air your home needs based on multiple variables. When the dehumidifier runs, it uses these calculations (we dial them in during installation) to bring in a certain amount of fresh air from the outdoors.
The fresh air enters through a small duct connected to the ductwork routing air into the dehumidifier. This outdoor air passes through a filter, and the dehumidifier removes the moisture. We can even configure the unit to “lock out” fresh air during temperature extremes – above 90 degrees or below 40 degrees, say.
Taking advantage of your dehumidifier’s fresh air option is a great way to circulate clean air throughout your home and ensure proper ventilation year round! It’s like opening a window except that you’re not letting in pollen or humidity.
How energy efficient is a whole-house dehumidifier?
When you install a whole-house dehumidifier, you’re committing to improved comfort and indoor air quality. In exchange for those advantages, you’ll likely pay a little more each month. A dehumidifier, after all, is an appliance that consumes energy when it runs.
But if you’re suffering from the effects of high RH, it’s so worth it!
Some of our clients don’t notice a major increase in their utility bills during the summer because the dehumidifier lets them set their thermostat to a higher temperature. For example, if you were setting your thermostat to 70 but the dehumidifier keeps you comfortable at 76, the increase in monthly utility costs usually won’t be significant.
Just remember that dehumidifiers aren’t energy efficiency upgrades – they’re comfort upgrades.
It might just change your life. Seriously.
After installing a whole house dehumidifier, life just seems… different.
You grow accustomed to low humidity levels, even in peak summer. You might feel unusually warm or clammy when visiting someone else’s house, going to a restaurant, or sitting in a waiting room. Your home literally becomes an oasis of comfort – you’ll wonder how other people put up with the high humidity!
If you’ve got allergies, you’ll notice that you’re not sneezing and wheezing so much. Low RH, after all, makes your home inhospitable to dust mites. And that’s a good thing.
All in all, a whole-house dehumidifier makes the muggy days more bearable. As summer approaches, could there be a more tempting possibility?