What is the best way to duct a dehumidifier?

Humidity control is becoming a bigger deal than it used to be.  As Joseph Lstiburek, PhD, PE said to me one time, &#;the gap between stupid and hurt is narrowing.&#;  Building codes and best practices are getting better, which is reducing the drying potential of homes.  The result is&#;or can be&#;moisture damage in walls, floors, and ceilings.  That can lead to expensive repairs, reduced comfort, and indoor air quality problems.  That means there are a lot of places where a dehumidifier is practically essential now.  So today, I want to go through four ways to duct a dehumidifier, in order of best to worst.

View Details

1. Independent ducting

The best way to duct a dehumidifier is to give them their own independent ducts.  The intake side is connected a grille (or maybe multiple grilles).  The supply side is connected to a grille or register (or maybe more than one).  It&#;s the simplest way to set up a whole-house dehumidifier.

Pros

• Simplest ducting method
• Easier to get the air flow right since there&#;s only one blower affecting the pressure in the ducts
• Dehumidifier does not need to be near air conditioner.

Cons

• May require more space for ducts
• Because the dehumidified air is warm, you have to find a place to send it where it won&#;t create comfort problems.

2. Dry air sent into air conditioning ducts

The second best way to duct a whole-house dehumidifier is to send the dry air into the air conditioning ducts.  This makes things more complicated because now you have two blowers moving air and creating pressure differences in the ducts.  You also have two choices.  The dehumidifier intake will have its own grille, but you can put the dehumidified air into either the return or the supply side of the air conditioner.

From the diagram below, you can tell which is the better choice:  the supply side.  I&#;ll probably do a whole article on the reasons behind that setup, but for now let me just keep it simple.  When you put the dry air into the return side, you&#;re taking away some of the air conditioner&#;s job.  Normally, the return air going into the air conditioner needs both cooling and dehumidification (sensible and latent cooling) for those of us in humid climates.  By putting dehumidified air into the return side, the entering air is drier.  The AC does less work, and the dehumidifier does more work.  So you end up using more energy and running the dehumidifier more than you would in the better configuration shown below.

Pros

• Dehumidified air gets tempered with conditioned air before entering the conditioned space.
• Dehumidified air gets distributed throughout the house or zone.
• Can reduce the amount of ducting needed compared to independent ducting

Cons

• May need to increase the supply duct size to handle the additional air flow
• Increased energy use if you put the dehumidified air into the return side

3. Bypass method

In this configuration, the dehumidifier intake pulls air from the return side of the air handler, dehumidifies it, and then sends it into the supply side to mix with the conditioned air.  Some of the air bypasses the air handler, hence the name.  This method can reduce the amount of ductwork needed.

Pros

• Can reduce the amount of ducting needed compared to independent ducting
• Dehumidified air gets tempered with conditioned air before entering the conditioned space.
• Dehumidified air gets distributed throughout the house or zone.

Cons

• Return and supply ducts may need to be increased to handle the additional amount of air flow.
• May need damper to prevent backflow through dehumidifier
• Will need a method to prevent short-circuiting when the air handler is not running.  This could be a second damper or having the air handler wired to keep running whenever the dehumidifier runs.

4. Injection Method

The final method here has both the intake and dry air sides of the dehumidifier connected to the same side of the air handler.  In the diagram  below, they&#;re connected to the supply side.  Putting them on the return side has the same problem of reducing the moisture removal capacity of the air conditioner mentioned in method 2 above.

Pros

• Can reduce the amount of ducting needed compared to independent ducting
• Dehumidified air gets tempered with conditioned air before entering the conditioned space.
• Dehumidified air gets distributed throughout the house or zone.
• Can increase dehumidifier efficiency because of cooler entering air

Cons

• Air handler must run whenever dehumidifier runs to prevent short circuiting.
• May need damper to prevent backflow through dehumidifier
• Increased energy use if you put the dehumidifier on the return side
• With dehumidifier on supply side,  dehumidifier capacity can be lower because of cooler entering air.
• With dehumidifier on supply side,  this method is appropriate only for hot humid climates with lots of air conditioner runtime.
• Not every dehumidifier can handle cooler entering air in supply-side injection method.

Final notes

For simplicity of design and operation, independent ducts are your best choice.  Connecting to the air conditioning ducts complicates things because you have two fans moving air and creating pressure differences.  You have to consider the relative air flows when you connect two duct systems because you want the dehumidified air flow to be less than the air conditioner air flow.  Some of the configurations above also require larger ducts,  dampers, and different control methods (i.e., running the air handler whenever the dehumidifier runs).

One other ducting issue I didn&#;t mention above is the ventilating dehumidifier.  The lead photo shows one ducted to bring in outdoor air as well as dehumidifying air from the home.  The R/A duct (for return air) is the intake for recirculated air from the home.  The F/A duct (for fresh air) is bringing in outdoor air.  They both go through the dehumidifier and get distributed to the house.  You can set up this type of system using any of the four ways to duct a dehumidifier described above.  You just have the additional duct bringing in the outdoor air.  The diagram above shows a ventilating dehumidifier set up using method 2.

There, that&#;s not so hard, is it?  Of course, we can always make it more difficult.  One way to do that is to throw an energy recovery ventilator (ERV) into the mix.  Fortunately, David Treleven has covered how to combine heating and cooling, dehumidifier, and ERV ducts in a great article on Green Building Advisor.

And finally, ducting a dehumidifier has the same requirements for good air flow as ducting for heating and cooling systems.

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of . You can follow him on Twitter at @EnergyVanguard.

Related Articles

The Confusing World of Dehumidifier Capacity

17 Steps to Better Duct Systems

An Energy Recovery Ventilator Is NOT a Dehumidifier

Lead photo of Santa Fe dehumidifier courtesy of Nikki Krueger

Comments are moderated. Your comment will not appear below until approved.

Updated: Sept 9,
Muggy. Sultry. Moist.

Ant Product Page

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 their 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.

As you&#;ll see in this article, our team goes to incredible lengths to deliver a highly effective, reliable, and long-lasting dehumidifier installation. If you&#;re considering a whole-house dehumidifier for your Atlanta-area home, get in touch today!

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 before September of , 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:

1. 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 grille, just like your main HVAC return.
2. 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.
3. 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 in 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 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 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 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 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 for 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 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 muggy days more bearable. As summer approaches, could there be a more tempting possibility?

If you want to learn more, please visit our website Ducted Dehumidifier(vi,ar,de).