This tech tip was written with the guidance of Nikki Krueger, the Director of Marketing & Business Development at Santa Fe Dehumidifiers. Nikki has been a longtime contributor to HVAC School as a recurring podcast guest and presenter at the HVACR Training Symposium. Most of the illustrations and instructions came from Santa Fe's Ultra V Series Manual.

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As fresh air ventilation has become a staple of commercial and residential construction in recent years, we’ve had to pay more attention to how we bring it in. In cool and dry climates, a heat recovery ventilator (HRV) is usually perfectly acceptable. An HRV doesn’t reduce humidity and wouldn’t fly in green-grass climates, but an energy recovery ventilator (ERV) would—if installed appropriately. In Florida, the latent heat exchange offered by ERVs alone isn’t sufficient for comfort, even with the best of installations. We need a strategy that can reach specific indoor relative humidity targets, remove moisture from the airstream, and drain the water out, which brings us to the ventilating dehumidifier (which can also be used with ERVs).

Unless you live in Florida, you’re spoiled for choice when it comes to ducting a ventilating dehumidifier. However, each configuration has its pros and cons. You may need to install check dampers to regulate flow. One configuration is exceptionally prone to sweating and can’t be used with a fresh air intake. Some configurations are even illegal in some places. Matt Bruner wrote a great tech tip about dehumidifier installations from a performance perspective a while ago, but we’ll look specifically at installation requirements and limitations and code compliance in this one.

How Ventilating Dehumidifiers Work

A typical whole-home dehumidifier has two ducts: a return and a supply. The return air may come from a dedicated return or an offshoot from the central HVAC’s ductwork.

The supply air typically discharges into the HVAC supply ductwork and mixes with air already in the ductwork.

Refrigerant-based dehumidifiers work like air conditioners. Air enters the dehumidifier and passes over an evaporator coil that cools it and removes moisture. Since the relative humidity (RH) is high after passing over the evaporator coil, the air passes over a warm coil to reheat it. The extra sensible heat doesn’t change the moisture content much at all, but it drops the RH. (Remember: Air is like a sponge. When you squeeze a sponge, it can’t hold much moisture, so it releases water. But when the sponge expands again, it feels drier.)

Optional Outdoor Air Intake

Ventilating dehumidifiers are essentially the same as normal dehumidifiers, but they have an extra return for an optional outdoor air intake. As with regular dehumidifiers, you can have either a dedicated return or duct it from the central HVAC’s ductwork. You can see both configurations below (using the examples from Santa Fe’s Ultra V155 manual):

Source: Santa Fe Ultra V Series Manual, pg. 16 & 17

Separate Ventilation and Dehumidification Functions

Ventilating whole-house dehumidifiers like the Santa Fe Ultra V155 can serve a dual purpose: they can supply outdoor air and provide dedicated dehumidification for the home. A ventilation controller will activate the fan of the unit and open the motorized power damper to deliver filtered outdoor air. The compressor of the dehumidifier is powered by a remote humidistat, which is often in a central location in the living space. 

Source: Santa Fe SmartAire™ Power Damper Manual, pg. 7

One misconception is that the ventilation air will always be dehumidified before entering the living space. That’s not typically the case, as this strategy would be very energy-intensive in humid climates. If ventilation is active while dehumidification isn’t, the dehumidifier’s fan will bring in outdoor air and temper it with return air from the home.

Source: Santa Fe Ultra V Series Manual, pg. 13

Without any additional moisture removal by the dehumidifier, the supply air’s temperature and humidity will be determined by that of the outdoor and indoor air mixing in the unit.

When the air conditioner can’t handle the latent load or there is no load on the home, the RH in the living space will rise above the setpoint, and the dehumidifier’s compressor will start.

Optional Duct Tee and Gravity Damper

You may also be able to send dry air to a specific area of the home, not just the central HVAC ductwork. Using a duct tee and a gravity damper, you can direct some of the air to a specific room or area of a home; this arrangement will be more common in basement or crawlspace installations.

Source: Santa Fe Ultra V Series Manual, pg. 16

Ducting Different Configurations

Ventilating dehumidifiers tie into the central HVAC system via the ductwork. Flex ducts, even if just for a small part of the entire duct run, are useful for reducing noise and vibration. Whether you decide to use all, partial, or no flex, you’ll want to aim to limit restrictions and turbulence.

You can prevent turbulence in flex ducts by minimizing turns. If you can’t avoid turns, use long, sweeping turns instead of tight ones. Pulling the flex tight and making sure it’s strapped well so that it doesn’t sag also goes a long way to reduce turbulence. We wrote about these practices and some other best practices for flex duct installation HERE.

The outdoor air intake is pretty self-explanatory; it provides a path from the outdoors to the dehumidifier and has a motorized damper that opens or closes on a timer or some other control. However, there is a bit more design flexibility with the supply and return ductwork. There are just a few basic principles to keep in mind:

• When you can install them, dedicated returns are usually best
• If the dehumidifier supply and return both tie into the central HVAC ductwork, they need a check damper (or for the HVAC fan to run at the same time)
• Avoid tying the dehumidifier supply into the HVAC return duct when possible
• In some areas, ducting the dehumidifier into or out of the return duct is forbidden by code

Here are a few different ducting strategies you may use with a ventilating dehumidifier:

Dedicated Return to HVAC Supply (Highly Recommended)

The ideal ventilating dehumidifier setup consists of an outdoor intake and a dedicated return from a central area of the home on the return side. The dehumidifier’s supply ductwork would then tie into the central HVAC supply (with a backdraft damper). This configuration doesn’t require a check damper AND is code-compliant even in states like Florida.

Source: Santa Fe Ultra V Series Manual, pg. 16

The return should be in a room that is NEITHER the bathroom nor the kitchen and that does not have its doors closed for long periods of time. In other words, try to have the return grille in a living room rather than a kitchen, bathroom, or guest bedroom that only gets used once or twice a year.

To make a dedicated return, you could either install a new return duct and grille or disconnect an existing return from the HVAC and route it to the dehumidifier.

HVAC Return to HVAC Supply

You could duct the dehumidifier return to the HVAC return and its supply to the HVAC supply ductwork in a bypass configuration. While this one uses similar principles to the configuration we just covered, you’ll need some extra hardware to make it work—and you can’t make it work at all in Florida.

The working principle here is that some air will bypass the air handler and go through the dehumidifier instead. Ventilating dehumidifiers filter, cool and dehumidify, and then reheat the air. Then, the dried and reheated air will re-enter the supply ductwork and mix with air that went over the air handler’s evaporator coil in the central HVAC ducts.

This configuration, however, requires the use of a check damper if there will ever be a situation where the HVAC blower will not run at the same time as the dehumidifier. If there is no check damper, the HVAC blower must run at the same time as the dehumidifier.

Source: Santa Fe Ultra V Series Manual, pg. 17

We also can’t use this configuration if the static pressure exceeds 0.5” WC. In this configuration, the dehumidifier has to overcome negative pressure in the return and positive pressure in the supply. Below, you can see how the dehumidifier CFM output takes a hit as the static pressure increases:

This configuration is also not allowed in Florida. Ducting the dehumidifier to the return in any form, whether as a supply or return duct or both, is forbidden by code. Be sure to check with your local building codes to make sure this design complies. Here is what the Florida Building Code (2020) has to say:

HVAC Supply to HVAC Supply

Supply-to-supply injection-type installations have the dehumidifier’s supply and return ducted into the central HVAC on the supply side. These configurations are very effective at removing moisture when the air conditioner runs in cooling mode. When air has already passed over the central HVAC evaporator coil, it’s cool and has near-100% relative humidity; when it goes over the dehumidifier coil, even more moisture gets squeezed out. However, there are some extra design factors we need to consider.

First, this setup is more likely to make the dehumidifier coil freeze. Since air passes over a cold coil, the air in the supply plenum is considerably cooler than the air in a room or a return duct. When chilly air passes over the dehumidifier’s evaporator coil, the coil is more likely to freeze. Dehumidifiers can cycle in and out of defrost if the coil constantly freezes, derating their performance. If the air in the supply plenum is below 55°F, you should not use a supply-to-supply configuration.

Second, when the system runs in heat mode, warm air will exit the air handler. Since the dehumidifier coil needs to be cold to pull moisture out of the air, it can’t get very cold if heated air keeps passing over it. The dehumidifier’s performance takes a hit. As a result, the supply-to-supply configuration isn’t recommended for climates where the HVAC runs in heat mode during the spring and fall (such as Mid-Atlantic states).

Just like when we use the configuration with the HVAC return and supply, we need a check damper in the main HVAC ductwork between the dehumidifier return and supply.

Source: Santa Fe Ultra V Series Manual, pg. 17

As you may have noticed in the diagram, you also won’t be able to use the optional fresh air intake with a supply-to-supply setup. Condensation on the dehumidifier is much more likely when you’re mixing cool air from the supply duct with warm, humid air from the outdoors. Moisture can cause property damage, so that’s a risk that manufacturers like Santa Fe advise against taking. If you want to add outdoor air, you will need a separate means of adding fresh air that ties into the HVAC return.

Dedicated Return to HVAC Return

Any form of installation where the dehumidifier supply ductwork ties into the HVAC return should be a last resort only. Dehumidifiers add sensible heat, and that’s not much of a problem when air mixes in the supply ductwork. However, when that warm air goes over the evaporator coil, it gets a lot harder for that coil to get cold enough for effective dehumidification.

Using a dedicated return for the dehumidifier return and tying the dehumidifier supply ductwork into the HVAC return isn’t recommended, but it’s not the worst installation.

Source: Santa Fe Ultra V Series Manual, pg. 18

As you can see, you can still use the outdoor air intake and don’t need a check damper since the dehumidifier has a dedicated return.

HVAC Return to HVAC Return (NOT Recommended)

In a return-to-return installation, the dehumidifier return ductwork attaches to the HVAC return. The dehumidifier’s supply ductwork also ties into the return, just closer to the air handler.

Source: Santa Fe Ultra V Series Manual, pg. 18

According to a 2018 study by the Florida Solar Energy Center and the University of Central Florida, this setup is by far the least effective for dehumidification, and some states even forbid it by code. The same Florida Building Code excerpt from earlier applies here, too:

As with other injection and bypass configurations, you will need a check damper in the central HVAC ductwork between the dehumidifier supply and return ducts.

No Central HVAC

In homes with only ductless units, such as some passive houses, there won’t be central ductwork for the dehumidifier, but that’s not a problem. You simply need to install a dedicated return and fresh air intake (if you’re using the dehumidification capabilities), and then you just add the supply ducts yourself.

Source: Santa Fe Ultra V Series Manual, pg. 18

Since dehumidifiers add sensible heat, delivering the supply air to a single room can cause that room to heat up and feel uncomfortable. Try to have at least three ducts with registers in different rooms to spread out the supply air across the home; Santa Fe recommends terminating the supply ducts as close to the ductless units as possible, so you can plan the duct design around the ductless unit distribution. Use 8” supply ducts for bedrooms and 10” ones for larger areas.

Installing a dehumidifier—especially a ventilating dehumidifier—can be a great upgrade for customers in humid climates. However, installing a dehumidifier requires a thorough understanding of the customer’s HVAC system and local building codes. Santa Fe makes the rest easy by publishing detailed guides with caveats for each configuration, control setup tutorials, and performance data. You can read more about their Ultra V155 ventilating dehumidifier in its data sheet and Ultra V Series manual; you’ll find a lot of similar information to what’s in this tech tip.

But of course, it’s still on us to understand why these configurations matter, make good judgment calls in the customer’s home, and consult local codes to make sure we’re dehumidifying and bringing in fresh air legally and responsibly.