I once walked into an older ranch house and immediately noticed something unusual: it didn’t smell like an old house.
The air simply smelled neutral.
I asked the homeowner what they had done differently. Two of the biggest changes were encapsulating the crawlspace and installing an Energy Recovery Ventilator, or ERV.
That experience stuck with me because it illustrates something homeowners don’t always associate with ventilation. Ventilation isn’t simply about bringing outdoor air into a house. It’s about controlling where the air comes from, removing odors, moisture, and indoor pollutants, filtering the air we bring in, and doing all of it without creating an unnecessary energy penalty.
Older homes often received plenty of outdoor air because they leaked. Air found its way inside through cracks around windows, gaps in the building enclosure, crawlspaces, attics and penetrations. The problem was that no one controlled where that air came from, how much entered the house or what it carried with it.
A modern high-performance home is different. Once we intentionally reduce those leaks, we also need to intentionally design how the house receives fresh air.
In Atlanta’s hot, humid climate, that’s where an ERV becomes an important part of the mechanical design.
Healthy indoor air starts with designing the building enclosure, ventilation, filtration and humidity control as one system.
What Does an ERV Actually Do?
An Energy Recovery Ventilator (ERV) continuously performs two jobs: removing stale indoor air and bringing fresh outdoor air into the house. Those two air streams pass through the equipment without directly mixing.
Inside the ERV is an energy-recovery core. As the incoming and outgoing air streams pass through that core, some of the heat and moisture energy is transferred between them. That allows us to ventilate the house without throwing away all of the energy we’ve already used to condition the indoor environment.
That recovery is particularly valuable in Atlanta. During the summer, we may be bringing in hot, humid outdoor air while exhausting cooler, drier indoor air. The ERV reduces the heat and moisture load associated with that required ventilation air before the HVAC and dehumidification systems finish conditioning it.
It’s important, however, to understand what an ERV doesn’t do. An ERV doesn’t eliminate the need for heating, cooling or dehumidification, and it isn’t a complete whole-house filtration system by itself.
It’s one component of a larger indoor-air-quality strategy.
An ERV exhausts stale indoor air while bringing in controlled outdoor air and recovering a portion of the energy between the two air streams.
Want to see how an ERV works? In this video from the International Builder’s Show, we look at how an ERV brings fresh outdoor air into the home while exhausting stale indoor air and recovering energy between the two air streams.
Why We Use ERVs Instead of HRVs in Atlanta
Homeowners researching ventilation systems will often encounter both ERVs and HRVs. An HRV, or Heat Recovery Ventilator, primarily transfers sensible heat between the incoming and outgoing air streams. An ERV transfers heat while also allowing a portion of the moisture energy to transfer through its enthalpy core.
For Atlanta, we generally choose an ERV because moisture matters so much in our climate.
During humid weather, the ERV can reduce the moisture load of the outdoor ventilation air before that air enters the rest of the mechanical system. It doesn’t remove all of that moisture, and the incoming air may still contain more moisture than the indoor air. The HVAC system or dedicated dehumidifier still has work to do.
But compare that with a leaky home, where hot, humid air can enter through hundreds of uncontrolled pathways with no energy recovery at all. The ERV gives us a much more efficient and predictable way to provide the fresh air the occupants need.
We’ve had good experience with equipment from manufacturers such as Broan and Panasonic, although the specific equipment depends on each house’s requirements. The important part isn’t the logo on the box; it’s whether the equipment is properly selected, designed, installed, balanced and commissioned.
The ERV is one component of a larger mechanical strategy that includes ventilation, filtration, humidity control and air distribution.
An ERV Helps With Humidity, but It Isn’t a Dehumidifier
One of the most common misunderstandings about ERVs is the idea that installing one will solve the home’s humidity problems.
It won’t.
An ERV can reduce the moisture penalty associated with required ventilation, but the amount of moisture transferred through the core is limited. During Atlanta’s humid months, the ventilation air entering the house still usually needs additional moisture removal.
That’s why SawHorse typically includes dedicated dehumidification in our high-performance homes.
This becomes especially important during Atlanta’s shoulder seasons. Think about a mild, rainy spring or fall day. The house may already be at a comfortable temperature, so there isn’t enough cooling demand to make the air conditioner run for long periods. Outdoor humidity, however, can still be very high.
If we rely on the air conditioner as our only method of moisture removal, indoor relative humidity can rise even though the indoor temperature feels comfortable.
A dedicated whole-house dehumidifier lets us manage that moisture independently of the cooling load. We generally target indoor relative humidity around 50%, while recognizing that normal conditions can vary seasonally.
Depending on the house, we’ll integrate equipment from manufacturers such as Aprilaire or Santa Fe into the mechanical strategy. Sometimes the dehumidifier works near a return-air pathway and returns dried air to the HVAC system. On other projects, we use a more integrated mechanical-space approach.
Every house is different, which is exactly why we design these systems instead of relying on a standard mechanical-room recipe.
An ERV reduces the moisture load associated with ventilation, but dedicated dehumidification allows us to control indoor humidity even when the air conditioner isn’t running enough to remove moisture.
Ventilation and Filtration Are Two Different Jobs
This may be the most important indoor-air-quality distinction in the article.
Some people assume that because an ERV contains filters, an ERV is all a Passive House needs for air filtration.
We don’t agree.
The ERV filters the outdoor air entering the ventilation system. That’s important, but it doesn’t continuously clean all the air already circulating inside the house.
Indoor pollutants originate indoors, too. Cooking, cleaning, furnishings, fabrics, pets, people and everyday activities can contribute particles and other contaminants. If we want to continually reduce those particles, we need to recirculate the home’s indoor air through an appropriate filtration system.
At SawHorse, we generally design around a minimum MERV 13 filtration rating for the whole-house system, with HEPA filtration added when the project or occupants warrant it. One version of our Mixing Room approach is designed to move indoor air through the filtration system approximately four times per hour.
This distinction becomes particularly important with ductless mini-splits. Mini-splits can provide excellent heating and cooling efficiency, but their built-in filters generally aren’t intended to serve as a robust PM2.5 whole-house filtration strategy. If we’re using ductless equipment, we need to consider how the home’s indoor air will be recirculated and filtered separately.
With a ducted system, filtration can be more straightforward because the air that’s already being conditioned is continually moving through a central pathway where we can install appropriate filtration.
Ventilation brings fresh air in. Filtration cleans the air already inside.
We want both.
The ERV filters incoming ventilation air, while a separate whole-house filtration strategy continually cleans recirculated indoor air.
A Small Prefilter Can Save an Expensive ERV Filter
There’s another simple ERV detail we use that homeowners may never notice unless someone points it out.
Most ERVs include their own internal filters, and replacement filters can be relatively expensive. Yet many ERV installations send outdoor air directly to those filters without first capturing larger debris.
We prefer to install an accessible prefilter ahead of the ERV when the system design allows it. Depending on the design, that may be a MERV 8–13 filter. The prefilter catches particles before they reach the ERV’s more expensive internal filters, helping keep the unit cleaner and potentially extending filter life.
A higher MERV rating isn’t automatically better in every location. As filtration efficiency increases, we also need to account for pressure drop. That often means increasing filter surface area to improve filtration without restricting the airflow the ERV was designed to deliver.
This is one of those small details that says a lot about mechanical design. We’re not simply asking, “Does the ERV run?”
We’re asking, “Can the homeowner maintain this system affordably, and will it continue delivering the intended airflow?”
An accessible prefilter can capture larger particles before they reach the ERV’s more expensive internal filters, while proper sizing helps avoid unnecessary airflow restriction. Credit: @Homeperformance
Where the Stale Air Goes Matters Too
In our Passive House designs, we typically use the ERV to continuously remove stale air from bathrooms and other appropriate areas where odors and moisture are generated. Fresh outdoor air is then introduced into the home’s ventilation and distribution strategy.
This balanced approach is different from a conventional bathroom fan that simply throws air outside, leaving the building to figure out where the replacement air will come from.
In a very airtight house, pressure differences become important. An exhaust device can create negative pressure if we remove significant amounts of air without intentionally providing a replacement path.
Phius projects have specific requirements for ventilation balance and building pressurization and depressurization. Larger intermittent exhaust loads may also require makeup air.
That’s why our Passive House bathrooms normally exhaust through the balanced ERV system rather than relying on conventional high-CFM bath fans. The ERV operates continuously, so we’re removing stale air throughout the day rather than waiting for someone to remember to flip a switch.
A boost mode can be incorporated for unusual moisture events or other conditions, but when the baseline ventilation is properly designed, continuous operation handles most everyday conditions.
Kitchen Exhaust Is a Good Example of Why Pressure Control Matters
A kitchen range hood presents a bigger challenge because it can move a substantial volume of air very quickly.
Imagine a hood exhausting 300 CFM from a very airtight home. That air has to come from somewhere. If we don’t provide a controlled replacement pathway, the house may pull air through whatever path offers the least resistance.
That’s exactly what we worked so hard to avoid when we built an airtight enclosure.
Instead, we can create a balanced makeup-air strategy.
If the kitchen hood removes approximately 300 CFM, the makeup-air system can introduce a corresponding volume of outdoor air to help avoid excessive depressurization. That doesn’t mean every hood and every house gets the exact same design. The airflow, equipment, enclosure tightness and project requirements all have to be evaluated.
The underlying principle, however, is always the same:
We decide where the replacement air comes from.
It’s another example of why airtightness isn’t about sealing people inside a house. Airtightness is about gaining control over air movement so ventilation and exhaust can be designed intentionally.
In an airtight home, high-CFM kitchen exhaust needs a planned source of replacement air. A balanced makeup-air strategy keeps us from relying on random leakage through the enclosure. Credit: Fantech.
Here’s what that looks like in practice. In this video, we look at a makeup-air system designed to provide replacement air when a kitchen exhaust hood is operating—allowing the home to control where that air comes from instead of relying on random leakage through the building enclosure.
The SawHorse Mixing Room Concept
On some projects, we take the integration of these systems a step further with what we’ve been calling the SawHorse Mixing Room.
The concept was influenced by Panasonic’s OASYS approach, but we’ve adapted and significantly modified it for the homes we design. We don’t use it on every project; it makes sense when the architecture, mechanical design and available space support it.
The Mixing Room is a dedicated mechanical space where we can bring together several of the systems responsible for indoor conditions: conditioning equipment, ERV, whole-house filtration, dedicated dehumidification and air distribution.
Instead of treating those pieces as separate appliances scattered around the house, the space allows us to think of them as a single indoor-air system.
In some versions, sensors in individual zones monitor conditions such as temperature, relative humidity, CO₂, VOCs and PM2.5. If one area begins to deviate from the desired conditions, the system can respond by increasing airflow to that zone.
The concept also provides something often missing in residential mechanical design: space.
When SawHorse designs a new house from scratch, we can coordinate the architecture and mechanical design so the equipment isn’t squeezed into whatever corner remains after the floor plans are finished. Filters need room to come out. ERVs need to be serviced. Dehumidifiers need access. Ductwork needs enough space to avoid unnecessary restrictions and awkward transitions.
A good mechanical system isn’t simply a list of equipment.
The architecture has to give that equipment an opportunity to perform properly.
How Much Fresh Air Does a House Actually Need?
We don’t determine fresh-air ventilation by guessing or by applying a single universal air-change number.
SawHorse uses the applicable residential ventilation standards and project requirements to determine the required whole-house ventilation rate. For Passive House projects, the certification process adds its own ventilation and verification requirements.
The exact formula isn’t the most important thing for a homeowner to memorize.
What matters is that the required airflow is calculated, the system is designed to deliver it, and the installed system is measured to make sure the required air actually reaches the appropriate locations.
That final step is commissioning.
Installing an ERV Doesn’t Mean the ERV Works
We have a real example that illustrates why commissioning matters.
On one project, the homeowner noticed that some of the exhaust locations didn’t seem to be performing properly. The ERV itself was operating, so simply looking at the equipment wouldn’t necessarily have revealed the problem.
We measured the airflow at the individual ports and confirmed the homeowner’s observation: some locations weren’t receiving the intended exhaust airflow.
Further investigation revealed a blockage on the exhaust side of the system. After removing it, we tested the affected locations again and achieved the desired CFM.
That’s exactly what commissioning is supposed to accomplish.
The question isn’t whether the fan turns on.
The question is whether the installed system delivers the airflow shown in the mechanical design.
At SawHorse, we measure and balance the ERV using the equipment’s test ports and verify airflow at individual supply and exhaust points.
Installing the equipment isn’t the end of the job. We need to verify that it actually performs as designed.
ERV commissioning includes measuring actual airflow at the supply and exhaust locations to verify that each space is receiving the CFM specified in the mechanical design.
What About CO₂, VOCs and PM2.5?
Mechanical ventilation helps dilute and remove pollutants generated inside a home, including odors, moisture, VOCs and occupant-generated CO₂. Whole-house filtration addresses another part of the problem by continually reducing airborne particles, including PM2.5, from recirculated indoor air.
In a conventional design, we encourage monitoring so homeowners can understand what’s actually happening inside the house rather than assuming the indoor air is good just because the house feels comfortable.
Our more advanced Mixing Room approach can go further by using sensors in individual zones to monitor temperature, humidity, CO₂, VOCs and PM2.5. That information can then be used to address conditions in specific areas rather than treating the entire house as though the air is identical everywhere.
This is where indoor-air-quality design starts to move beyond simply meeting a ventilation requirement.
At SawHorse, we have developed our own indoor-air-quality standards that, in some respects, go beyond the minimum requirements of Passive House certification—particularly regarding whole-house filtration and recirculation.
Passive House provides an excellent framework.
We still ask whether there are ways we can make the house better for the people living in it.
Monitoring helps us understand indoor conditions that can’t be judged by temperature alone, including humidity, CO₂, VOCs and fine particles. Credit: Swidget.
Why ERVs Make Sense in Both New and Older Atlanta Homes
That old ranch house I mentioned at the beginning illustrates why controlled ventilation isn’t limited to new Passive Houses.
The homeowners had improved the crawlspace and added an ERV, and the result was something anyone could experience without looking at an energy model: the house smelled neutral.
An ERV doesn’t magically fix every cause of an old-house odor. Moisture problems, crawlspaces, water intrusion, materials and other pollutant sources still have to be addressed at the source.
But once those problems are controlled, mechanical ventilation provides a consistent way to remove stale indoor air and replace it with intentionally sourced outdoor air.
In an older home, this can improve indoor conditions.
In a new high-performance home, controlled ventilation becomes even more important because we’ve intentionally removed many of the leaks the old house relied on for accidental ventilation.
That’s really the progression:
A conventional old house gets air wherever it leaks.
A high-performance house gets air where we designed it to come from.
An ERV Is Only One Part of Healthy Indoor Air
If there’s one thing homeowners should take away from this article, it’s that installing an ERV doesn’t automatically create healthy indoor air.
A well-designed strategy requires several systems to work together.
The ERV provides controlled outdoor air and removes stale indoor air. Whole-house filtration cleans recirculated indoor air. Dedicated dehumidification manages Atlanta’s moisture when the air conditioner doesn’t have enough cooling load to do the job. Makeup air prevents large exhaust equipment from creating excessive depressurization. Monitoring tells us what’s actually happening, and commissioning verifies that the system we designed is the system we actually built.
That’s why we don’t see an ERV simply as another appliance.
We see it as one component of the house’s air-management system.
In Atlanta’s hot, humid climate, that systems approach matters. We need outdoor air for healthy indoor conditions, but that air also brings heat and moisture. We therefore want to control how much enters, recover as much energy as practical, remove the remaining moisture, filter the air and verify that the finished system performs as designed.
The result isn’t a house that can’t breathe.
It’s a house where we decide how it breathes.
Frequently Asked Questions
What is an ERV?
An Energy Recovery Ventilator is a mechanical ventilation system that simultaneously exhausts stale indoor air and brings fresh outdoor air into a home. An energy-recovery core transfers a portion of the heat and moisture energy between the incoming and outgoing air streams without directly mixing them.
Is an ERV better than an HRV in Atlanta?
SawHorse generally uses ERVs rather than HRVs in Atlanta because moisture transfer is particularly valuable in our hot, humid climate. An ERV doesn’t eliminate the incoming moisture load, but it can reduce the amount of heat and moisture the HVAC and dehumidification systems have to manage.
Does an ERV dehumidify the house?
Not by itself. An ERV can reduce the moisture load associated with ventilation, but it does not replace a dedicated dehumidifier. We typically include separate dehumidification in high-performance Atlanta homes so humidity can be controlled even during mild, damp weather when the air conditioner isn’t running very much.
Does an ERV filter all the air in the house?
No. The ERV primarily filters the outdoor air passing through the ventilation system. SawHorse uses a separate whole-house recirculating filtration strategy to continually clean the air already inside the home.
How do you know an ERV is working correctly?
It has to be commissioned. We measure and balance supply and exhaust airflow and verify actual airflow at the appropriate locations. Testing can uncover problems—such as duct blockages—that wouldn’t necessarily be apparent simply by seeing that the ERV is running.
Continue Learning About Building Science
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See how ventilation fits into a larger mechanical strategy that includes right-sized heating and cooling, filtration, humidity control, duct design and independent commissioning.
What Is ACH50? Why Airtightness Matters for Atlanta Homes
Learn why airtightness makes controlled ventilation possible and how blower-door testing helps us measure the performance of the building enclosure.
What Makes a Home Healthy? Our Science-Based Approach to Building
See how the building enclosure, ventilation, filtration, humidity control and material choices work together as part of a larger healthy-home strategy.
Explore the Complete SawHorse Building Science Series →
Considering a Healthier, High-Performance Atlanta Home?
Ventilation is only one part of creating a healthy indoor environment.
SawHorse integrates architecture, building science, HVAC design, ventilation, filtration, humidity control, testing and construction so these systems can be designed to work together from the beginning.
The goal isn’t simply to install efficient equipment. It is to create an indoor environment that remains comfortable, healthy and predictable throughout Atlanta’s changing seasons—and then verify that the completed systems perform as intended.
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