One of my builders have decided to add fresh air vent to his houses. Houses are in Oklahoma with high moisture content air and hot/warm summer weather. Is this going to cause moisture problem in the house? Also how do you incorporate the vent when modeling this in Rem calculations?

Thank you

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Primer on ventilation options - pros & cons: 

As for rem-rate they have a ventilation area where you put in whatever mechanical system you are using for it or are we just talking about a hole in the wall?

Thank you for getting back with me
The vent is 4x4 inch connected to the return air duct.
I look at the equipment page in 15.7 version but couldn’t find anything about the vent.
Can you please help?
Thanks again

The amount of CFM pulled through that fresh air duct will be completely unconditioned. Obviously adding that amount of load to the A/C and Furnace. Let's assume it is 50-80 CFM of unconditioned air coming into the return whenever the central fan is operating.

If you are worried about added humidity during the summer -- you can either allow the A/C system to dehumidify this additional load, install a dehumidifier inline that pulls from the outside, or install an ERV system to partially condition your fresh air.

UltraAire has a nice small ducted dehumidifier with the option for fresh air intake. (I don't work for them, I'm just pointing them out as an efficient option). That would help with the moisture but not with the incoming cold air during winter.

Pulling in pure outside air will assist with IAQ. However, it comes with higher operational costs. There is no free way to bring unconditioned air into the home :)

An ERV can be tied to the furnace fan to operate in unison. Introducing fresh air into your HVAC return duct at a temperature/moisture close to the interior condition.

Fresh air taps from the outside to the return are the cheapest option for bringing in fresh air. However, they come with their own set of operational cost and interior comfort problems.

Thank you so much, great information
Sorry I meant thank you Luke

Allow me to add to what's already been said. Typically I recommend an ERV or HRV for ventilation, depending on climate and other factors. Either will bring incoming air temperature closer to inside air, but the ERV also reduces moisture content of the ventilation air, thus avoiding the need for a dehumidifier in most cases (ultimately, that's going to depend on envelope tightness, internal moisture loads, effective use of spot exhaust, and how much moisture the A/C removes, which depends on HVAC design, especially sizing A/C close to the actual load).

Although HRV/ERV installation guides typically show an option to connect to HVAC ducts, I don't recommend doing that as it often creates issues with air balance, especially if HVAC operates at low static, as is best practice.

CFIS (central fan integrated supply) ventilation systems are passive, i.e., outside vent connects directly to the return duct, as you described. Here's a white paper on CFIS from Building Science Corp. We had some in-depth discussions on pros and cons of CFIS in the Home Performance Forum (search the HP Forum Archives for CFIS, linked in the main menu above, if you want to read more about this.

As noted by others, CFIS requires conditioning of the incoming air. If you don't run the HVAC blower except when there's a heating or cooling load, there will be many hours when there's no ventilation, especially during spring and fall. OTOH, if you use a cycle timer to run the blower off-cycle to draw in ventilation air, then you'll likely to end up needing a central dehumidifer, which probably costs more than a good ERV in first-cost. More importantly, you end up paying more to run the blower off-cycle, plus the cost of operating the dehumidifier, which is not insignificant in your climate.

Lastly, the entries in REM are very different for CFIS versus ERV. Once you decide which way you want to go, we can provide specific instructions on how to set it up in REM.

David is absolutely correct that ERV systems independent of the furnace fan will always perform better. Best case scenario for efficient fresh air -- installing the HRV/ERV to extract independently from bathrooms and the kitchen. While simultaneously supplying to the living room and bedrooms. This has the highest upfront costs but it also allows you to eliminate energy wasting bathroom fans.

Linking to the furnace ducting can create issues with airflow balancing and fresh air distribution. But, it is a least expensive way to install an ERV in a retrofit...Which was my reason for recommending that strategy. If you are doing a new construction with open walls and room to route ducting; Always install the ERV to operate independently and run continuously (boosting the ERV for bathroom use, heavy cooking, raised occupancy etc).

It's certainly possibly to connect an HRV/ERV to a furnace or A/C duct system and make it work. The installation guide specifies a certain distance to space the connections from the blower to minimize interference, although the potential for interference at a given separation depends largely on blower static relative to ventilation system static. It's easy to test this.

Luke wrote:

> (ERV)  "...extract independently from bathrooms... allows you to eliminate energy wasting bathroom fans."

It's not a good idea to rely on an ERV as primary exhaust for high-use bathrooms. By design, the ERV will recycle a high percentage of shower moisture back into the fresh air stream and distribute to the house (because shower exhaust will almost certainly have higher dew point than incoming fresh air).

I've raised this issue repeatedly here and other home performance online communities over the past six years, after my brother forked out north of $20,000 to remediate serious mold damage to his home, the direct result of an ERV as only exhaust in his master bath. Once he got rid of the mold, I had him install a conventional exhaust in the bath and there's been no recurrence of the mold.

When you stop to consider how an ERV works, the risk should be obvious, although most ERV manufacturers and virtually the entire home performance industry seem to think this configuration is OK. Panasonic is the notable exception. The install guide for its Whisper Comfort ERV prohibits use in high moisture areas such as a bath or laundry. Kudos to Panasonic. Other manufacturers should take note.


I'm not sure I can agree with the idea that an ERV operating continuously will distribute moisture from periods of showering in any quantity that would lead to an interior moisture issue.

Generally, most ERV systems are pulling air from 3-5 locations. So, even with a shower operating -- the humid air would be diluted by the other extraction locations before reaching the ERV core. Further, periods of showering are probably only taking place 3% to 5% of each day. The other 95% of the time...your ERV should be operating without the high moisture from showering. During the small amount of time showers take place....the ERV is only going to exchange a percentage of moisture. I'll estimate most systems will transfer between 25% and 75% of moisture. Whatever percentage is not transferred will be sent into the lower moisture air stream. During winter operation (dry exterior condition) some of the moisture will come back in with the fresh air to help maintain a comfortable interior, the rest will be exhausted from the residence. In comparison, an HRV would be directing 100% of moisture in one direction or the other. I would argue some moisture control is always going to be better than no moisture transfer.

During a humid summer exterior condition (like the original poster), an ERV will block some of the exterior moisture from entering the home with his fresh air. An HRV would bring in 100% of the exterior moisture with his fresh air. Blocking some moisture from entering, in this scenario, leads to lower interior dehumidification and cooling loads.

The Panasonic whisper comfort ERV is only meant to exhaust from one bathroom (I think). They will not have any dilution from other exhaust locations, and maybe this is why they don't recommend a single bath use?

An ERV is a passive heat/moisture transfer machine. If humidity is high inside, and humidity is high de-humidification can take place through the ERV. As the moisture content is too high in both conditions. An active dehumidifier would be needed in that scenario or rely on the cooling system to dehumidify.

From a standpoint of several thousand ERV installations..I can say that generally most houses that directly exhaust from bathrooms do not have issues with the interior shower moisture causing problems to the larger home because of moisture transfer in the ERV. However, there can always be outlying conditions and scenarios where anything is possible. There is no silver bullet across the board explanation for why one project might experience a unique issue.

IAQ is much more than temperature, humidity levels and observed odors. What long-term, measured evidence led this builder to come to this "solution"? Indeed, what long-term, measured evidence leads any IAQ professional to recommend/implement their "solutions"? If you don't measure you are guessing, or at worst, going strictly on some firm's marketing "one size fits all" logic. If you implement a "solution", has the issue actually been eliminated, or does it reappear cyclically/seasonally? Or have different issues emerged (a la David Butler's brother)? You don't know if you don't measure; you're just guessing (and not at an educated level).

Thank you so much


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