More Building Science
We come at residential design and construction from very different backgrounds—a professor of business and a practicing architect. Our knowledge and experience are complementary when we collaborate on projects to analyze the economic and ecological impacts of competing building methods and materials. Our goal is to apply the analytical tools of economics and finance to uncover practical design solutions for homes. We recently took this approach to a study of balanced ventilation, specifically the use of an energy recovery ventilator (ERV) or heat recovery ventilator (HRV) to provide fresh air.
Our opinion is that the best way to configure a residential ventilation system is integrate an ERV or HRV with the heating and cooling ductwork, assuming the house has a forced-air system. This is the most economical approach. This approach also allows for better filtration than an ERV or HRV can provide on its own because they do not work with fine particulate filters. And finally, a whole-house duct system can better distribute fresh air to all areas of the home, especially bedrooms, where it is needed the most.
In this article we’ll discuss the need for ventilation based on the carbon dioxide (CO2) levels found in most homes, the challenge of integrating an ERV or HRV into a common duct system, and some potential solutions that allows us to keep the systems integrated.
CO2 and indoor air quality
All inhabited buildings need ventilation. In addition to historically-known indoor pollutants (CO, VOCs, molds, etc.), CO2 has more recently emerged as a threat in buildings with increasingly tight thermal envelopes. Whereas most indoor pollutants accumulate slowly, CO2 rises rapidly from respirating inhabitants. This places CO2 on the critical path to a solution.
While not the most dangerous of indoor pollutants, if enough air is exchanged to keep CO2 in a healthy range, other indoor pollutants will be kept in check. Air pollutants disperse to all areas of a home, so this requires a whole-house ventilation system. ERVs and HRVs offer balanced ventilation (equal supply and exhaust) while minimizing energy loss. Best practice is to draw the exhaust air from bathrooms, creating negative pressure where desired, while minimizing thermal envelope penetrations from individual bath fan vents.
We prefer that the ERV or HRV is integrated into the homes forced air duct system. This way, all incoming supply air passes through a high-efficiency particulate air (HEPA) filter mounted on the return side of an air handler, and a UV light on the coil—our preferred system design. This avoids diminished cognition from high CO2 levels by exchanging air, reduces vulnerability to breathing problems and disease caused by NO2, O3, and VOCs by flushing indoor air, and it reduces allergies by cleaning the air of pollen and other irritants. With this arrangement, all of the air circulating in the home, both existing and freshly introduced, is effectively filtered and cleaned. However, it requires that the HVAC blower is operating.
Integrating the systems
When ERVs and HRVs are integrated in this way, manufacturers’ specify an installation protocol that delivers fresh air into the return side of the ducted forced air system, and upstream from the filter (see illustration from Honeywell installation manual below). That is ostensibly for two reasons: first, it would route outside air through the filter before distribution and, second, the fresh air infusion point will be at negative pressure when the blower fan is operating, and that avoids the potential for air pressure to push against the ERV or HRV fan. That specification works effectively when the air handler blower is on, which then operates in tandem with the ERV or HRV fan and draws the incoming fresh air through the filter before distribution to the house and its occupants.
The problem arises when the ERV or HRV is operating but the blower fan is not, and this commonly occurs in most homes. Whether the ERV or HRV is run continuously or cycles on because of a CO2 sensor or timer, it will operate when needed to exchange air, and that is independent of the blower fan which operates on thermostat conditions.
Forced air will flow to the path of least resistance. During conditions when the ERV or HRV is operating but the blower fan is not, the incoming fresh air back-feeds through return air ducts, out through return grilles, and into the house unfiltered. In the absence of an operating blower fan, the filter creates resistance, and the freshly injected air flows out of the return grilles, causing two specific problems. First, the raw outside air is not filtered before entering the home, which in high pollen seasons will aggravate allergy sufferers. Second, the air exchange may not occur in the spaces where it is most needed. The highest need for air exchange is in bedrooms, where occupants spend the largest number of consecutive hours, and often behind closed doors. While some forced-air systems are designed with returns in bedrooms, most installations in the U.S. employ just a few central returns in the large common areas. That design, when an ERV or HRV is integrated and active without the blower fan, does not adequately ventilate bedrooms.
One of our recent projects illustrates this problem. The home had a forced-air system with a MERV 13 filter, and occupants with allergies. When the HVAC blower fan is off, fresh air from the ERV exits return grilles and not supply registers, and the outside air enters the home unfiltered. The HVAC design employed three central returns, and bedrooms received no air exchange when doors were closed, and only minimal and insufficient air exchange with doors open. By installing ERVs to equipment specifications, outside air is injected into the home unfiltered when the blower fan is not operating. This occurs regardless of whether returns are central or distributed. And if there are no returns in bedrooms, the system does not sufficiently exchange air in those spaces where it is most needed.
We engaged the ERV manufacturer about injecting the fresh air into the supply side of the blower, but they responded that it would void the warranty because of concern with opposing fan pressures. Blower fans are typically much stronger than ERV and HRV fans. In addition to possibly damaging the weaker fan, HVAC-conditioned air could back-feed through the ERV or HRV and be wasted to the outside.
We prefer integrated ventilation as opposed to stand-alone systems for two critical reasons. First, ERVs employ a very coarse internal filter that does not scrub pollen and other airborne pollutants, and their fans are not sized sufficiently to push air through heavier HEPA filters that catch small particulate matter. Consequently, a stand-alone ventilation system would create more suffering for occupants with allergies. Second, the supply of fresh air needs to be ducted to all indoor spaces, and especially to bedrooms. A stand-alone system with that much ducted distribution is redundant and wasteful when a forced-air HVAC system could have been employed in complement. An integrated system avoids these concerns, but we need a solution to the back-feeding problem detailed above.
Solutions and tradeoffs
One solution could be to run the blower fan at all times, but that is indiscriminate and energy wasting. We recommend adding a control to automatically operate the blower fan whenever the ERV or HRV runs. This unspecified fix requires a relay switch and wiring, and it solves the problem for less than $100. Unfortunately, ERV and HRV manufacturers do not note this limitation or provide instructions to avert the problem. As we collect data on homes with mechanical ventilation, we have not discovered this remedy elsewhere.
There are scientific reasons for the International Code Council’s requirement that buildings employ mechanical ventilation, but it creates several measurable tradeoffs. First, there is additional upfront installation cost. Second, there are higher energy costs for operation. And finally, there are higher energy costs to do more conditioning because while ERVs and HRVs recover some of the energy in exhausted air, it is not full recovery. Still, the comprehensive energy needs of operating an ERV or HRV in a tight house today are lower than trying to condition the leaky buildings of the past. Mechanical ventilation is best integrated with a distributed forced-air system to serve all conditioned spaces, even when doors between rooms are closed. This may be an additional cost, depending on what other conditioning systems are considered.
ERVs and HRVs use energy when they operate, so they should not operate more than is necessary to keep CO2 levels below 950 ppm. Recall that if ventilation successfully caps CO2 levels, other indoor pollutants will also be adequately mitigated. Rather than running an ERV or HRV indiscriminately, it can be designed to operate on a sensor or timer switch. In either case, the ERV could be called into action when the blower fan is not operating, and that creates the dual problems of unfiltered air and inadequate distribution. The tradeoffs of excessive ERV or HRV use are fewer years of operable equipment life, more pollen and other outdoor airborne pollutants pulled inside, more energy wasted in operation of both the ERV or HRV and other HVAC equipment, and possibly contributing to moisture problems in high humidity climates or seasons.
The case house noted above, which is average in size and number of occupants, requires the ERV to run only 30 hours each week, or 18% of the time. A timer was installed with ERV run times skewed heavily toward overnight hours to keep bedroom conditions healthy. Because the exhaust pull is from bathrooms, with booster switches, routine bathroom use adds additional ERV operation beyond the scheduled 30 hours/week.
The bottom line
Any house built with detailed attention to air infiltration must add mechanical ventilation. Most home dwellers in temperate climates would benefit from an ERV or HRV, but they are not required by many local codes and are still too rarely installed. The data indicate that new and recently-built homes in the U.S. have an undiagnosed CO2 problem when not augmented with effective ventilation. For reasons discussed in this article, mechanical ventilation is best integrated with a whole house forced-air ducted HVAC system, but there is a crucial link missing from installation specifications.
The simple but critical solution is to control the HVAC blower to operate whenever the ERV is in action. This protocol effectively filters outside air before distribution to living spaces, and it effectively exchanges air from all supplied rooms. We further argue that continuous and indiscriminate use of mechanical ventilation is inefficient, energy wasting, and introduces net negative returns. Effective and efficient residential ventilation is achievable with either sensor or timer activation, and we’ve demonstrated the critical importance of linking the operation of the ERV or HRV with the HVAC blower.
James Leaman, Ph.D., is associate professor of business at Eastern Mennonite University. Charles Hendericks, AIA, CSI, CGP, LEED AP, is principal at Gaines Group Architects.
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
27 Comments
"a MERV 13 HEPA filter"
Note to the authors: If you are using "HEPA" in a nonstandard way, you might want to mention that. I do not believe there is such a thing as a "MERV 13 HEPA filter." Best information I can find is that for something to qualify as HEPA, it would need to be at least MERV 17. See, for example, https://www.iso-aire.com/blog/what-are-the-differences-between-a-merv-13-and-a-hepa-filter.
Thanks aunsafe2015; helpful clarification.
GBA has long taken the stand that two things should be avoided if possible; installing an ERV on a forced air furnace, and installing high resistance filters on forced air systems. What changed?
MERV 13 is not a crazy level of filtration to put on most forced air systems, but a real HEPA filter (MERV 16+) certainly is.
I think this article is confusing and potentially misleading readers.
GBA does not promote a single way of building, they share relevant articles that may provide food for thought, even if they go against conventional advice. I don't know any building science experts who think adding an ERV to a forced air furnace is best practice or even good practice, but it can be reasonably effective if other approaches are not practical for some reason. I think the author lays out a good argument for why they choose this route. I also don't know any building scientists who would call MERV-13 a HEPA filter.
Agreed, however in this case the author is a GBA Editor, therefore more speaking on behalf of GBA and less of just covering an alternative viewpoint. My impression of this article was more of a recommendation for doing it this way, which seems in contrast with GBA articles of the past.
I think it may have been good to mention this is not the best approach, but can work if no other options exist and is executed properly.
Hi James, Charles,
This is a very timely article for me as an earlier post (https://www.greenbuildingadvisor.com/article/ducting-hrvs-and-ervs) discouraged a hybrid ducted system due to lack of general awareness in the trades and overall install complexity and I was ready to accept that maybe its worth the expense of full ducted to make commissioning as error proof as possible.
While the post gives me hope, is there anything that the community has learned since 2018 that may have shifted the thinking towards this and away from fully ducted ventilation?
Thanks, jdailyjr, for your comment,
The community continues to learn about the cross-section of spatial need for fresh air and dispersion of fresh air from various ducted designs. I think we are also more sensitive to resource use (both ecological and financial), which encourages solutions through a single ducted system. Finally, much greater accessibility of CO2 sensing/meter systems have helped refine ventilation system design both from a distribution and intermittency perspective.
Amy word about commissioning? How would you commission such a system with a variable speed blower? Thanks
My experience with commissioning has only been with commercial buildings. Have you seen commissioning in residential projects? Speaking as an architect and not an engineer or commissioning agent, I would imagine one could design a test using the lowest fan speed to evaluate performance of the system to verify compliance of the system to test design compliance?
I agree that if you can get enough ventilation at low HA fan speed you should be ok at higher air velocities. The creation and implementation of a standard commissioning procedure is a MUST for the diffusion of such integrated systems. It would guide the average HVAC installer that does not hold a degree in engineering and help home owners understand if their system is working the way is intended. I am sure there is some smart guy in GBA that could start developing that ;)
Commissioning is common on high-performance homes, and should be standard practice on all homes.
Panasonic ERVs include contacts to switch the G (fan) wire on a furnace/air handler. Maybe other brands do, too. See wiring diagrams on pg.6 of https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Panasonic-FV-10VE2-Install-Instructions.pdf
The main problem with integrating an ERV with an existing HVAC system in a humid climate (like Harrisonburg), in my experience, is that you end up running the fan when the A/C compressor is off. That re-evaporates the moisture on the coil back into the room air. I ended up with persistently higher relative humidity, which I tried to control with the dehumidification mode on my A/C. The energy penalty was significant. I've since ducted the ERV separately and it's much better in almost every regard. Air distribution — unsurprisingly — isn't as good without the air handler running whenever the ERV is.
I've added a filter box with a MERV 13 filter to the fresh air duct, so I'm getting good filtration without needing to rely on the filter in my central system.
ERVs can be tough in a retrofit scenario. I think for most households that don't want to spend (or have) the money to properly duct an ERV, just stick in a bath fan on a timer and be done with it. It's not as elegant as an ERV, granted, but much more attainable for typical housing.
Good points but leaving the coil damp for prolonged time does not sound a good thing either in terms of air quality and in a humid area a. dehumidifier is a must anyway
Our operating and study environment is west-central Virginia (Harrisonburg region), and thus far we have not encountered high indoor humidity from integrating the ERV with a ducted HVAC system (without dehumidification). If humidity rises significantly from such a setup, I would ask 1) whether the ERV is adequately scrubbing humidity, and/or 2) is the ERV running more than is necessary to cap CO2 at healthy levels?
Curious about the MERV 13 filter box on your ERV...given the significant airflow resistance, have you tested a level of lost efficacy? Does your ERV (brand and model) advise for or against filtering in addition to its own?
The ERV doesn't "scrub" humidity when it's more humid outside. There will always be a gradient from high to low for sensible and latent energy. So, when it's 85º and 60% RH outside and 75º and 50% RH inside, the ERV will (slowly) warm and humidify the indoor air.
My ERV is set to run continuously at the ASHRAE 62.2-2016 rate and I have a few CO2 monitors to confirm that I'm getting adequate air changes, especially in bedrooms. I'm not wedded to the 62.2 rate, it just happened to be pretty close most of the time.
Panasonic sells an optional internal MERV 13 OA filter. It's tiny and expensive. The filter box takes a 10x20" filter that has a much larger surface area and the filters are cheaper, too. The pressure drop is insignificant, and lower than the factory filter that I'm not using.
Very helpful feedback. How common do you think your setup is in the industry; are you aware of systems other than your own that have employed MERV 13 filtering directly with the ERV?
James, all ERVs I am aware of have filters. Usually MERV-8 is standard and up to MERV-13 is available. Do you have experience with stand-alone balanced ventilation systems?
Michael, yes, with both stand-alone and integrated systems.
Zehnder uses a MERV 13 on the outdoor intake side, and has an option (if using their distribution system) for adding a MERV-15 filter on the room supply side. Presumably this arrangement would cause the (more expensive) MERV-15 to last considerably longer.
Bravo -keep it real - so glad you mentioned the humidity problem. Ervs are not air conditioners not are they dehumidifiers ! Too much of gba/ Hugh performance home dialogue is northeastern climate centric (double stud walls, ervs, Hardie board cladding and no mention of stucco, etc) vast majority of homes need dehumidification esp,in basements aside from even properly sized air conditioning. And all ervs should have a recirc mode. NB also vast majority of hvac contact rose evn reputable lens, wont want your erv messing with their hvac system sizing ducting. Look if you have your erv ducting to just dedicated two outlets and two dedicated returns , that’s not a hard ducting project in new house;and NB ervs can easily increase humidity load in many climates in summer even with 90% actual efficiency; if it’s 80m% rh outside and only 60 inside, your erv is still delivering air that is mroe humid than in your house ! One other benefit of a senate ducted erv, you cna run constantly and get good overall air exchange circulation even when hvac not running , esp if you locate the supply and returns far apart and at different heights.
Good info but one issue not addressed is operating an ERV unbalanced to provide positive indoor pressure that compensates for the negative pressure caused by exhaust fans (bath or range). I think I’ve seen only one ERV designed to do this (forget the make) and it was limited to a couple hundred CFM I believe. I guess part of the problem is if you operate it unbalanced, you also compromise the energy exchange ability and basically turn it into a glorified intake fan.
Some day I’m hoping for a smart system that can harmoniously integrate exhaust fans, central HVAC and ERV in a retrofit… probably a pipe dream though.
You can roll your own if you're willing to rabbit hole a bit but no "smart" system exists yet out of the box. I've sorted a setup that does run an HRV imbalance for makeup air, reacts dynamically to kitchen exhaust (kitchen exhaust automated to induction range use and speed varying based on induction power use) and does react to CO2, Radon and VOC.
James mentions: "Recall that if ventilation successfully caps CO2 levels, other indoor pollutants will also be adequately mitigated. Rather than running an ERV or HRV indiscriminately, it can be designed to operate on a sensor or timer switch."
What I've learned is that is not necessarily true, particularly if occupants are away. After observing our system over winter and summer with CO2, VOC and radon monitoring (one sensor per floor, 3 floors) I had to modify controls a bit so that the system ramps down vent levels, but stays at 50 CFM longer until CO2 levels are below 450 ppm, radon stays below 120 Bq/m3, and VOC levels stay below 200.
The big enabler for me was figuring out that I could control EC fans from AC Infinity and Terrabloom (Fantech as well, by another gent) using 0-10 V control, wireless, via two devices that are relatively new to the market: https://community.hubitat.com/t/success-0-10-volt-control-of-ac-infinity-or-any-ec-fan-motor-using-leviton-zs057-d0z-zigbee-dimmer-or-zooz-zen54-zwave-0-10v-dimmer/104450
Previously I was only monitoring CO2 and the system would turn off prematurely so VOC, and radon levels would rise noticeably if we were away. After a few days away and returning, the air in the house was noticeably a bit stale. Several contributors here have mentioned the same issue, and I did pretty much confirm this after nearly a year of looking at live data. With a few sensors and EC fans controlled, the system in play varies balanced air flows from 50,65, 75 and 100 CFM. It imbalances to provide 80-90 CFM of makeup when required by the kitchen exhaust. The house is fairly tight...
I would never install an ERV into the HVAC system ducting. There are too many issues that occur from improper installation and maintenance and lack of commissioning. In North Alabama, we fight humidity most of the year. We want bathroom fans, in short, straight, hard-ducted runs to the exterior and with timers to make sure the fan removes the moisture once the occupant leaves. We want multi-stage, inverter driven heat pumps with oversized returns and jumper ducts in every bedroom. We recommend installing ventilating dehumidifiers with its own dedicated duct system along with fresh air vented straight into the return side of the HVAC system. In some times of the year, due to high humidity, we close the damper down on the straight air supply. Ventilation coupled with humidity control in hot, humid areas can be done but it is best to keep it simple.
Do we have any statistical data showing how ERV/HRV are installed? My bet is that shared duct systems are actually more common than dedicated duct systems due to cost limitations in spite of the known issues. In my opinion, these issues are all potentially fixable with good guidance and protocols for installation AND commissioning. I am actually surprised that no one has done it yet.
Todd makes sense…re ventilating dehumidifier, assume you duct its exhaust into the hvac supply ducting as far away from air handler as possible.? Are you concerned about back drafting when the .>>more powerful hvac blower turns on?
I was left wondering re the potential for the bigger return blower fan damaging the smaller ERV/HRV fan. I assume that the smaller fan can be disabled and just use the return blower.
I also assume that the ERV/HRV filter can be removed to eliminate that much resistance and simply rely on the return filter instead.
I assume that the return blower can be operated by CO2 sensor to insure operation even if heating or cooling is not needed and thus the return blower wouldn't normally activate.
Feedback welcomed.
How bad of a solution is it to run a high efficiency blower 24/7 at low speed (and thus low power)? Doesn't that solve the problems mentioned here?
Alternatively, with respect to damaging the ERV fans- would that still be a problem if it is inserted into the supply with an injection port style? https://www.greenbuildingadvisor.com/article/integrating-hrvs-with-air-handlers
Log in or create an account to post a comment.
Sign up Log in