Ensuring Fresh Air in Bedrooms

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Ensuring Fresh Air in Bedrooms

New CO2 measurements raise concerns about bedroom air quality

Posted on May 18 2018 by Martin Holladay

Green builders try to make their homes as tight as possible. To ensure good indoor air quality, green building programs (and many residential building codes) require new homes to have a mechanical ventilation system.

The aim, of course, is to provide enough fresh air to keep occupants healthy. To gauge whether a ventilation system is effective, many researchers measure indoor carbon dioxide (CO2) concentrations. (When humans and pets breathe, they consume oxygen and exhale carbon dioxide. The more people in a room, and the lower the ventilation rate, the higher the CO2 level is likely to be.)

There is some controversy around the question of how high CO2 levels need to be before the situation becomes worrisome. That said, there is increasing evidence that levels over 1,000 ppm raise concerns. (For more on this topic, see Stuffy Offices Lower Cognitive Function.)

CO2 levels are easier to measure than many other indoor air pollutants, so researchers often measure CO2 levels to determine whether a whole-house ventilation system is delivering enough fresh air to individual rooms.

Since high CO2 levels are associated with low ventilation rates, a high CO2 level is a warning sign that something may be wrong with indoor air quality.

People spend a lot of time in bed

Ideally, a ventilation system will provide fresh air to every room in a house. That said, some rooms are more important than others. For example, if a house has a pantry, it is probably occupied for only a few minutes a day. A bedroom, on the other hand, is usually occupied for at least 8 hours a day — making bedroom air quality far more important than pantry air quality.

Researchers interested in indoor air quality have long worried about the amount of fresh air reaching bedrooms, especially in homes with exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. systems. It’s usually assumed that homes equipped with an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or an ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. — especially if the system has dedicated ventilation ductwork — has fewer problems with bedroom air quality than homes with exhaust-only ventilation systems.

Aiming to keep CO2 under 1,000 ppm

The latest researcher to look at bedroom air quality is Brian Just, a mechanical engineer who manages Efficiency Vermont’s residential new construction team. He reported his findings in a presentation, “The drawback of breathing: Nighttime carbon dioxide levels in bedrooms in 22 Vermont homes,” at the recent NESEA-sponsored BuildingEnergy conference in Boston.

Before delving into his findings, Just discussed the question of what level of indoor carbon dioxide is concerning. He told the audience in Boston, “It’s true that other stuff is worse than carbon dioxide — for example, formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen.", particulates, and radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles.. But CO2 is easy to measure, and it’s a decent proxy for lack of ventilation. So how much is too much? Carbon dioxide isn’t considered toxic until it reaches 40,000 ppm, but ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. and some building codes usually target 1,000 ppm.”

Just cited several studies showing reduced cognitive functioning at CO2 levels above 1,000 ppm, including a 2016 study by Joe Allen. Just concluded, “I say that anything above 1,000 ppm is too much. At that level, sensitive individuals may experience symptoms. And at that level of CO2, other pollutants may also be elevated.”

Door open vs. door closed

Just obtained some equipment to record indoor CO2 levels — a TSI VelociCalc 9565-P meter and data logger with an accuracy of ±3% or 50 ppm, whichever is greater. The meter takes a reading every minute.

Before undertaking his study, Just decided to measure CO2 levels in his own bedroom. Occupancy consisted of “two people plus a dog.”

Just shared the results of his six-day monitoring exercise in the form of a graph. He added two red arrows to the graph: Arrow #1 indicates when the bedroom window was opened, and arrow #2 indicates the CO2 spike that occurred after the bedroom door was closed.

Just explained, “I procured some equipment and tested it out in my own home. What’s clear is that closing the door made a huge difference. When the door was closed, the CO2 shot up to 1800 parts per million.”

Each bedroom was monitored for four nights

Just obtained permission to monitor CO2 levels in 22 Vermont homes. At each of these homes, he first performed a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. The homes ranged from relatively tight to quite leaky (more than 3 ach50). He also inspected the existing ventilation systems and measured the volume of each bedroom.

Finally, he inspected the existing heating systems. Ten homes had heating systems (for example, a furnace or an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps.) that moved air mechanically, while twelve homes had heating systems (for example, a boiler or wood stove) that didn’t move air mechanically.

After the blower door test, there were four nights of CO2 testing in the bedrooms. Participating homeowners all agreed to alternate the position of the bedroom door each night, so that the bedroom door was closed for two nights, and open for two nights. The homeowners also agreed to keep their bedroom windows closed. (The test was conducted during the winter.)

Homeowners reported the occupancy level of each bedroom. School-age children and adults were each counted as 1 person; infants, toddlers, and dogs were each counted as 0.5 person.

Most bedrooms had high levels of CO2

Measured carbon dioxide levels were high. All of the bedrooms had CO2 concentrations exceeding 1,000 ppm on at least one of the four nights. At the majority of homes, the CO2 level was significantly lower on nights when the bedroom doors remained open; yet only one of the 22 homes stayed below 1,000 ppm on both open-door nights.

Graph of CO2 levels in one of the monitored bedrooms

Just said, “The ‘door closed’ levels were significantly worse than the ‘door open’ levels. There were big CO2 spikes with the doors closed. At 19 of the homes — that is, 86% of the homes — the CO2 level sometimes exceeded 2,000 ppm on the door-closed nights.”

At 32% of the homes, the CO2 level exceeded 3,000 ppm.

The above graph shows all of the data from 22 homes.

Common theories are debunked

At the presentation in Boston, Just addressed three common reactions to his findings:

  • Maybe leakier buildings have better air quality than tighter buildings.
  • Maybe homes that have heating systems with fans — that is, heating systems that move air around — have better air quality than homes with heating systems that don’t have fans.
  • Maybe large bedrooms, or bedrooms with few people in them, have better air quality than small bedrooms or bedrooms with more occupants.
  • As it turns out, the data don’t support the first two theories. “What about building tightness?” asked Just. “Some people think this isn’t much of a problem in older homes, but that’s not true. The leakier homes also have this problem.”

    Just continued, “The theory that mechanically moved air helps with air quality is also untrue.” In a published article on his study, Just wrote, “[T]here was little apparent correlation between airtightness of homes and CO2 concentrations. Comparing homes with heating systems that mechanically ‘move’ air … with those that do not … yielded similar results. Data did not show that one system resulted in ‘better’ indoor air quality.”

    Just told the Boston audience, “When it comes to the number of occupants and bedroom volume, the theory, to some extent, is true. Carbon dioxide levels did go up with more occupants. If you double the number of people in the bedroom, you get higher CO2. So at our house, the dog is now out of the bedroom.”

    In his article, Just wrote, “[T]here was a tendency for peak CO2 level to be higher in homes with more people and smaller bedrooms. However, with doors open — i.e., reduced ability to ‘trap’ air inside the bedroom as people exhale CO2 during the night — occupant density and bedroom volume had little effect.”

    Good ventilation systems help

    The last theory that Just discussed is that homes with a balanced ventilation system (which usually means an HRV or an ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.) have better indoor air quality.

    Just said, “Yes, this is probably true — especially if the ventilation system was installed following best practices.”

    Just reported there was a “dramatic” difference in CO2 levels between the homes without decent ventilation systems and the three homes that (a) had ventilation systems with automatic controls and (b) ventilation rates equal to at least 50% of the ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. standard.

    The CO2 levels were lower in the three homes with the best ventilation systems. “Intentional ventilation helps if the system is properly designed,” said Just. “But none of the homes in the study would have passed the Vermont ventilation code. I’ll repeat: nobody meets code. Of the 22 homes, 10 failed the spot ventilation test, and 21 failed the whole-house system test. None of the homes passed both tests.”

    Testing CO2 levels in homes with balanced ventilation systems

    A year after completing his first monitoring study of Vermont bedrooms, Just decided to conduct a follow-up study focusing on homes with above-average ventilation systems. Three of the homes in the new study were equipped with either an ERV or an HRV. The first was a new house with a Zehnder ComfoAir200; the second was an older house with a Zehnder ComfoAir 350; and the third was an older house with a Broan ERV.

    These three homes had much better results than the first batch of 22 homes. In the three homes with balanced ventilation systems, CO2 peaked at 1,200 ppm.

    Just also did some testing in a house with four occupants (two parents and two children). The house had a pair of Lunos fans installed in the master bedroom. Each child’s bedroom had one half of a pair of Lunos fans. “The Lunos fans are helping, but the CO2 is still getting up in the 1,800 ppm range,” said Just. He guessed that the relatively poor performance is due to the fact that the owners keep the Lunos fans on low speed. “The medium speed would have better results, but the medium speed is noisier,” said Just. “Almost no one uses the Lunos fans at high speed. High speed is too noisy to use.”

    The children’s bedrooms had even higher CO2 levels than the master bedroom — especially when the door was closed. Under that situation, CO2 levels rose above 2,300 ppm.

    What about the CERV?

    Finally, Just measured CO2 levels in three homes with CERV units. These demand-controlled ventilation appliances are set to ventilate whenever the CO2 levels rise to 1,000 ppm. (For more information on the CERV, see A Balanced Ventilation System With a Built-In Heat Pump.)

    “The results are boring,” said Just. “The unit comes on for a few minutes whenever the CO2 rises to 1,000 ppm. The CERV doesn’t run often but really works well.”

    Just’s takeaway points:

    • Recent evidence shows that CO2 levels above 1,000 ppm are undesirable. High CO2 levels can be an indicator of other pollutants in the air.
    • Most homes have suboptimal ventilation. Most existing ventilation systems leave a lot to be desired.
    • Consider fresh air distribution (especially distribution to bedrooms) when designing a ventilation system.
    • A central unit with dedicated ducting performs better than a Lunos system.
    • A good balanced ventilation system is a no-brainer for new construction. Such a system belongs in every home.
    • Although Just didn't emphasize the point, the data are clear: nothing beats the CERV when it comes to performance.

      Until recently Just has been advising people, “If nothing else, at least keep your bedroom door open at night.” Yet he told the Boston audience that he became more reluctant to share that advice when he learned that fire safety advocates have a campaign urging everyone to keep their bedroom doors closed at night. The campaign is called “Close before you doze.”

      Which advice is best? You decide.

      Martin Holladay’s previous blog: “Building a Passive House for $163 per Square Foot.”

      Click here to follow Martin Holladay on Twitter.

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    Image Credits:

    1. Image #1: Rileyroxx / Flickr.com

    May 18, 2018 8:20 AM ET

    A responsive system seems best
    by Skip Harris

    General rules of thumb never work as well as a measure-and-respond system (CERV, for example). Next best might be a balanced system that will bring in all fresh air through bedrooms and out through bathrooms.... but then one needs to keep the doors open or cut high.

    May 18, 2018 10:07 AM ET

    O2 Chamber
    by Brian Knight

    Thank you for the excellent summary of Mr. Just's findings. This helps prove that pop singer Michael Jackson was genius.

    May 18, 2018 10:13 AM ET

    Edited May 18, 2018 10:30 AM ET.

    Response to Brian Knight
    by Martin Holladay

    Since I don't make a habit of reading tabloid newspapers, I was unfamiliar with the reports that Michael Jackson was rumored (at one time) to sleep in a hyperbaric chamber delivering 100% oxygen. So I had to do some Googling.

    Although your comment is tongue-in-cheek, I'd like to emphasize that there is no medical evidence that healthy people should sleep in a hyperbaric (above atmospheric pressure) environment, nor is there any evidence that 100% oxygen is the right gas to breathe.

    In the 1950s, hospitals put premature babies in incubators that were hooked up to oxygen tanks that delivered 100% oxygen. Many of these babies became blind after developing retrolental fibroplasia as a direct result of the 100% oxygen environment. The blindness is permanent. Hospitals no longer use 100% oxygen for premature babies.

    OK -- on to more serious issues.

    May 18, 2018 10:09 PM ET

    Does anyone make CO2 meters
    by Calum Wilde

    Does anyone make CO2 meters that can tie in with HRV/ERVs? That seems like it should have been an obvious step long ago. I'd love to have a system that ran based on feedback not assumptions.

    May 19, 2018 5:42 AM ET

    Edited May 19, 2018 5:43 AM ET.

    Response to Calum Wilde
    by Martin Holladay

    Lots of web sites sell what you're looking for. Google "CO2 controller."

    These devices are mostly purchased by people who grow plants indoors. (I wonder what species of herb they are growing?) The idea of these devices is to energize an appliance when the CO2 is too low -- so that the appliance can inject more CO2 into the grow room.

    To use this type of controller to control a ventilation system, you would first plug a relay into the CO2 controller, so that the relay turned an appliance off, not on, when the controller detected that the CO2 levels are low. Not too hard to set up.

    Here are links to one device that costs $221:

    AutoPilot CO2 controller

    AutoPilot CO2 controller instruction manual

    May 19, 2018 10:01 PM ET

    Edited May 19, 2018 10:09 PM ET.

    If CERV is reading this.
    by Andy Kosick

    I realize that their goal is to sell CERVs, but since there are a lot of HRV/ERVs out there that aren't going anywhere anytime soon, the company should package their sensor technology (WIFI and all) as a retrofit kit for existing HRV/ERVs. I have a feeling there would be market for it. I'm picturing a short section of duct with the touchscreen right on the side of it.

    I've felt for sometime that the most significant short coming in balanced ventilation is the controls, they seem to be either totally inadequate or ridiculously over complicated.

    May 21, 2018 6:51 AM ET

    Edited May 21, 2018 6:51 AM ET.

    Response to Andy Kosick
    by Martin Holladay

    Looking into the topic more deeply, I realize that demand-control ventilation (using CO2 sensors) is fairly common for commercial systems. There isn't any reason (other than cost) that commercial demand-control ventilation equipment couldn't be used for a residential system.

    Here is a link to a manufacturer of demand-control ventilation equipment:
    CO2 meter.com - Demand-controlled ventilation

    Here is a link to an article about code requirements for commercial ventilation systems: Demand Control Ventilation

    May 22, 2018 1:36 PM ET

    Is 1000 ppm the right standard for sleeping?
    by Reid Baldwin

    The studies I am aware of have set the target of 800-1000 ppm because that is the level at which cognitive function seems to begin decreasing. When I am sleeping, how much cognitive function do I really need?

    May 22, 2018 1:49 PM ET

    CO2 sensors
    by Trevor Lambert

    Optional CO2 sensors that integrate with the HRV/ERV are already standard offerings for the premier brands. Zehnder, UltimateAir and Jablotron all have them. I'm sure there are others.

    May 22, 2018 1:50 PM ET

    Edited May 22, 2018 2:02 PM ET.

    Response to Reid Baldwin (Comment #8)
    by Martin Holladay

    During his presentation, Brian Just cited one study that showed deleterious cognitive effects resulting from sleeping in rooms with CO2 above 1000 ppm. The Danish study was "The effects of bedroom air quality on sleep and next-day performance" by Strøm-Tejsen, Zukowska, Wargocki, and Wyon.

    According to the study's Abstract, "Objectively measured sleep quality and the perceived freshness of bedroom air improved significantly when the CO2 level was lower, as did next-day reported sleepiness and ability to concentrate and the subjects' performance of a test of logical thinking."

    May 22, 2018 2:08 PM ET

    Response to Trevor Lambert (Comment #9)
    by Martin Holladay

    Thanks very much for that information. The paragraph shown below comes from the User Manual for the Zehnder ComfoAir 200.


    Zehnder ComfoAir CO2 sensor.jpg

    May 22, 2018 3:22 PM ET

    Placing the CO2 sensor
    by Reid Baldwin

    I believe the CERV measures the CO2 in the air that is flowing through the ductwork, That works in a system like CERV that alternates between recirculation and ventilation, but wouldn't work in an ERV that alternates between off and on unless it came on periodically just to measure. Another issue is that if the ductwork is set up the normal way, it is measuring the CO2 of the bathrooms rather than the rooms that are occupied for long periods.

    A CO2 sensor in the family room wouldn't do a good job of turning on the ventilation when the bedroom levels get high, especially with closed doors. Similarly, a bedroom sensor wouldn't turn the ventilation on when you have a family room full of people. You probably need multiple sensors each of which can turn on the ventilation. Maybe the boost mode controllers for bathrooms should include a CO2 sensor.

    May 23, 2018 11:43 AM ET

    CO2 as a proxy
    by Jeremy Good

    Do you have support for this statement?

    Most researchers agree that CO2 levels (which are easy to measure) are a good proxy for levels of other contaminants in the air that may be more difficult to measure.

    This statement seems to be the safer one to make:

    ... high CO2 levels are associated with low ventilation rates ...

    Looks like some good info in this study. And particularly interesting to see some common assumptions/myths challenged. My own less scientific testing has shown that closed-door CO2 doesn't drop appreciably when the air handler fan (only) is running, but it does during a cooling call (same fan speed). This is for an A/C-only system; my heating is via boiler. My guess is that you need a temp delta to get a convection going to actually mix the air in the room and dilute the CO2. That seems to align with Just's second myth. (My house is/was so leaky that the stack effect obviates any CO2 problem in the winter.)

    My plan is to install an ERV after the attic is drywalled. In researching residential units up to ~100 cfm, it seems like there's a gap in the market for CO2 sensors and more sophisticated controls. I've used NDIR CO2 sensors in an IAQ prototype -- they are cheap. A Wi-Fi connected controller with a CO2 sensor could run as a standalone device or connect to a Nest or Ecobee via their respective APIs additional benefits.

    May 23, 2018 12:09 PM ET

    Return air registers in bedrooms
    by David Baerg

    I find it interesting that closing the door produced a CO2 spike even in homes with a forced air system. That suggests that there isn't much mechanical air movement into the room.
    -were they running the furnace fan on continuous low speed?
    -what was the weather like during the study? If heating demand was low and the fan was not set to run continuously, that would lead to poor movement.
    -were the door undercuts blocked by carpeting?
    -were there cold air returns in the bedrooms?
    I'm skeptical that the door open effect is due to CO2 diffusing out of the room. Wouldn't it be more likely that opening the door would allow the furnace fan to push air into the room? If that's the case, it suggests that bedrooms should have cold air returns or, failing that, significant door undercuts.

    May 23, 2018 12:32 PM ET

    Response to Jeremy Good (Comment #13)
    by Martin Holladay

    The two quotes you pulled from the article point in the same direction. I agree with you that the second of the two quoted passages is probably more technically accurate. The fact remains that if your bedroom has high CO2 levels, your ventilation system isn't doing its job -- and if there are other air pollutants of concern in your home, there's a good chance that your ineffective ventilation system is doing a poor job diluting the other pollutants, too.

    May 23, 2018 12:36 PM ET

    Response to David Baerg (Comment #14)
    by Martin Holladay

    I don't have answers to most of your questions. There's always a chance that Brian Just will see your comments and respond.

    I am hesitant to speculate, or to reach conclusions based on speculation. It's best to simply report the data.

    Just reported that CO2 levels are much higher in bedrooms when the door is closed than when the door is open. It's up to readers to decide whether that information is useful, or whether it's worth acting on that information when they go to bed each night.

    May 23, 2018 2:14 PM ET

    Response to Martin (comment # 16)
    by David Baerg

    I would agree that this is a very good starting point for further research. I think we need to go deeper than suggesting that people keep their doors open if there are good reasons to keep them closed (fire safety - as long as there is a smoke detector in the bedrooms - privacy, noise). Figuring out why opening the door reduces CO2 concentration is the next step to developing strategies to keep CO2 levels low without requiring the occupants to keep their doors open. I would start by answering the above questions, measuring the flow at the registers with the door open and closed, measuring the door undercut and recording if there is a return register in the room. Are returns in bedrooms common in the US? It's been common here since 90s.

    May 23, 2018 3:56 PM ET

    CO2 as a proxy
    by Jon R

    "Carbon dioxide is useful as an indicator of general air quality only in buildings where there are significant metabolic or combustion sources of carbon dioxide. "


    "Particulate matter ≤ 2.5 μm in aerodynamic diameter (PM2.5), acrolein, and formaldehyde accounted for the vast majority of DALY losses caused by IAPs considered in this analysis, with impacts on par or greater than estimates for secondhand tobacco smoke and radon."


    May 23, 2018 7:11 PM ET

    Great piece
    by Dan Kolbert

    I've long worried we don't really know what we're doing re ventilation - glad that studies like Brian's are starting to help.

    May 24, 2018 8:38 AM ET

    Misc. notes
    by Brian Just

    There is some thoughtful discussion here. A lot of the questions raised already have great answers given by others. I'll add a few notes:
    - I hesitate to make sweeping discussions based on 30 or so homes, but well-designed ventilation is the one variable that consistently resulted in lower CO2 levels.
    - We did not monitor heating periods, though if you look at data for individual homes (see the tail end of the slide deck I used for the BuildingEnergy Boston conference, available on the BuildingEnergy website) you can see behavior on some homes that could indicate furnace cycling. Note, however, that even if a furnace fan helps, you only get that benefit when heating/cooling or just running the fan (and as supply/return locations, undercuts, etc. allow). I wouldn't count on this strategy year-round, and even an ECM blower is going to consume substantially more electricity than most ventilation-specific equipment.
    - Regarding a closed loop system, anecdotally, colleagues tell me that the CO2 feedback sensors in commercial systems don't hold calibration well. In any case, I question whether it's a real need. See the image below Martin's "Testing CO2 levels in homes with balanced ventilation systems" header. None of these homes had CO2 feedback and all performed well. All had fresh air supply in the bedroom. All had a central system that consumed just 10-20W (one was a home in the 2016-17 study that retrofitted their home last year - the pre- and post-retrofit data shows a stark difference). While the CERV or custom system with CO2 feedback may work best for some, good performance can be achieved without it. You could always consider buying or borrowing a low-cost desktop CO2 monitor that you can move around over the course of a few days or weeks if you want to fine tune a home’s system. Some ventilation systems have a “party” or boost mode that addresses the problem of large gatherings.
    - The part around fresh air delivery to bedrooms seems key. In fact, the home with the second-highest CO2 peak in the 2016-17 study was a home with an HRV with decent flows, but where the nearest supply was in the hallway just outside the master bedroom. Again, compare to the three homes in the previous bullet, where door open / door closed nights are nearly indistinguishable and CO2 peaks are modest.
    - If you install an HRV or ERV, I recommend investing a few hundred dollars extra on a high efficiency one with a high SRE, vs. an economy model. Back-of-the-envelope calculations show that payback is short, but the more substantial benefit might be better tempered air. These systems only tend to work if people like them and keep them turned on. Also consider a unit that can filter to MERV 13 or HEPA (not all have this capability) if present or future occupants decide they want this for health or other reasons. Good speed control, filter-loaded notification, and swappable ERV/HRV cores are other nice features.
    - Joe Lstiburek has often spoken of ASHRAE 62.2 and overventilation. Consider Building Science Corporation's "Standard 01 - 2015", https://buildingscience.com/documents/special/ventilation-new-low-rise-r.... Unlike ASHRAE, it accounts for balanced (or not) and distributed (or not) systems; for a thoughtfully-designed balanced system it will yield a lower flow rate – which in theory could yield quieter operation, better tempered air, and less energy waste. Of course, testing air quality (as mentioned above) is a great strategy to fine tune a system to an individual home and occupancy.
    - A final note is that many homes in the original study never dropped below 700-800ppm (or higher) CO2, even during the daytime. Contrast that with the homes with balanced ventilation that dropped to near-atmospheric CO2 levels relatively quickly once occupants left the room for the day.

    May 24, 2018 11:16 AM ET

    Brian Just's final comments
    by Malcolm Taylor

    Make me think the new ventilation requirements in our building code got it right. They require a balanced ventilation system, with supply to each bedroom and to the living area, and exhaust from the kitchen and bathrooms. No sensors or complications to worry about.

    May 26, 2018 12:44 PM ET

    Challenging assumptions
    by Thomas Nedelsky

    "Most researchers agree that CO2 levels (which are easy to measure) are a good proxy for levels of other contaminants in the air that may be more difficult to measure." I think this statement means that there is a high correlation between CO2 and say, formaldehyde levels. I think this article suggests otherwise. In any event this a testable assertion. If Just were so inclined I would love to see him run a study on this and let us know the results.

    May 26, 2018 4:09 PM ET

    Edited May 26, 2018 4:10 PM ET.

    Response to Thomas Nedelsky
    by Martin Holladay

    Thanks for your comments. I have edited the problematic sentence; the new wording is more accurate.

    As I noted in Comment #15, "If your bedroom has high CO2 levels, your ventilation system isn't doing its job -- and if there are other air pollutants of concern in your home, there's a good chance that your ineffective ventilation system is doing a poor job diluting the other pollutants, too."

    May 28, 2018 12:07 PM ET

    Balanced ventilation exhaust distribution
    by Brian Knight

    Sorry to add to your work load there Martin and thanks for the further comments Brian. For IAQ monitoring, definitely prefer the idea of commissioning over adding room-by-room technology, even if calibration issues improve. Will more strongly consider the added technology of HEPA, filter indicators and swappable cores moving forward.

    We have always installed balanced systems in our homes. We always run supply ducts to bedrooms and living areas but exhaust is usually pulled from air handler trunklines or convenient, central location while being mindful of short circuiting. We prefer dedicated spot ventilation for kitchen and baths. Running additional exhaust ducts for these rooms would add measurable costs for questionable benefits (if occupants use spot ventilation like they should).

    I'm confused by Malcolm's comment about new ventilation requirements. It seems Section 403 added more language to balancing systems but looks like 403.3.2.3 still allows kitchen and bath spot ventilation only. It also seems 403.3.2.1 allows outdoor air be delivered via return side of air handler, not dedicated to bedrooms as they probably should. What am I missing?

    May 28, 2018 3:33 PM ET

    Brian knight
    by Malcolm Taylor

    Sorry, I should have been more clear. I was referring to the new ventilation provisions in our code here in British Columbia, which brought in mandatory full-time mechanical ventilation. While it does still allow exhaust only systems for one storey houses under 1800 sf, the location of supply and exhausts for both those and balanced systems seem to alleviate the problems Brian Just has found.

    Jun 5, 2018 7:34 PM ET

    Challenging assumptions
    by Thomas Nedelsky

    Martin, some one might get the sense from your revised statement that if you go from poor dilution of CO2 to good dilution of CO2 you will have good dilution of, say formaldehyde. I know that is not your claim but I am curious to what degree that is the case. Needless to say lots of variables at play here.

    Jun 6, 2018 6:02 AM ET

    Response to Thomas Nedelsky
    by Martin Holladay

    In fact, I think it's logical to assume that if you are comparing two cases -- Case A without mechanical ventilation in the bedroom, and Case B with mechanical ventilation in the bedroom, with the following data points: Case A = elevated levels of CO2 that are worrisome, and Case B = much lower levels of CO2, such that they are below the level of concern -- then there is a very high likelihood that the levels of formaldehyde will be lower in Case B than Case A.

    I'll admit that the study described on this page didn't show that, but I think it's not a crazy presumption.

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