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Community and Q&A

Study Concludes Exhaust-Only Ventilation Best

GBA Editor | Posted in Mechanicals on

The Consortium for Advanced Residential Buildings (CARB) evaluated three whole-house ventilation strategies in a non-profit development of efficient homes in Chicago in 2005. The homes were constructed with structural insulated panels (SIPs), heated with condensing furnaces, and utilized sealed combustion water heaters, and efficient lights and appliances. The blower door tests showed 300-350 CFM50 for 1700 SF of conditioned space.

They compared an ERV interlinked with the air handler fan (it did not sufficiently reduce CO2 without the air handler operating), an AirCyclerâ„¢ supply ventilation system that fed the air handler, and an exhaust only (bath fans on timer control).

They monitored the homes for six winter months, including temperature, humidity, and carbon dioxide concentrations outdoors, in two bedrooms, and in the living room; fresh air supply flow rate, temperature, and humidity; exhaust air flow rate, temperature, and humidity. And they calculated total energy costs including electricity consumption for ventilators and air handlers, and natural gas consumption or savings over an identical house with no central ventilation.

The exhaust fans were the only units that continued to flow at the same rate at which they were commissioned (the ERV averaged higher and the AirCycler averaged considerably lower). The exhaust-only system also maintained the lowest levels of indoor CO2.

Conclusion:

Among the systems monitored, it is clear that the exhaust only system is the least costly to operate; costs were less than 50% of those of the other systems. Looking at carbon dioxide concentrations and air distribution, SWA believes that data show that the exhaust system does provide adequate distribution and is the most appropriate system for these homes.

It is worth reiterating that the exorbitant energy costs for the Air Cyclerâ„¢ and ERV systems would be significantly reduced with electrically efficient, well-sized furnaces or especially (in the case of the ERV) with a dedicated duct system for ventilation.

This study certainly does not suggest that exhaust-only systems are appropriate for all applications. These small homes had an efficient exhaust fan in the center of both floors (each approximately 850 ft2). The effectiveness of exhaust strategies in larger homes requires more investigation, and the need for active air distribution may grow as house size increases.

Nevertheless, this study shows that exhaust-only ventilation certainly can be a very effective, low-cost ventilation option.

Link to study

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Replies

  1. Riversong | | #1

    Some data from the study:

  2. User avatar
    Carl Seville | | #2

    Interesting study, but what about humid climates where depressurization can cause problems by drawing excess moisture into the structure?

  3. mike eliason | | #3

    the ERV returns were in the basement. the ERV didn't have dedicated ducts. the ERV had a paltry 70-75% recovery rate.

    Among the systems monitored, it is clear that the exhaust only system is the least costly to
    operate

    except for the modeled annual costs of an inefficient ERV w/o air handler are less than all other options.

  4. Riversong | | #4

    Carl,

    Clearly, this was a cold-climate study (though Chicago is very humid in the winter).

    HVAC requirements are entirely different in hot/humid climates where an ERV is almost a necessity.

  5. Riversong | | #5

    the ERV had a paltry 70-75% recovery rate.

    Actually that's a good rate for recovery efficiency:

    "Depending on the volume of air flow and the temperature of the outdoor air, HRVs and ERVs typically recapture 60-85% of the outgoing air's sensible heat - after accounting for the unit's own energy use. This efficiency is termed the sensible recovery efficiency. Total recovery efficiency is typically substantially lower. High efficiency HRVs and ERVs should have HVI-rated sensible recovery efficiencies of at least 76%. This level of efficiency is required to achieve an Energy Star rating for homes with electric heat."

    Among the systems monitored, it is clear that the exhaust only system is the least costly to operate except for the modeled annual costs of an inefficient ERV w/o air handler are less than all other options.

    Only marginally so, and at $0.65 per therm for gas and electricity costs at $0.10 per kWh. Since the ERV costs were mostly electricity use, different local energy costs could skew this either way.

  6. Damon Lane | | #6

    Shouldn't the comparison have been to an HRV instead of an ERV in Chicago's climate?

  7. Riversong | | #7

    Damon,

    I would have spec'ed an HRV, but the only difference that might have made is in indoor relative humidity levels, and the report didn't comment on that.

  8. Stephen Eggerman | | #8

    Robert, were there any passive air supply holes in the house?

  9. Riversong | | #9

    No make-up air inlets. And the report indicated that this seemed to work in a small house, with 850 SF per floor and one fan centrally located on each floor, but may not work in a larger or more complex house.

  10. Jonathan Lantz-Trissel | | #10

    I'm building what I plan to be a very efficient 30' x 56' house in the Shenandoah Valley of Virginia with all the living space on one floor (limited loft/storage above). The house is basically a 24' wide 4/12 shed roof and peaks at 20' in height on the south end for two stories of properly protected windows to provide solar gain and passive cooling on summer nights.
    I want to build closer to passive house standards in terms of air-tightness and insulation values than the architect originally designed the house for (I'm planning on 9 in. polyiso on roof and mid R-30s on the walls, about double code for our zone).
    Assuming I will need some active ventilation, do I spend the money on a HRV given the larger footprint of the house or can I do the exhaust only option with vaulted ceilings spread across that much area? Also the south half of the house is all open and the north half has beds/bath/utility. A 1/2 bath on west wall, kitchen on east, and the full bath in the middle of the north side. I figure those 3 would be my exhaust fans (kitchen only when cooking). Maybe air inlets in the bedrooms and living area, can't make up my mind based on all the discussions. Any recommendations?

  11. Greg Duncan | | #11

    I think this headline is very misleading. The study actually indicates that a well-designed ERV system, even using 2004 tech, costs less to operate than exhaust-only ventilation.

    To quote from the study:
    Summary and Conclusions
    Among the systems monitored, it is clear that the exhaust only system is the least costly to
    operate; costs were less than 50% of those of the other systems. Looking at carbon dioxide
    concentrations and air distribution, SWA believes that data show that the exhaust system does
    provide adequate distribution and is the most appropriate system for these homes.
    It is worth reiterating that the exorbitant energy costs for the Air Cycler™ and ERV systems
    would be significantly reduced with electrically efficient, well-sized furnaces or especially (in the
    case of the ERV) with a dedicated duct system for ventilation.
    This study certainly does not suggest that exhaust-only systems are appropriate for all
    applications. These small homes had an efficient exhaust fan in the center of both floors (each
    approximately 850 ft2). The effectiveness of exhaust strategies in larger homes requires more
    investigation, and the need for active air distribution may grow as house size increases.
    Nevertheless, this study shows that exhaust-only ventilation certainly can be a very effective,
    low-cost ventilation option.

  12. mike eliason | | #12

    it'll be tough to meet PH w/ exhaust only, you take a huge hit on the space heating demand, like 50-70% higher space heating demand over HRV.

    i don't think you can use make up air w/ PH

  13. Riversong | | #13

    Greg,

    You just repeated what I had already posted under the "headline", which accurately reflected the conclusion of the study.

    And, as I already stated in post #5, a more efficient ECM air handler motor might have lower annual operating cost in that particular market but...

    Only marginally so, and at $0.65 per therm for gas and electricity costs at $0.10 per kWh. Since the ERV costs were mostly electricity use, different local energy costs could skew this either way.

  14. Riversong | | #14

    Who the hell is talking about PH?

    As I've stated many times on this forum, the PH standards are arbitrary and excessive and are not cost-effective to meet in an affordable home. They take a couple of good ideas (air tightness and superinsulation) to a ridiculous extreme. Extremism is never a solution, but merely creates new problems.

    you take a huge hit on the space heating demand, like 50-70% higher space heating demand over HRV.

    Obviously, you didn't read the report or even the cost comparison charts I attached below. Of the three commonly-used ventilation strategies, taking into account heating savings and electrical consumption, exhaust-only was the least expensive.

    It's also necessary to factor in the equipment and installation costs of the ventilation system and the payback time, which would be infinite if the operating costs were higher than an exhaust-only system.

    In a Review of Residential Ventilation Technologies by Lawrence Berkeley National Lab (2005):

    The results show that a single port exhaust system is the least expensive to purchase and install. This is supported by Wray, et al (2002) who also found a mechanical exhaust system the least expensive to operate.

  15. mike eliason | | #15

    robert,

    jonathan @10 mentioned building closer to the passivhaus standard. and then you responded about responded about building to PH-levels of airtightness.

    so, that is who the hell is talking about PH.
    frankly, i think your position on PH is arbitrary and ridiculous. and the 'not cost-effective' bit is debatable.

    i wasn't responding to the report (which i did read, you should really stop lying) - i was putting the space heating demand info out there for jonathan and others who are thinking about building to PH standard. when doing a PH, an exhaust only system appears to add about 50-70% to the space heating demand over a high efficiency ERV/HRV, based on the projects we've modeled in the PH software.

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Robert,
    This is an old (and somewhat flawed) study. I reported on the results in an article in the June 2005 issue of Energy Design Update.

    The study found, as you report, that exhaust ventilation systems work well and are inexpensive to operate. That's a useful finding.

    However, the results on the ERV system aren't particularly useful, because the houses had inappropriate furnaces and ducting. When I interviewed one of the researchers, Robb Aldrich, he admitted as much. The results of this study reinforce a message I have been delivering for years: if you are going to install an ERV or HRV, be sure to install dedicated ventilation ductwork, or else be sure that your air handler or furncace has an ECM blower.

    Here are excerpts from my June 2005 article:

    "Unfortunately, the builder did not adopt two of the recommendations made by SWA consultants. The consultants specified furnaces equipped with efficient electronically commutated motors (ECMs), but the recommendation was rejected. (According to the contractor, the $1,000 upgrade charge for an ECM-equipped furnace was a budget buster.) ... Electricity use was unexpectedly high at both House 1 [the house with the ERV] and House 2 [the house with the central-fan-integrated supply ventilation system]. ‘The ventilation and whole-house electricity monitoring has provided some eye-opening results,’ Aldrich said. ‘We immediately observed that the electricity consumption in the homes was very high. Upon further investigation, it was found that the furnace fans were responsible for a large part of this.’

    "In November 2004, the two houses using the furnace fan to distribute ventilation air consumed an average of 347 kWh for ventilation alone — more than three times the average monthly electrical production from the homes’ PV systems. Each South Chicago furnace draws between 700 and 800 watts in fan-only mode. More efficient furnaces equipped with ECM blowers usually draw 250 watts in fan-only mode, while the maximum power draw of the ERV is only 102 watts. If House 1 had been equipped with dedicated ventilation ductwork, ventilation air could have been distributed with the ERV’s energy-efficient fan. In New England, where hydronic heat prevails, such ductwork (typically 6-inch-diameter galvanized ducts) is commonly installed in homes equipped with ERVs. Since the South Chicago homes lack dedicated ventilation ductwork, the only way to distribute the ventilation air is with the power-hungry furnace fan."

  17. Riversong | | #17

    Mike,

    Nobody on this thread but you is talking about the extreme case of a PH. Poster #10 said he wanted to move the house "closer to passive house standards in terms of air-tightness and insulation values than the architect originally designed", which is simply saying tighter and better insulated than code minimum. And the Shenandoah Valley has less than a 4,000 HDD climate, so a non-recovery ventilator does not incur much of a heating penalty and your numbers are not relevant.

    And I'd suggest you be more careful about confusing a legitimate conclusion based on someone's post with "lying". If you can't make a rational argument, then you'd be wise to not say anything at all.

  18. Riversong | | #18

    Martin,

    You can claim this study to be "flawed" only if you expected it to be a best-case study of a tiny minority of homes in the US. It was not, and that's what makes the findings relevant and important.

    I believe that most new US homes have air handler HVAC systems, not hydronic heat, and the mechanical equipment installed in these Chicago homes is likely typical of what most homes get. It's very hard to justify a dedicated supply and return ducting system for ventilation when one is already installed for heat and cooling, and I suspect that most builders will choose the simplest and least costly system that meets ASHRAE 62.2 standards - which, by the way, don't require uniform distribution, only a minimum exchange rate.

    And ECM furnace is a nice upgrade, but most builders and homeowners are going to prefer to put that $1000 (or the $2000 cost of dedicated ventilation ducting and a heat recovery ventilator) into something else.

    And these were not sloppily built homes - they had about 1.4 ACH50 - and yet were well served by a simple, reliable and efficient exhaust-only system.

  19. Riversong | | #19

    Jonathan,

    If you're going to air condition the house, then you should use an ERV for humidity control, which could be integrated into your air handler if you're using a furnace or heat pump.

    But, if not, then an exhaust-only system could work fine with fresh air inlets in the bedrooms and high on the south wall in the living space. The high inlets would mix incoming air with the warmest air at the peak and help redistribute it. If located under the south roof overhang, they would also bring in the sun-warmed outside air rising up the south facade.

    If the house is going to be approaching PH tight, then you have to use all sealed-combustion appliances, no woodstove or fireplace, a small kitchen range hood and might need dedicated make-up air for the laundry if a clothes dryer is used.

    If you'd like a more detailed consultation, you can reach me at HouseWright (at) Ponds-Edge (dot) net.

  20. User avatar GBA Editor
    Martin Holladay | | #20

    Robert,
    Unfortunately, you are probably right that the systems installed in these Chicago homes are typical of what most homeowners get.

    All the more reason for progressive builders to visit GBA and learn best practices for the installation of HRV and ERV systems.

  21. Jonathan Lantz-Trissel | | #21

    Thanks for your response Robert. We are not installing air conditioning and would probably use a ductless split system if we did given our heavy timber framing and dislike of ducted air. So dedicated ventilation ducting is planned and will be fairly easy given the location of my storage areas. The kitchen is a little trickier, but do-able.
    Harrisonburg is 4300 HDD, but has been over 4600 the last three years (5100 in 2010, the highest in 30 years of data). I like the intakes at the top of the south wall if I go with the exhaust only option, but they stand to lose me a lot of heat as well given the higher temperatures and pressures that close to the roof. I'm trying to avoid those kinds of penetrations in the thermal envelope.
    I don't have the money to go all the way to PH standards, but I like the emphasis on putting money into the thermal envelope over higher tech solutions like PVs, solar water heating, etc. Those things can be added later with proper design, but your not going to yank out Marvin windows to replace them with Thermotechs at year 5 or 6 or 15.
    I'm going to try to find someone to do some modeling for me with the PHPP if it can fit into my budget. That might help answer my exhaust question in part, but I do like KISS in general.

  22. mike eliason | | #22

    robert,

    several people on this thread are talking about PH in various forms. your 'legitimate conclusion' happened to be a blatant lie. i suggest you heed your own advice for once.

    jonathan,
    PHIUS maintains a list of PH consultants, that should be revised once the latest round of examination results are released:
    http://web.me.com/polytekton/CPHCstatesMap/CPHCstates.html

    since the project is already designed, you should be able to find someone in VA that can model your house in PHPP relatively quickly and without much expense. in all the projects we've looked at, even when migrating to sub-4,000 HDD climates (e.g. memphis, charlotte), there is still a fairly significant space heating demand loss incurred by not utilizing a ERV/HRV per PHPP - about a 40-60% loss.

  23. Riversong | | #23

    Mike,

    Your personal attacks and misrepresentations only undermine your own credibility and that of the PH movement which you claim to represent.

    And your claim of a 50%-70% or a 40%-60% increase in space heating demand might be credible in a PH-insulated structure, but not a more appropriately insulated one, and it's meaningless if it doesn't include total energy consumption (heating and fan electrical consumption), and then compare that to the additional installed cost and maintenance of the ventilation system to determine payback.

    frankly, i think your position on PH is arbitrary and ridiculous. and the 'not cost-effective' bit is debatable.

    It matters little what you think. My position on PH is based on 30 years of pioneering super-insulation design and building, a thorough understanding of building science and hygrothermal engineering (both of which I teach), and on the fundamental principles of Bau Biologie and healthy living.

    Perhaps you'd care to share your credentials, which I would hope are more impressive than your grasp of basic grammar and sentence structure.

  24. John Brooks | | #24

    Martin:

    All the more reason for progressive builders to visit GBA and learn best practices for the installation of HRV and ERV systems.

    I admit to knowing little about HRV's ..
    but this install in an unconditioned basement does not look "right" to me.
    shouldn't the HRV be in the conditioned space?
    and if they are allowed "outside" shouldn't the ducts AT LEAST be insulated?

  25. User avatar GBA Editor
    Martin Holladay | | #25

    John,
    The best location for an HRV is definitely inside the home's conditioned space, in a location where changing the filter (or accessing the filter for cleaning) is convenient.

    In a mild climate, it's possible to install the HRV outside of the conditioned space, but its efficiency will suffer.

    You're right, of course, about insulating any ductwork outside of the conditioned space.

  26. John Brooks | | #26

    It was NOT a mild climate and it was a featured project at GBA
    http://www.greenbuildingadvisor.com/homes/old-house-gets-superinsulation-retrofit

  27. User avatar GBA Editor
    Martin Holladay | | #27

    John,
    A quick search for manufacturer's installation instructions came up with this:
    "The HRV should be located in a conditioned space and in close proximity to a fused power source."

    The instruction is for a Carrier HRV. Here is the link:
    http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/hrv-07si.pdf

  28. TJ Elder | | #28

    Last weekend I toured a house with a Lifebreath HRV and was surprised by how much noise it generated. There was quite audible fan noise in each bedroom coming from the supply vents. I haven't noticed this issue being raised about HRVs but it could be a factor for some people.

    An unrelated issue raised by a contractor I spoke with recently is that exhaust-only ventilation could cause moisture problems if negative pressure pulls water into walls during rainy weather. This is a concern here in the Pacific NW, where rain is more typical than snow in the winter. Robert Riversong has often suggested that negative pressure in the winter is good because cold (infiltrating) air has low absolute humidity, but here the outside is wet during winter. It does seem plausible that infiltrating air could bring liquid water into walls.

  29. Riversong | | #29

    Thomas,

    The slow rate of infiltration from the slight negative pressure of a bath exhaust fan cannot pull liquid water into the envelope. Driving wind can push rain through claddings, but not through the rest of the envelope unless there is an opening.

    100% relative humidity outside air cannot condense in the thermal envelope since it is warming as it enters and reducing its RH.

    Negative pressure is problematic only in hot/humid climates with air conditioned homes.

  30. User avatar GBA Editor
    Martin Holladay | | #30

    Thomas,
    Check out a psychrometric chart. Unless it's quite warm outdoors, winter air contains little moisture because it is cool -- even if it is raining.

    Moreover, any ventilation strategy requires outdoor air to be introduced into the house. This is true whether you have an exhaust system, a supply system, or a balanced system.

    Of course, an ERV can, in some conditions, reduce the moisture level in the incoming air stream -- but in general, not during the winter.

  31. mike eliason | | #31

    robert,

    can you quantify what you believe to be the difference between a PH-insulated structure and a more appropriately insulated one is?

    because i can't. a PH-insulated structure can potentially utilize less insulation than some of the pioneering projects you've worked on, given the right glazing.

    i've never claimed to represent PH.

    speaking of credibility... it was either a complete fabrication when you wrote

    Obviously, you didn't read the report or even the cost comparison charts I attached below.

    or you willfully forgot responding to my post on flaws in the report.

    for someone who claims i'm losing credibility with 'personal attacks' - you seem to be quite the hypocrite.

  32. Riversong | | #32

    Mike,

    You have made this forum into a venue for personal revenge because I have legitimately, fairly and accurately challenged almost everything you've posted on this website, because your statements have exhibited a consistently poor comprehension of building science principles and building practices.

    By so doing, you demean yourself further. I suggest you discontinue your self-abuse and allow the rest of us to discuss substantive issues in a constructive manner.

  33. ROY HARMON | | #33

    How does an ERV recover energy through ventilation?
    Does the Air Cycler pre condition air that is brought into the system?
    Most of the older homes that I've worked on don't seem to need these types of systems. Are they specific to air tight homes?

  34. Riversong | | #34

    Roy,

    All new homes must meet ASHRAE 62.2 minimum whole house ventilation standards. This typically requires something in the range of 60 cfm continuously 24/7 to maintain indoor air quality and evacuate excess humidity (tight houses tend to be too humid).

    No code requires heat recovery ventilation or a distribution system, but many builders of very efficient homes will install a heat-recovery or energy-recovery ventilator (HRV or ERV). They work by cross-flow or counter-flow channels that allows incoming fresh air to pick up much of the heat of outgoing stale air. ERVs additionally transfer outgoing (or incoming) humidity to the opposite air stream.

    In homes with ducted HVAC systems, an HRV/ERV can be connected to the primary ductwork to avoid duplication, though this often requires running the air handler fan in order to distribute the fresh air when the heat pump or furnace is not operating.

    Using separate ducting to remove and supply ventilation air is more effective and often less costly to operate, since the fan is smaller and more efficient. But a central HRV will almost never be as effective at removing moisture in a bathroom as a dedicated bath exhaust fan.

  35. J Chesnut | | #35

    Roy,
    ERVs transfer heat and humidity between incoming and outgoing air. HRVs only transfer heat.

    Response to Thomas' post #28 -
    Thanks for sharing your observations of the Lifebreath HRV concerning the noise.
    I worked on a project that installed a Lufta HRV unit and used flex ductwork. The Lufta unit includes a muffler and with the small interior surface of the flex ductwork the resulting noise is vastly diminished.
    Anyone know if noise is a factor with Venmar HRVs?

  36. J Chesnut | | #36

    But a central HRV will almost never be as effective at removing moisture in a bathroom as a dedicated bath exhaust fan.

    I don't know if this is true for all makes but HRVs include a ramped up exhaust setting to deal with removing elevated moisture levels produced by showers and cooking. The observations I have of these units that have dedicated ductwork (i.e. not tied in with heating ductwork) is that there ability to exhaust moisture is sufficient.

  37. ROY HARMON | | #37

    Thanks Robert and J ,
    Graphics help, so heat exchange cores are basically mixing boxes without other components, and blowers or fans do the work. The key, as always is getting the proper balance or design for a given home. Appreciate the lesson.

  38. J Chesnut | | #38

    Roy,
    In addition, sometimes another source of heat exchange is introduced to this basic system to further precondition incoming air. For example, semi-passive systems like a hydronic ground loops or earth tubes.

  39. Riversong | | #39

    so heat exchange cores are basically mixing boxes

    Not mixing boxes. Except for incidental leakage, there is no mixing of incoming and outgoing air - only heat transfer through metal plates, or moisture transfer through a membrane.

  40. ROY HARMON | | #40

    Got it, makes sence~ starting to understand the pricing a bit more now.
    Thanks again Robert

  41. User avatar
    Elden Lindamood | | #41

    All this got me to wondering, and I have just enough knowledge to be dangerous. So I have a few questions.
    Is it possible that a very tight house with an exhaust only ventilation system wouldn't technically meet ASHRAE 62.2 because the infiltration rate would be below the exhaust rate minimums?
    For instance, if you ahd a 1200 square foot 3br house, whith an infiltration rate of .6 ach@50, that would be about 110 CFM. Although the house would only require 45CFM by 62.2, what is the expected depressurization that would result from one centrally located exhaust fan?
    And would it be able to pull 45CFM via natural ventilation paths alone?
    Is the 45CFM requirement linked to a measured pressure?
    I realize this is somewhat hypothetical, but just curious.

  42. User avatar GBA Editor
    Martin Holladay | | #42

    Elden,
    It's perfectly possible to meet ASHRAE 62.2 requirements with an exhaust-only ventilation system. Of course, any mechanical ventilation system needs to be commissioned after installation; that means air flow must be measured.

    If a house requires 45 cfm according to ASHRAE 62.2, the exhaust fan should be designed and commissioned to move 45 cfm. If the house is so tight that an exhaust fan requires makeup air, then a makeup air inlet would need to be installed.

  43. User avatar
    Elden Lindamood | | #43

    Thanks Martin. I guess that kind of what I was getting at with my question. If the envelope conditions dictated that a makeup air inlet needed to be installed, then I why would you not install an HRV or ERV to either provide that air, or balance the ventilation? I acknowledge that the conditions would not be typical, but I was exploring the premise that it is certainly within the realm of possibility. Especially within the realm of possibility for a Passive House.
    I was kind of also seeking verification of a plausible scenario where a blanket statement about the "best" solution of an exhaust only system would not be true. I tend to challenge "one-size-fits-all" solutions, which is seemingly what you were doing with the initial post.

  44. User avatar GBA Editor
    Martin Holladay | | #44

    Elden,
    Q. "If the envelope conditions dictated that a makeup air inlet needed to be installed, then I why would you not install an HRV or ERV to either provide that air, or balance the ventilation?"

    A. Because the installed cost of an HRV or ERV is much higher than the installed cost of an exhaust fan. However, an HRV or ERV provides excellent ventilation, as long as you can afford the installation cost.

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