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Why Is This Obsolete Supply-Only Ventilation Method Still Used?

Ventilation Duct Outdoor Air Return Plenum

The small duct in the photo below is supposed to bring in outdoor air for ventilation in this new home.  It’s a simple method that consists of a duct running from an outdoor wall cap to the return plenum of the HVAC system. A decade ago, it was commonly recommended as the easiest and least expensive way to ventilate a home in a humid climate. When I worked at Southface, however, we made the transition away from it for some very good reasons.

The small duct in the photo below is supposed to bring in outdoor air for ventilation in this new home.  It’s a simple method that consists of a duct running from an outdoor wall cap to the return plenum of the HVAC system. A decade ago, it was commonly recommended as the easiest and least expensive way to ventilate a home in a humid climate. When I worked at Southface, however, we made the transition away from it for some very good reasons.


Aside from the obvious problem of the sharp turn and the constriction from the strap, both of which reduce the air flow in the system shown above, this ventilation strategy has serious problems. They are:

  • With no damper to close it off, this method is really just duct leakage. Yeah, the leak is from the outside, but it’s still little more than a duct leak.
  • With no electronic damper and controller, this method will result in too little ventilation for much of the year.
  • If the damper fails, malfunctions, or doesn’t get installed, that duct could be the source of a lot of extra ventilation air, which then would be air leakage. It would exchange air even when the system is not running because it’s just an open pathway to the outdoors.

Bringing in outdoor air is a good thing. It needs to be controlled, though, so the type of system that connects a duct to the HVAC system needs to have some other components:

  • An electronic damper, to close the duct when ventilation air isn’t needed and to open it when it ventilation is needed
  • A controller to open and close the electronic damper and to turn on the air handler when the house needs ventilation air but not heating or cooling
  • A balancing damper to set the proper amount of ventilation air

The diagram below shows how it’s supposed to be done. You can’t see the controller here, but that’s what opens and closes the electronic damper.

mechanical ventilation positive pressure central fan integrated supply

This type of system is called central fan integrated supply (CFIS) ventilation. It’s tricky, but it can work well if designed, installed, and commissioned properly. Without those other components, though, it’s just extra duct leakage and air leakage that will rarely provide the right amount of ventilation air.

It’s a bit odd to be writing about this because I didn’t think anyone was doing it anymore, but the house where I took the photo above is a brand new house. Apparently they never got the message that you need to have control of holes that go to the outside of the building enclosure. I applaud this HVAC contractor’s installing a mechanical ventilation system at all because this home is not part of a program or code that requires it, but it’s time to relegate this method to the scrap heap of ideas that don’t work.


Related Articles

The 2 Primary Causes of Reduced Air Flow in Ducts

The Great Ventilation Debate – Live at Affordable Comfort!

Can Occupants Be Trusted to Control Their Home’s Ventilation System?


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This Post Has 21 Comments

  1. Even with the motorized
    Even with the motorized damper, our company never recommends supply-only ventilation. We very rarely are able to measure sufficient ventilation using this method, and even when we can, we are having to set the motorized damper to run for an inordinate amount of time. Once you plug in the actual electricity required to run the air handler (yeah, 0.1 watts is not an acceptable input) into your energy modeling software, you are looking at a fresh air system that costs a LOT of money to operate. Considering Southface’s inexplicable stance on exhaust-only ventilation, you’re left with balanced systems as the only option left. Affordable housing be damned.

  2. So, the $64,000 question is:
    So, the $64,000 question is: What controls (modulates) the position of the electronic damper? 
    A $32,000 question (on special today, buy one question, get second question for half price): Is owner required to continuously operate the air handler blower, with all that entails for energy consumption and condensate re-evaporation?

  3. Matthew V.
    Matthew V.: Yeah, I’ve written about the difficulties of CFIS before: Why I’m Not a Fan of Positive Pressure Mechanical Ventilation. I do believe that there’s a way to do it well, but maybe it’s the KISS principle that will be the ultimate doom of this method. 
    Curt K>: Armin Rudd, formerly of Building Science Corp., patented a device that does this. AirCycler and AprilAire sell versions of it. Ideally, the air conditioner will be running most of the time the the damper is open, so the latent load gets handled on its way in. In the swing seasons, especially in a place like Florida, you could still have problems. 

  4. Third time a charm. 
    Third time a charm. 
    We have been using a Honeywell system with a Y-189 motorized damper that has an ASHRAE switch setting(s) and we adjust the controller based on RED calculations and another spreadsheet to meet our ASHRAE specifications and we have been successful especially on a tight home. Only drawback is in client ed.

  5. Thanks for taking on this
    Thanks for taking on this topic, Allison. Although supply only is far from ideal, cost considerations will keep it around for some time to come. Doing it right means controlling the source, amount, and timing of the ventilation air. 
    Unfortunately here in NC, code actually stipulates that it is OK to have uncontrolled ventilation air. There is even a ‘green’ program that does not require controlled ventilation. 
    Matt, there are some innovative supply ventilation strategies that tie into bath exhaust fans. When there is no call for heating or cooling, the bath fan is turned on to make up the difference. 
    I’m not familiar with Southface’s stance on exhaust only. I will have to look into it.

  6. While I really like Armin, I
    While I really like Armin, I have always disliked this system since I first saw it without a controller in an old house in Fresno over 25 years ago. For it to work you need enough static pressure at the location in the return where the outside air intake is.  
    Even if you get it right that pressure changes as the filter gets dirty. Then you have to run that ridiculously inefficient air handler(efficiencies in the single digits or low teens see
    Then there is the fact that you have to run the air handler (and use the duct system) in the swing months adding to leakage and conduction losses. If you insist on using this method, you should at least add a BPM/ECM motor and the two dampers Allison mentioned, and Armin’s controller; then know it will only work as designed some of the time.  
    Allison what is Southface’s position on exhaust only. I presume it has to do with bringing in outside moisture into the walls and having it cool and condense. It is really too bad that the Eastern part of the US has that problem :^} 
    With respect to other strategies, I am impressed with all the new offerings from Panasonic.

  7. I take issue with your
    I take issue with your calling this method “duct leakage”. It is filtered and conditioned (dehumidified) air which is far from the typical effect of duct leakage. And it promotes a slight positive pressure which is desirable in a hot-humid climate — and undesirable for those of you in a heating dominated climate, you need to identify your climate. 
    Because of my climate, I would prefer not to discuss exhaust-only ventilation at all, except in an academic sense. 
    In my hot-humid climate, the first little bit of CFIS ventilation should cost little or nothing in operation, as outside air infiltration would be reduced by nearly the amount of CFIS supply. This is for a system sized to meet maybe half of the ASHRAE ventilation amount. In measuring actual AC usage, any increase in energy consumption is not measurable. This is a special case, not suited to every house nor every climate of course. 
    What I really admire about the object of your scorn, CFIS ventilation, is  
    1) I had nothing before, and HVAC professionals declined to recommend anything, 
    2) it has no moving parts, no electrical items to be professionally installed or maintained 
    3) it plays a role in reducing infiltration of outside air, which chronically is 70-75F dew point where I live 
    FWIW I have a *manual* closure method for the winter and shoulder months. I know such a butt-simple CFIS method does not meet the ASHRAE recommendation, but that is based on guesswork anyway and most houses in my region have nothing at all for intentional ventilation. 
    So I would argue CFIS is not all bad, and other methods add expense and complexity. Thank you for listening.

  8. I would like to add a comment
    I would like to add a comment from Paul Raymer I got on a similar post over on HomeEnergyPros:  
    “So one thing you might consider is the AirCycler’s latest controls. The G2 which operates the air handler blower that you can set up with the pipe to the outside, etc. It will monitor the run time of the air handler for ventilation operation. If the air handler has not run enough to satisfy the ventilation requirement, the ventilation function is transferred to the FanConnect control which turns on a bath fan to complete the need. The power hungry HVAC fan doesn’t run any more than it did before. In the spring and fall, the bath fan will be the primary ventilation system, using much less power. 
    The hardest part of this is running the wires between the two controls. And it’s still hard to know how much ventilation air you are accomplishing, but its a thought.”

  9. Interesting discussion. I
    Interesting discussion. I agree that supply-only/CFIS ventilation is fraught with peril that only the most diligent contractor is likely to address in practice. 
    In moderate climates, an inline fan such as Fantech FR-110 or FR-125 can be configured as a simple supply ventilation system, as long as the additional latent load is addressed (but that’s true with any non-enthalpy recovery approach). This strategy provides a controlled, consistent amount of ventilation while avoiding the high cost of a balanced system, which in any case, cannot possibly be justified in milder climates. The filtered air could be delivered near the main return, and then set air handler to operate x minutes per hour during extended off-cycle periods. This requires a t’stat that supports this feature as well as an ECM blower with low-wattage (typically 50%) fan-only mode, both fairly common these days. That said, I don’t think it’s necessary to jump through hoops to evenly distribute ventilation supply air. In my mind, ventilation is more about removing the bad stuff than delivering ‘good’ air, which is why spot exhaust is still necessary.

  10. Well, part of the gang is
    Well, part of the gang is paying attention. It seems that the first need is to be clear and consistent with terminology. I think Alison is calling the obsolete system the one WITHOUT motorized damper and a fan cycling/damper control. He seems to approve of the one that I first dubbed central-fan-integrated (CFI) supply or CFIS in a 1998 ACEEE paper with that name in the title. A bonafide CFIS ventilation system has a motorized damper and fan cycling/damper control, except the motorized damper part is less important in warm-dry climates because the amount of extra air exchange due to no damper has much less impact than in warm-humid or cold climates. Something that people often forget is the clear benefits of knowing where the outdoor air is coming from, being able to filter the outdoor air all the time and condition it much of the time, fully distribute it even to closed rooms, and provide added value of recirculation filtration and floor-to-floor/room-to-room thermal comfort mixing. After more than 15 years and many tens of thousands of successful houses with bonafide CFIS, with production builders no less, it is hard to imagine how pictures of kinked ducts or lack of appropriate specified components can be assessed against it, or any particular system for that matter. But, it’s still a free country!

  11. All obsolete. The HRV is the
    All obsolete. The HRV is the current standard, soon to be replaced by the ERV in most climates.  
    Mute subject.

  12. I have been waiting for
    I have been waiting for someone to claim HRV or ERV is the answer. In what climate sir? 
    When indoor air is 77F, 58 dew point, and outdoor air is 85F, 75 dew point, what good will come of your HRV? 
    And if you think ERV is going to be the entire answer, please tell me what percentage of humidity will be removed from the incoming air. And what CFM you will use vs. a proper supply only system. My theory is you will be introducing extra humidity to the house in a hot-humid environment, and that is *assuming* the ERV is set up to do what it can do. 
    Rather than just say the magic word “ERV”, I wish to see a discussion of what can go wrong with these things. From what I am told, in a humid climate it very often does.

  13. I designed a system, using a
    I designed a system, using a Fantech fan, as David mentioned, to use as a supply air strategy, and got code approval for it here in coastal NC. It brings in outside air at approximately 30% of the total flow, tempers that with the other 70% of the flow coming from inside the house, filters it (please don’t forget that all of these systems need a filter that removes pollen!), and delivers it to the supply trunk line. 
    Sounds like madness? It’s the same way the dehumidification ventilators work when the dehumidifier is not actually running (like 95% of the time), which is where the idea came from. It’s better than the duct-to-return, no matter how you do it, but not as good as an ERV (cheaper though). 
    M.Johnson, ERV’s are great for your climate. We use them for most of our projects. I’m not sure what you mean by “what can go wrong with them”. I know that they are often blamed for humidity problems, when the real problem is elsewhere. We also see them configured with wildly-off flow rates. In a humid climate, any ventilation except a dehumidification ventilator will result in an increase in more latent load. It’s not a problem if it’s done correctly and all the other ducts are in a row (envelope & duct issues, HVAC sizing, HVAC system selection, proper configuration, blah, blah).

  14. I just had my home air sealed
    I just had my home air sealed and insulated, exterior walls and attic, and basement rim joists spray foamed. My contractor just got back to me and said he needed to install ventilation into the home since the blower door tests called for it since the home was air sealed and insulated very well. He recommended the central fan integrated supply (CFIS) ventilation since it was the least expensive and would allow me to have fresh air ventilation into the home, which is what the goal is now. He did recommend other methods to accomplish this goal, but focused on the FCIS since it was most cost efficient. And he said since my furnace has a energy-efficient ECM blower, keeping the fan on all the time wouldn’t cost a lot and wouldn’t create a lot of wear and tear on the motor since the motor was an advanced motor. After reading this article I am not sure what to do. Any recommendations? Should I not go this route?

    1. John, is the contractor
      John, is the contractor installing a motorized damper and controller with the CFIS ventilation system? If so, it’s fine. My complaint in this article was about the simpler CFIS system that leaves those components out.

  15. John For once I get to
    John For once I get to disagree with Allison. Don’t do it. It even the more efficient ECM/BPM running all the time will use way more energy than it needs to. It will cost a good bit more by the time you put in an automatic damper and a fan control. Put in an efficient bath fan such as the one from Panasonic FV05-11***

  16. I’m unwavering in my
    I’m unwavering in my determination that the main air handler blower mustn’t ever be called upon to run independently of compressor operation during cooling season anywhere excess humidity is a concern.

    I insist that the water on the air handler coil be allowed to drain away, undisturbed by passing air, until the next call for cooling occurs. In round numbers, a modern high efficiency air handler coil has about 100 square feet of area, all soaking wet, per ton of installed system capacity. That’s a lotta water.

    Other than that, I’m open to whatever ideas anyone has as to efficient, cost, and comfort-effective mechanical ventilation. I tend toward having two or more two speed Panasonic bath fans run in low speed speed all the time. It’s far from perfect but pretty good.

  17. First, let’s agree that we
    First, let’s agree that we want to condition our homes 24/7. If we can agree on this we will next agree that we want them to be comfortable the whole time.

    Finally, we can agree that we want to do all this at a reasonable installation and operating cost.

    If you are in the house you need fresh air, all the time. This air must be “conditioned” unless, in the unlikely event, that the outdoor air is perfectly comfortable. Further the air brought into the house must displace air being exhausted.

    Since we create excessive CO2 and moisture by breathing, bathing and cooking exhaust fans will be in order. Incidental exhaust loads such as showering are not a concern but an HRV or ERV integrated with a central heating/cooling system is the logical choice for exhausting stale air and delivering fresh air.

    Should the “fresh” air not meet our IAQ standards for filtration, ambient temperature or humidity we have the perfect tool to condition it. The ERV transfers valuable conditioned air (temperature) and humidity (sensible) so that all is not lost but (recovered) or transferred from the exhaust air stream to the fresh air supply.

    Depending on the operating conditions, climate and appliance about 50% of the energy in the conditioned air is captured and returned to the home before it gets outside.
    I have had my old-school American Standard condensing furnace operating with “fan on” for over 10 years. Electric motors want to run. A fan will use more energy and wear out faster stopping and starting than they will in full-time operation. The new ECM motor are no exception except in the fact that they will last longer and use less energy do it.

    As for fresh air. The ERV transfers sensible and latent heat. The latter is moisture not always addressed by a standard DX cooling system. More especially if the cooling system was not designed correctly. Even if this is the case, in certain climates and construction a whole house dehumidifier may be in order.

    In any case the careful control of air movement through the envelope is essential to indoor air quality and occupant comfort. The use of an HRV or ERV dependent on climate is a useful tool to save energy and assure IAQ.

  18. The rationale for a CFIS
    The rationale for a CFIS system is to piggyback on the air handler or furnace blower to bring in filtered fresh air and condition it “on the spot”. However, a cycle timer and automatic damper are necessary to ensure adequate ventilation during long periods when there’s no call for heat or cool. But here’s the problem with that: in order to meet the 24/7 ventilation target when the furnace or AC isn’t needed, the intake would need to be dramatically up-sized. For example, if target is 60 CFM and cycle timer is set for 10 minutes an hour, then the intake would need to be sized to supply 360 CFM! This saves considerable blower energy, but it would seriously over-ventilate during heat and cool calls, especially during the hottest and coldest weather. Ouch!

    The fact is, most CFIS systems are adjusted to a flow rate that’s close to the target ventilation rate. In that case, a cycle rate of 10 or 15 minutes per hour will seriously under-ventilate much of the time. Note that even during mid-winter, a properly sized furnace will only operate continuously during the coldest for part of the day. Same in mid-summer. Over the course of a year, the home receives far less than the prescribed ventilation rate.

    Running the blower 24/7 would provide the correct ventilation, but that’s a stupid strategy for the reasons cited by Curt and John Proctor. If you live in a humid climate, your blower will re-evaporate moisture from the coil back into the airstream. This not only risks raising RH too high, but wastes the energy your system expended to condense the moisture in the first place. And it wastes a *lot* of energy to run the comparatively large central blower 24/7. Even an efficient ECM blower may consume on the order of 1,000 kWh extra per year if set to continous (depends on several factors). Variable speed ECM blowers typically operates at 50% of fan flow during off cycles. That would reduce the penalty (although not by half), but then you end up with the same issue as a cycled CFIS — either the CFIS will seriously over-ventilate during hottest and coldest hours, or it will seriously under-ventilate during the off-cycles. And if your furnace and/or A/C is multi-stage, this on-versus-off cycle variation can get even worse.

    John, you didn’t say where you live. If you’re in a humid climate zone, see the above comments by Skye and myself for a much better supply-only ventilation strategy (install a separate SMALL, low-wattage supply fan and filter). If you live in a cold or dry climate zone, consider going with an exhaust only ventilation system. The best solution though would be a recovery ventilation system.

  19. “First, let’s agree that
    “First, let’s agree that we want to condition our homes 24/7. If we can agree on this we will next agree that we want them to be comfortable the whole time. ”

    I disagree vehemently! My family has had the good fortune to enjoy mild New England weather for the past couple weeks and windows are and have been wide open – no heating, no cooling, and no mechanical ventilation needed.

    In Florida that strategy works for a month or more each spring and fall.

    We should encourage our owners / clients to be mindful of outdoor temp and humidity and take advantage when appropriate.

  20. Note that I did not say 24/7
    Note that I did not say 24/7/365.

    If we stay on topic, the issue is the shoulder months. Here in Minneapolis we experience the worst weather anywhere, summer and winter with moisture, in all it’s forms, ever present.

    The new ECM fans typically draw 80 watts on low-fan mode. You must move air to condition it either actively or for sensor controls. You may use humidity, temperature, CO2 or some combination thereof to control IAQ.

    In most of my hydronic based designed we use dedicated bath exhaust with push button 20 minute timers and minutes-per-hour exhaust to properly expel humid shower and cooking loads while maintaining a healthy home.

    Each strategy is determined by climate and construction but using uncontrolled (cracks and holes) fresh air is too old-school to be seriously considered except in the mildest climates.

    As for duct loss. With the new energy code it blower test, duct loss is a thing of the past.

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