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Energy Solutions

Our Top-Efficiency Heat-Recovery Ventilator

We expect our state-of-the-art HRV from Zehnder to provide fresh air to our home, with very low energy consumption, for years to come

Barry Stephens is installing the condensate drain on our Zehnder ComfoAir 350 Luxe heat-recovery ventilator (HRV). The appliance distributes fresh outdoor air to bedrooms and living areas, while simultaneously exhausting stale air from bathrooms.
Image Credit: Alex Wilson
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Barry Stephens is installing the condensate drain on our Zehnder ComfoAir 350 Luxe heat-recovery ventilator (HRV). The appliance distributes fresh outdoor air to bedrooms and living areas, while simultaneously exhausting stale air from bathrooms.
Image Credit: Alex Wilson
Zehnder HRVs are larger than most, but that helps them achieve very low sound ratings and very high efficiency. Zehnder's small-diameter plastic ducts fit into 2x4 walls. Our ducts were run through the access ceiling system in our house. An exhaust port that can easily be adjusted during balancing. This ceiling-mounted supply port, or diffuser, can be adjusted by simply turning the lower disk. This air-supply wall register is a little more difficult to adjust than ceiling-mounted diffusers.

In last week’s blog I reviewed some of the general strategies used for ventilating buildings — or not. This week, I’ll zero in on the types of balanced ventilation in which heat is recovered from the outgoing airstream to preheat the incoming fresh air.

Two fans, two airstreams

As noted last week, balanced ventilation requires two fans: one bringing fresh air into the house and one exhausting indoor air. By balancing these two fans and the airflow through their respective ducts, the house is maintained at a neutral pressure — which is important for avoiding moisture problems or pulling in radon and other soil gases.

In a heat recovery ventilator (HRV), the two fans are in the same box, and they force air through a heat-exchanger core made of a corrugated plastic or aluminum. There are several types of heat exchanger cores in HRVs, and these affect efficiency and cost.

HRVs can have cross-flow heat exchangers or counter-flow heat exchangers. With cross-flow, the incoming and outgoing air streams are typically at 90° angles to each other. The heat transfer efficiency is good but not great: typically 50% to 70%.

With a counter-flow heat-exchange core, there is a longer pathway across which heat exchange occurs, so the efficiency is typically higher.

Our Zehnder HRV delivers between 2.6 and 3.3 cfm per watt

The HRV we installed in our new house is a Zehnder ComfoAir 350 Luxe. This is a Swiss-made, highly efficient HRV utilizing a counter-flow heat exchanger. In fact, testing by the Home Ventilating Institute (HVI) shows it to be the most energy-efficient HRV available. The American division, Zehnder America, is off to a rapid start, with about 800 installations in North America since its launch several years ago, according to business development director Barry Stephens.

There are various ways to measure the efficiency of HRVs. Apparent sensible effectiveness (ASEF) is the most commonly reported number for heat transfer efficiency. The HVI-listed ASEF of our Zehnder unit is 93% — which is among the highest in the directory (though not quite the highest).

Another measure reported by HVI is the sensible recovery efficiency (SRE). This is a measure that corrects for waste heat from the fan motor that may be going into the incoming airstream, cross-flow leakage from the outgoing to the incoming airstream, and case leakage or heat transfer from the outside of the box to the airstream inside. These factors make it seem as if the heat transfer efficiency is higher than it really is; thus the SRE number is more accurate. With our Zehnder ComfoAir 350 the SRE is 88% — the highest that I found in the HVI Directory.

In reviewing the HVI list of certified products, I found some other HRVs with higher ASEF values, such as a Broan-NuTone model with a listed ASEF of 95%, but that product had a SRE value of only 58%. With that product and most other HVI-listed models that have very high ASEF values, the SRE values are considerably lower, indicating that waste heat from high-wattage fan motors or other losses are boosting the ASEF values.

Another measure of efficiency is how much air is moved per unit of electricity consumed. Here we can look at the cubic feet per minute (cfm) of air flow per watt of electricity consumption. With this metric, the Zehnder ComfoAir really shines, achieving a remarkable 2.58 to 3.25 cfm per watt (depending on the fan speed). The Energy Star criteria for HRVs to be listed as EnergyStar is 1.0 cfm/W, and most good HRVs have air-delivery efficiencies only in the 1.0 to 1.5 cfm/W range. I was able to find only a few others with cfm/W values exceeding 2.0.

(Several Panasonic exhaust fans have higher cfm/W numbers; for example, the Panasonic FV-05VK3 is rated at 12.4 cfm/W. However, these fans do not provide any heat recovery.)

Plastic ducts

Nearly as exciting as the superb energy performance of Zehnder HRVs is the ducting that is provided with them. The company produces ComfoTube ducting with a 3-inch outside diameter and 2.5-inch inside diameter. The outer surface is ribbed for strength and the inside is smooth for optimal airflow and quiet operation. The material is 100% high-density polyethylene, which is the most environmentally friendly plastic, in my opinion.

The ducting diameter is small enough to fit in 2×4 interior walls. Because the airflow rate through the ducts is relatively low and sharp bends are eliminated, the airflow delivery is very quiet. In fact, noise control is a key feature of all Zehnder products, and this is one reason the HRV itself is so quite large.

While some ducting systems for heating and ventilation are branched — with larger trunk ducts stepping down to smaller distribution ducts — Zehnder ComfoTube ducts are designed to be installed in a “home run” configuration, with a single, continuous duct extending from each supply and return diffuser all the way to the HRV. This feature also helps control noise, though it can make for a complicated spaghetti-like installation.

Three operation settings

Our HRV has three speeds, plus an extra-low “away” setting. Labeled 1, 2, and 3, the primary settings can be custom-set to deliver between 29 and 218 cubic feet per minute (cfm). As configured on our system, Setting 1 consumes 18-20 watts, Setting 2 consumes 30-35 watts, and Setting 3 consumes 80-85 watts. The Away setting uses just 7-10 watts.

There is a frost-protection cycle that goes on periodically in cold weather to prevent condensate from freezing in the heat exchanger core. This draws about 800 watts. The need for this can be greatly reduced by adding a ground-loop preheater. This circulates an antifreeze solution through a simple ground loop (tubing that can be buried along the house foundation during construction).

A pricey appliance

In my opinion, Zehnder makes the best HRVs and ERVs (energy-recovery ventilators) in the world. But you pay for that quality and performance. The system we have, a Zehnder ComfoAir 350 Luxe with ten supply ducts and ten return ducts, with their respective registers and two remote controllers (for the upstairs and downstairs bathrooms), costs about $6,000. The geo-exchange loop, which we did not include, adds another $2,000.

While this is a lot to spend on ventilation, this integrated whole-house ventilation system obviates the need for separate bath fans, which can cost $300 to $600, installed, and some of that extra cost will be recovered over time through energy savings during operation compared to standard HRVs.

The super-quiet, highly dependable operation is a nice bonus.

In next week’s blog I’ll talk about commissioning our HRV system.

By the way, Eli Gould (the designer-builder of our home) and I will be leading a half-day workshop at the NESEA Building Energy Conference in Boston on Tuesday, March 4, 2014. In this workshop, “What Would the Founder of Environmental Building News Do? Adventures on the Cutting Edge of Green Building,” we’ll be reviewing product and technology choices, describing lessons learned, presenting data on performance, and discussing, in a highly interactive format, some outcomes from this project that can be applied much more affordably in deep-energy retrofits. This should be informative and a lot of fun. Registration information can be found here.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.


  1. wjrobinson | | #1

    Your friend Bruce Brownell
    Your friend Bruce Brownell says his customers... enjoy cracking a window for fresh air.

    $8000 dollars vs cracked window at odd 30 cents per vent period along with the life experience of being connected to the planet verses to a machine.

    Warning: Bruce does mix science and a bit of over-marketing... relax, enjoy, visit one of his builds before you poo poo. Bruce was a continuous insulation fanatic way before the days of GBA. His homes take it to the limit with fully six sides sheathed in two carefully placed and taped layers of rigid. He is a detail type and demands the details be executed. Great guy, Pioneer for sure in energy reduction.

    Alex, great system, glad you could get one or afford one.

  2. jackofalltrades777 | | #2

    Money Well Spent
    Mechanical ventilation is a must on newer and more airtight homes. 2012 IRC/IECC requires it. Leaving a window open is not an option for most people since unwanted entry by intruders is always a possibility. Besides, most working people forget to open and/or close a window. On a 20F winter day, opening a window and letting the heated conditioned air out is not a wise thing to do. Recovering energy through the heat exchanger is the better road to go down for ventilation.

    The other cost savings involved in a HRV/ERV mechanical venting unit is that one doesn't have to poke R-0 holes into their roofs to vent the bathrooms. This saves on labor costs and the less holes in ones roof, the better. Roofs typically leak at the penetrations. Not to mention, a bathroom fan vent becomes a 24/7/365 open chimney that leaks conditioned air.

    Spending $6k on a top-of-the-line HRV unit with 20 air ducts is money well spent and it is not that huge of an expense for a new build. Subtract the cost of individual bathroom fans and the subsequent inefficiency they cause, and the unnecessary R-0 holes in your roof, an HRV/ERV is a wise investment for the health and well-being of the homes occupants.

  3. RedDenver | | #3

    aj builder
    Opening windows is definitely the lowest up-front cost to freshen air since the cost is already built-in if there are windows. But not everyone can use open windows. Many people are sensitive to a number of outdoor pollens and are miserable during certain times of the year if the windows are open. In an air-tight home without a mechanical air system, what do they do?

  4. DEnd2000 | | #4

    Does anyone make a configurable bypass ERV?
    I was looking at the Zehnder ERV, and noticed that some of their models offer a bypass option so that at certain times of the year or for certain conditions the air streams bypass each other. In my climate this could be desirable as you could in theory bypass the outgoing airstream when you take a shower say during the summer when you don't want to reintroduce the humidity from the shower into the incoming airstream (as an ERV to my understanding passes humidity from higher to lower across some form of membrane or rotating absorbent). Does anyone make a system that could bypass just the bathroom(s) outgoing air and still get some dehumidification of incoming air? Would the added mechanical complexity and cost be worth it?

  5. GBA Editor
    Martin Holladay | | #5

    Response to Donald Endsley
    Q. "Does anyone make a configurable bypass ERV?"

    A. Not that I know of.

    Q. "Would the added mechanical complexity and cost be worth it?"

    A. Probably not.

  6. Alex Wilson | | #6

    Bypass option
    If I'm understanding your question properly, yes, the ComfoAir 350 Luxe offers a Bypass option. Here's how it's described in the user manual:

    "2.1.3 Bypass for free cooling
    The bypass is often used during hot days in the summer season. By allowing colder outside air in at night, the indoor temperature of the dwelling can be kept low during hot days. The bypass works automatically: simply set the required comfort temperature."

    It looks like I set the "comfort temperature" on the control panel (where other program functions are set); it sounds easy enough, though I haven't gone into these program functions yet. I may have to put in a call to Zehnder America to understand what comfort temperature to set.

  7. GBA Editor
    Martin Holladay | | #7

    Response to Alex Wilson
    I understood Donald's question about a "configurable" bypass to refer to some type of damper that would allow a homeowner to bypass a single exhaust duct (from one bathroom, for example) while leaving the rest of the home's exhaust ducts operating normally.

  8. DEnd2000 | | #8

    Response to Alex
    Martin is correct I was referring to a damper. Though it would likely be a bit more complex than that. I'm in a mixed humid climate, as has been stated here at times to keep humidity under control a dehumidifier may be needed. With an ERV if you ran it with bathroom intakes not bypassed at times of high humidity (ie 100% bathroom humidity, 80% outside humidity) you would be reintroducing bathroom humidity to the house. Bypassing just the bathroom exhausts the humidity reintroduction would be basically eliminated, and since you are still passing a portion of the rest of the house's air (which I'm assuming would be a lower humidity level than the incoming air) through the ERV this would still somewhat reduce the humidity load in the incoming air. That would somewhat reduce the load on a dehumidifier if one was needed.

  9. wjrobinson | | #9

    Those of you that post the
    Those of you that post the need to be in homes with closed windows.. .you don't leave your homes?

  10. RedDenver | | #10

    Response to aj builder
    You do know that people with allergies are affected most when certain plants are in bloom, don't you? That means days or weeks when opening a window would make their life miserable. You're making a poor strawman argument here.

    They leave their homes when they need to and take meds to minimize the symptoms. But why design a house they cannot always live in comfortably when we have the technology to overcome that?

  11. jklingel | | #11

    good unit, but....
    I am just installing one of these (350) and I completely agree that it was one easy unit to install. I looked at others and absolutely dreaded running metal ducting for them. Installing Zehnder's ComfoTube is a piece of cake, and the silencers, manifolds, etc, have gone together very nicely. The rigid foam fresh and exhaust air pipes fit snuggly together and eliminate any need for insulating tubes that might be made of other material; nice feature. There is, however, a small issue that Zehnder should address. There is unacceptable burring on the steel "manifold tubes" (my term) that accept the plastic ComfoTubes. There are two slots cut into a steel tube wherever the ComfoTube meets steel; these slots accept clips that hold the tube in place and make a very clever, secure connection. These burrs need to be removed before you insert the tube and O-ring or you could seriously damage the O-ring. Deburring should have been done at the factory. I can not speak for the operation yet, but aside from the burring, this unit is certainly a well-engineered piece of equipment.

  12. schreib77 | | #12

    Is there no price too high to attain the ASHRAE standard for mechanical ventilation? Gilding the lily here, from my point of view. I have the money to buy this expensive alternative but most folks either do not or refuse to spend so much.

    A solution for even the worst case home (ie very tight, new) is extremely simple. Here is mine: built new home in Minnesota, 2700 sq ft one level, hit only 1.0 on blower door test, designed and GC'd myself. Mn regs insist ONLY on balanced mech. ventilation while conforming to the ASHRAE CFM calculation. For me: 50 CFM continuous, 100 CFM intermittent. For less than $100 I passed code with the following solution: leave a bath fan on continuously and run a blower on far side of home FEEDING air into the home. Both on 7 day timers (plug mounted) so I can regulate time ON/OFF per hour. Our main bath fan is manually controlled; normally leave it on for 10 to 20 minutes during showering. A toilet room fan on humidistat, turns itself OFF. Main range vent can pull in 700 CFM but is only used on HI when multiple burners ON-- always open a kitchen window a bit when range is used. This "system", basically a couple of blowers and human manual control of localized exhaust, easily and efficiently functions to distribute air throughout the home and control humidity and odor. (It is also foolishness to provide re-heat to range ventilation when it is used so seldom.) My "system" is only 50 cfm, so it is really foolishness to try to disperse such a tiny flow equally over a 2,700 sq ft floor space. (e.g. Zehder's solution might attempt to equally(?!) distribute 7 CFM feeds to various points, crazy.) This was simple and without all the expense of ventilation ducts or an HRV. I literally saved thousands of dollars, Zehnder provides the proof! In the winter it is nearly impossible to keep the humidity high enough so REMOVING moisture is not an issue.

    2nd issue with this article: It totally ignores the need for the other HALF of what is needed for HEALTHY air: controlling CO2 concentration while still costing more than 3 times what a reasonable fresh air recovery system should cost and ignoring the fact that both humidity and CO2, like other gaseous elements, disperse readily to equalize themselves within a building space. To solve the rest of this issue for my home I did the following:
    -- acquired a dual humidity & CO2 sensor and utilized it to regulate the ON/ OFF time for my two "mechanical ventilators".
    -- Moving it around the home allowed long term trends to be understood and the importance of ppm uniformity attainable vs desired across the space.
    -- In the summer we leave our windows open a LOT! This spring I plan to install a whole house(3600 CFM) attic fan to draw air in during night hours and providing almost free cooling. We will close up the house by mid-day or so in the really hot 3 weeks per year we "see". I only use my Mini-split during the REALLY(!) hot 90°F+ / high humidity days. As a result, the $100 "mech ventilation" only is used from October through April.

    For Minnesota, my ideal solution for most homes:
    An HRV with 70%+ efficiency and one 4 or 5" duct to send the warmed, fresh air to far side of home. Mount unit in Boiler / furnace room on other side of home or where runs for other 3 ducts(cold, Fresh air IN, Stale air OUT, stale air Feed) can be kept short. This HRV must include a portable (not mounted) humidity / CO2 sensor for portable location by user and ability to regulate ON/OFF time for HRV, be lightweight, small. Installed cost must be less than $1,800.

  13. GBA Editor
    Martin Holladay | | #13

    Your cheap solution -- a bath fan plus a fresh-air supply fan on the other side of the house -- may not be lowering the CO2 in your bedrooms when you're sleeping. For data on this issue, see "Ensuring Fresh Air in Bedrooms."

    1. schreib77 | | #15

      Hello Martin, It appears you are implying that with two humans sleeping it will raise the CO2 overnight and this system will not keep up or will have a big lag. Makes sense.
      Middle of our house is around 850 ppm as a general average and we are comfortable without feeling odd. My CO2 data shows significant reduction with fan rate change from 20% ON to 100% ON. I have not checked the bedroom lately. Will check and report in.
      Question for you: Apparently max CO2 for offices is upwards of 800 ppm for workers and somewhere around 600 ppm or so may be a target area for home CO2. Cannot recall exactly. Have you ever heard of any REALISTIC spec for a home CO2 level and / or effects of levels above 1000, above 1400 ppm etc.? Have you any data sources from folks with Heating ventilators for CO2 ppm?

      1. GBA Editor
        Martin Holladay | | #16

        My article cites studies and guidelines that target 1000 ppm as the level of concern. In you haven't read the article yet, check it out, as well as this article: "Stuffy Offices Lower Cognitive Function."

        1. schreib77 | | #19

          Hello Martin, I believe I read your article a while back and that is likely one of the few I have read. It was instrumental in pointing out the importance of CO2 the first time for me and why I spent $300 on a good meter! I had never seen this article you provided the link for but that is a very good article too. I noticed, however, it is dated 3 years ago. Are there still no or almost no residential studies? It seems to me that all states that are pushing tighter homes with new building codes should revise them to also require a CO2 meter if mechanical ventilation is installed.

          Finally, it would be nice to know HOW significant the effects were. and details about the statistical analysis on the study in your link.

          NOTE: please provide link to YOUR article. . . thx.

          1. GBA Editor
            Martin Holladay | | #20

            Q. "Are there still no or almost no residential studies?"

            A. There have been residential studies, and in fact my article reports on the findings of one researcher, Brian Just.

            Q. "Please provide link to YOUR article."

            A. I have done so twice -- once in Comment #13, and once in Comment #16. Here is the link for the third time: "Ensuring Fresh Air in Bedrooms."

      2. Expert Member
        1. schreib77 | | #25

          Thanks just what I was looking for Malcom.

  14. Trevor_Lambert | | #14

    "I literally saved thousands of dollars, Zehnder provides the proof"

    This is a classic false dichotomy, in which you present the most expensive alternative to your system as the only alternative. It's great that you're happy with your system and your decision. It would be interesting to see your energy bills after a year, as well as some CO2 readings throughout the house. You mention taking those readings, I'm curious as to what kind of numbers you were getting. Those potential issues aside, I personally would not trade the few thousand dollars I spent on my ducted HRV for the comfort issues of having ac 80-120CFM stream of -20C air drafting through the house.

  15. schreib77 | | #17

    I was responding to this article alone not the entire universe of choices for air exchangers so I drew from the costs elicited by Zehnder. Even if the cost of another system was $2,100 I would still have saved thousands-- two of them. I did not ignore all other alternatives I simply took advantage of the ripe example offered me!
    FYI: examples of false dichotomy:

    Above I offered a typical value for CO2 levels to Martin. So, Trevor, what are YOUR CO2 levels or do you not have a meter to provide that data? -- after spending thousands? You see, that is the problem I point out in my comment. Humidity ONLY is being addressed by the companies that make these expensive air "tricklers". Most (all?) of these units are being thrown into new homes while failing to monitor the effect on CO2 levels. CO2 is a major variable for providing healthy air. I would love to hear from others as to what ventilation levels / # occupants/ and resultant CO2 levels are! Pile on folks.

    ONE more. Well, each person has his own comfort level one can handle for COLD. Mine, like you probably imagine, is higher than yours. When was the last time you camped in a tent on 2 ft of ice at -22°F? 10°F? 35°F? Yep, done that. So my blood may be a bit thicker so I simply cannot rationalize spending thousands on moving 50 CFM of air back and forth while feeling just a slight coolness walking by my boiler room. We put our onions and potatoes in there for cold storage even with the boiler excess heat.

    1. Trevor_Lambert | | #23

      I have my flow rates set to maintain <650ppm. I think that's about 100-120cfm with three people and a small dog in the house.

      At the cfm levels you're talking about, you can get an HRV in the $350-500 range. Duct should be less than that, if you installed it yourself.

      1. schreib77 | | #24

        By the way, after doing additional checking I found that my levels are more like 900 to 950 ppm but I see almost no increase in the BR overnight. -- Martin and Trevor.

        As to your points, I agree Trevor. I just need to find the "right" one and may also need to consider how I will need to handle CO2 spikes from running the dang FP. Probably have to boost CFM to 100 and allow occasional boost cycles. Whatever the case it is too painful to rationalize spending so many thousands of bucks as this vendor offers and then, not even have CO2 monitoring included. I have put out requests for info on tight homes with wood stoves to see how others' handle the CO2 rise. I will share the info . . .

        By the way, does your CO2 monitor control the HRV or monitor a room space only?

        1. Trevor_Lambert | | #27

          I have two monitors, one in the master bedroom and one in the center of the main floor (open concept, all one room aside from the mechanical room and a 2-piece lavatory). The HRV is programmed to increase ventilation if either of those monitors go above a set level. This is a fail safe, as once I had the rates dialed in, the only time the levels go above the target is if we change our schedule and I forget to manually change the HRV setting (e.g. someone stayed home on a weekday, we had company over for a lengthy dinner party).

  16. GBA Editor
    Martin Holladay | | #21

    As I said before, researcher Brian Just has collected good data to show that HRVs connected to dedicated duct systems are effective at lowering CO2 levels -- more effective than other ventilation systems. Once again, here is the link: "Ensuring Fresh Air in Bedrooms."

    Ventilation equipment manufacturers do, indeed, offer equipment that responds to changes in CO2 levels. One example is the CERV. For more information on the CERV, see "A Balanced Ventilation System With a Built-In Heat Pump."

  17. schreib77 | | #22

    Thanks for your patience I found the link!, 3rd times a charm.

    NO. I had not read that, great article and comments also. I learned a lot and validated some things.
    -- having a portable CO2 meter that can be moved around makes more sense than trying to control the HRV with it. I would set the HRV flow so I minimize living room and bedroom CO2 levels below 800 to 1000 ppm.
    --- It would be nice to see a good statistical study on bedroom CO2 levels and health / sleep "goodness", next day tiredness.

  18. AndrewsPropertiesLLC | | #26

    Hi Alex,

    Thanks for the post and sharing your experience. I am currently building a new construction home in New Hampshire, coincidently only about 20 min from Zehnder HQ... I have been researching the Zehnder ComfoAir 350 over the past couple weeks which is how I came across your post. You mention the cost of the unit coming in around $6k. Did that include install or was that just the unit from the manufacturer? I have submitted for a quote from them so should learn soon, but figured I'd inquire here as well. I am considering handling the install myself and having Zehnder come out for commissioning. Any additional feedback you could share would be greatly appreciated. Thanks for your contributions to the forum, I have found the info here invaluable in my process of designing and building a healthy and efficient home for my family.

  19. Saralowel | | #28

    As a small business owner, I found this blog particularly interesting. I do agree that there are great benefits to having your AC units maintained. Hot AC-less downtime is bad for business. I certainly learned that the hard way. I highly recommend

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