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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. Aj Builder, Upstate NY Zone 6a | | #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. Peter L | | #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. Jason D | | #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. Donald Endsley | | #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. User avater 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. User avater
    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. User avater 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. Donald Endsley | | #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. Aj Builder, Upstate NY Zone 6a | | #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. Jason D | | #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. John Klingel | | #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.

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